Vitamin D Deficiency


I didn't think I would be deficient in vitamin D due to my habit of exercising outdoors every day. I was wrong. Do not assume you are vitamin D sufficient; ask your doctor to check your vitamin D levels.

Summary

Links Between Vitamin D and Diabetes

The Details

About Vitamin D
Reviews of Vitamin D and Type 1 or Type 2 Diabetes

Studies on Vitamin D and:


Summary

Links Between Vitamin D and Diabetes

There are many hundreds of studies on vitamin D in relation to diabetes. There is evidence from observational studies that having adequate vitamin D levels is protective against type 1 diabetes, and some evidence that vitamin D deficiency could contribute to the development of type 1 and perhaps type 2 diabetes. Further studies that include measurement of vitamin D levels in people before they develop diabetes, and intervention trials that involve ensuring adequate vitamin D levels could confirm these findings. Vitamin D deficiency is also linked to an increased risk of insulin resistance and gestational diabetes, and may affect diabetes management and the development of diabetes complications in all types of diabetes.

While we await the results of more intervention trials, taking vitamin D supplements offers a safe way to protect against vitamin D deficiency-- and hopefully protect against the development of diabetes or its complications as well. Pregnant women should also make sure they are sufficient in vitamin D. Vitamin D deficiency during pregnancy is linked not only to gestational diabetes but also preterm birth and additional health effects on the offspring. Ensuring vitamin D sufficiency during pregnancy is safe, simple, and cost-effective (Genuis 2015).

There is a trial that is open to enrollment to see if vitamin D and omega 3 supplements can help prevent the development of type 1 diabetes (Ricordi et al. 2019). For information on enrollment, see the grassroots health D*Action project, or preventt1d.org.

There are a number of controversies surrounding vitamin D, for example: what levels constitute "deficient" or "insufficient" or "sufficient;" how to accurately measure vitamin D; how to supplement with vitamin D (which type, how much, how long); and how much of a role vitamin D plays in non-skeletal disorders like diabetes. Yet everyone does agree that, as the title of one commentary states, "vitamin D deficiency is not good for you" (Mathieu and Van der Schueren, 2011).

The Details

About Vitamin D

Vitamin D is known as "the sunshine vitamin" because our skin produces it when exposed to sunlight. Vitamin D can also be obtained through diet. Dietary sources include fish and fish oil, eggs, and fortified foods. In people with limited sun exposure, dietary sources become more important (Norris 2001). Vitamin D supplements are also a source of vitamin D, and are cheap and easy to obtain.

There are different types of vitamin D (calciferol). Two major types are vitamin D2 (ergocalciferol), and vitamin D3 (cholecalciferol). Calcifediol (25(OH)D), is a pre-hormone of vitamin D, and used to measure vitamin D levels in the body. I mostly just use the term "vitamin D" throughout this page to reduce confusion; specific details of which types of vitamin D were used can be found in the abstract of each study.

Vitamin D is technically not a vitamin but actually a hormone, since it is a chemical signal produced by the body and transported through the blood to other parts of the body (vitamins cannot be made by the body, and must be obtained in other ways). When exposed to UV radiation, substances in the skin are converted to pre-vitamin D3, which slowly becomes vitamin D3.

Vitamin D Deficiency

Different organizations and authors define "vitamin D deficiency" and sufficiency in different ways. I have used the definitions that the authors themselves used in their studies. 

The Endocrine Society uses these levels to denote vitamin D deficiency and insufficiency, and also provides recommendations for supplementation at different ages (see Holick et al. 2011 for details):
  • Vitamin D deficiency: 25(OH)D below 20 ng/ml (50 nmol/liter) 
  • Vitamin D insufficiency: 25(OH)D of 21–29 ng/ml (525–725 nmol/liter)
For more on vitamin D deficiency, see the section below, An Epidemic of Vitamin D Deficiency.

Which Evidence is Strongest?

I have organized each section below based on the type of study. Trials, in which people are given vitamin D supplements and followed over time, provide the strongest evidence, especially randomized, controlled trials (RCTs), and large, double-blind RCTs in particular (which are rare).

Observational studies include those in which vitamin D-related measurements are compared in people who do or do not have/develop diabetes, and do not provide as strong evidence as trials. There are many types of observational studies. Longitudinal studies, which follow people over time, are stronger than cross-sectional studies, which provide a snapshot in time. For example, a cross-sectional study may find an association between vitamin D deficiency and an increased risk of diabetes, but cannot determine which came first-- if the diabetes may have caused the vitamin D deficiency or vice versa, or if some other factor contributed to cause both. Thus there are limitations to human observational studies, which is why laboratory studies (e.g., of animals or cells) are also important.

Systematic reviews and meta-analyses combine the data from multiple individual studies, essentially with the aim of making a larger study, but can also lose details of individual study findings. Reviews can include trials, observational, and/or laboratory studies. (When individual studies are large in themselves, I have tried to note that in the text).

The type 1 diabetes section is also organized by timing of exposure, beginning in the womb, since the timing of exposure may make a difference in the risk of later developing diabetes.

Vitamin D exposure is approximated in many different ways in the studies below. Some researchers measure blood levels of vitamin D directly; this is the strongest measure. Other researchers approximate levels by using surveys to ask people about their diet, or whether or not they take vitamin D supplements, or by providing supplements during a trial and asking whether or not people take them. (Past recall is notoriously poor). Some approximate vitamin D level by estimating sun exposure or daylight hours or latitude, this is also a poor measure of vitamin D-- especially since vitamin D deficiency is high even in sunny countries.

This page describes hundreds of studies, but by no means all of the studies on this topic. These aim to be representative of the science in general, with enough detail for those interested.

Reviews of Vitamin D and Type 1 or Type 2 Diabetes

Reviews of Vitamin D and All Diabetes

Most reviews of the evidence on vitamin D and type 1 and type 2 diabetes find that observational studies show associations between vitamin D deficiency and an increased risk of diabetes, but that more evidence is needed from intervention trials to be conclusive (Altieri et al. 2017; Harinarayan 2014; Mathieu et al. 2005; Papandreou and Hamid, 2015). Vitamin D supplementation is linked to improved outcomes in the various diabetes complications of type 1 and type 2, although again, more randomized, controlled trials are necessary (Gembillo et al. 2019).

According to one review, "Data from epidemiological and association studies clearly indicate a correlation between vitamin D deficiency and a higher prevalence of both forms of diabetes. In animal models, vitamin D deficiency predisposes to type 1 and type 2 diabetes, whereas high doses of vitamin D ... prevent disease. Large scale, randomized, blinded prospective studies however, remain lacking... We propose, in particular, to avoid vitamin D deficiency in individuals at risk of developing T1D or T2D (Mathieu 2015). 

Reviews of Vitamin D and Type 1 Diabetes

Epidemiologic (human) evidence suggests that vitamin D deficiency can play a role in the development of type 1 diabetes. Numerous studies have investigated the role of vitamin D as a potential treatment in people with new-onset or established type 1. This free full text review presents the current knowledge on the immune system effects of vitamin D and summarizes the intervention studies investigating its use for prevention or treatment of type 1 diabetes (Infante et al. 2019).

Others have reviewed the mechanisms involved in vitamin D deficiency and type 1 diabetes development, progression, and its complications, including its effects on the immune system (Chakhtoura and Azar, 2013; Rak and Bronkowska, 2018).

One analysis calculated that if everyone in the U.K. took vitamin D supplements, 374 cases of type 1 diabetes (out of 1357 total predicted) could be prevented (Zipitis et al. 2016).

Reviews of Vitamin D and Type 2 Diabetes or Insulin Resistance

Most reviews of the evidence on vitamin D and type 2 diabetes or insulin resistance find that observational studies show associations between vitamin D deficiency and an increased risk of diabetes, but that more evidence is needed from intervention trials to be conclusive (Al-Shoumer and Al-Essa, 2015; Garbossa and Folli 2017; Lips et al. 2017; Muñoz-Garach et al. 2019; Wallace et al. 2016). Vitamin D deficiency may also contribute to the development of related metabolic diseases, including metabolic syndrome (Park et al. 2018).

Reviews of the mechanisms connecting vitamin D to type 2 diabetes or insulin resistance shows that a variety of mechanisms may be involved, including inflammation, beta cell death or dysfunction, oxidative stress, and epigenetics (Berridge 2017; Leung 2016; Palomer et al. 2008; Szymczak-Pajor and Śliwińska, 2019).

Certain populations may be more at risk of vitamin D deficiency and its effects, including, for example, obese adolescents. Most observational studies of vitamin D in obese adolescents have found associations between low vitamin D levels and insulin resistance, and clinical trials have reported beneficial effects (Peterson et al. 2014).

Type 1 Diabetes

Vitamin D can affect the function of the immune system, and in this capacity may be protective against autoimmune diseases (Norris 2001). Vitamin D deficiency is also associated with an increased risk for other autoimmune diseases (Lapillonne 2010; Skaaby et al. 2015; Baeke et al. 2010).

The D*Action Project to Prevent Type 1 Diabetes is currently conducting a trial to see if vitamin D supplements can help prevent the development of type 1 diabetes, and is currently open for enrollment. There may be other ongoing trials as well.

Interestingly, vitamin D may interact with other environmental factors linked to type 1 diabetes, such as viruses (Federico et al. 2018) or environmental chemicals (see the section Vitamin D and Environmental Chemicals below).

Can a Mother's Vitamin D Levels During Pregnancy Affect the Later Risk of Type 1 Diabetes or Autoimmunity in her Children?

Trials

I do not know of any trials investigating this issue.

Observational Studies

Meta-Analyses

Most meta-analyses say "no" or "we don't know" if a mother's vitamin D levels during pregnancy affect her children's risk of type 1 diabetes:
  • A systematic review and meta-analysis of data from 30 observational studies on offspring growth (not all addressed type 1 diabetes) found no association between maternal blood vitamin D levels and the offspring developing type 1 diabetes, although low vitamin D levels were associated with low birth weight, increased risk of small for gestational age birth, and elevated weight at 9 months (Santamaria et al. 2018).
  • A meta-analysis of 3 observational studies on maternal vitamin D intake during pregnancy found no association with type 1 diabetes in the offspring, and concluded that there is not enough evidence to determine whether or not there is an association between on maternal intake of vitamin D and risk of type 1 in the offspring (Dong et al. 2013).
Individual Observational Studies

Some observational studies say "yes," that higher vitamin D levels or related factors in the mother may be linked to a decreased risk of type 1 diabetes or type 1-related autoimmunity in her children: 
  • In Norway, cod liver oil taken during pregnancy was associated with a lower risk of type 1 diabetes in offspring (this association may also be due to the omega-3 fatty acids present in this oil; see the Nutrition page for more on this topic) (Stene et al. 2000).
  • In Norway, higher maternal blood vitamin D levels at delivery and in umbilical cord blood were associated with a lower risk of type 1 diabetes in the offspring, with some influence by genetic risk (Tapia et al. 2019).
  • In Norway, higher maternal blood vitamin D levels during pregnancy were associated with a lower risk of type 1 diabetes in the offspring by age 15. The children of mothers with the lowest vitamin D levels had more than twice the risk of type 1 than those of mothers with the highest levels (Sørensen et al. 2012).
  • In Norway, higher maternal blood levels of vitamin D-binding protein, especially in the third trimester, was associated with a lower risk of type 1 diabetes in offspring by age 15 (Sørensen et al. 2016).
  • In Denmark, more maternal exposure to sunshine during the 3rd trimester was associated with a lower risk of type 1 diabetes in boys at age 5-9 (Jacobsen et al. 2016a).
  • In the U.S., maternal dietary intake of vitamin D via food (but not supplements) during pregnancy was associated with a lower risk of the appearance of type 1-related autoantibodies in children (Fronczak et al. 2003).
Some say "maybe" or "it depends:"
  • In Finland, maternal first trimester blood vitamin D levels were not associated with the children's later risk of type 1 diabetes, although a large percentage of both groups (mothers of children with and without diabetes) had low vitamin D levels (Miettinen et al. 2012). However, the same authors found that the mother's vitamin D-related genes were associated with the offspring's later risk of type 1 diabetes, instead of the vitamin D levels per se. It is possible that these genetic effects are only important if a mother is vitamin D deficient (Miettinen et al. 2015). A reanalysis found that some genes were more important than others (Miettinen et al. 2017).
  • In Sweden, mothers' use of vitamin D supplements during pregnancy was associated with a reduced risk of autoimmunity in the offspring at 1 year of age, but not by 2.5 years of age (Brekke and Ludvigsson, 2007).
  • In Italy, newborn vitamin D levels were not associated with type 1 diabetes development up to age 10 in Italian children, although in a subset of immigrant children (who tended to have lower vitamin D levels), there was an association (Cadario et al. 2015).
Other observational studies say "not really:"
  • In Finland, maternal intake of vitamin D via food or supplements during pregnancy was not associated with the development of type 1 diabetes or related autoimmunity in their offspring (Marjamäki et al. 2010).
  • In Finland, vitamin D levels in umbilical cord blood were not associated with type 1 diabetes development in the offspring (Mäkinen et al. 2019).
  • In Sweden, mothers' use of vitamin D supplements during pregnancy was not associated with the development of type 1 diabetes in their offspring (Granfors et al. 2016).
  • Data from two large population-based studies from Denmark found that despite a high incidence of low vitamin D levels, vitamin D levels at the time of birth were not associated with the offspring's development of type 1 diabetes years later (Jacobsen et al. 2016b).
  • Another large study, using data from Denmark and Norway, found that "normal variation" in maternal vitamin D levels were not associated with the offspring's development of type 1 diabetes (Thorsen et al. 2018). 
  • The large, international TEDDY study found no difference in offspring autoimmunity risk between those whose mothers took vitamin D during pregnancy and those that did not (Silvis et al. 2019).
So, in conclusion:

While the jury is still out, it wouldn't hurt to get vitamin D levels checked during (or before) becoming pregnant, to make sure to avoid deficiency during pregnancy.

Can Vitamin D Levels in Infancy and Early Childhood Affect the Later Risk of Type 1 Diabetes or Autoimmunity?

Trials

I do not know of any trials investigating this issue.

Observational Studies

Meta-analyses of observational studies find that vitamin D supplementation during early life may be protective against the development of type 1 diabetes:

Meta-Analyses
  • A meta-analysis of the data from 5 observational studies found that type 1 diabetes risk was significantly reduced in people who had been given supplements of vitamin D in infancy. Higher doses appeared more protective (Zipitis and Akobeng 2008). 
  • A meta-analysis of data from 8 observational studies found that vitamin D supplementation during early life is associated with a lower risk of type 1 diabetes (Dong et al. 2013). 
Individual Observational Studies

Some observational studies say "yes:"
  • In a study spanning 30 years, Hyppönen et al. (2001) found that regular dietary supplementation of 2000 IU vitamin D per day in the first year of life was associated with an 80% reduced risk of type 1 diabetes in Finnish children.
  • In a Europe-wide study, the EURODIAB Substudy 2 Study Group (1999) found that vitamin D supplementation in infancy was associated with a reduced risk of type 1 diabetes.
  • The large, international TEDDY study found that higher with childhood vitamin D levels had a lower risk of type 1-related autoimmunity, especially children with certain genes (Norris et al. 2018).
  • In Egypt, vitamin D supplementation in the first year of life was associated with a lower risk of developing type 1 diabetes (Awadalla et al. 2017).
  • A long-term study of German children found that vitamin D levels were lower in children with multiple islet autoantibodies and in children with type 1 diabetes than in autoantibody-negative children. However, vitamin D deficiency was not associated with faster progression to type 1 diabetes in children with multiple islet autoantibodies (Raab et al. 2014). This finding suggests that low vitamin D levels may play more of a role in the development of autoimmunity than the subsequent development of type 1 diabetes.
While others say "no:"
  • In genetically at-risk U.S. children, neither vitamin D intake and nor blood vitamin D levels during childhood were associated with the development of either type 1 diabetes or related autoimmunity (Simpson et al. 2011).
  • In Finland, vitamin D levels during childhood (age 3 months on) were not associated with the development of type 1 diabetes or related autoimmunity (Mäkinen et al. 2016).
  • In Sweden, vitamin D supplementation during infancy was not associated with autoimmunity at 1 or 2.5 years of age (Brekke and Ludvigsson 2007).
  • In Finland and Estonia, blood vitamin D levels were not associated with islet autoantibodies (Reinert-Hartwall et al. 2014).
  • In Norway, neither vitamin D supplements nor cod liver oil during the first year of life were associated with the risk of developing type 1 diabetes by age 15 (Stene et al. 2000).
In conclusion:

So while not all studies agree, many have found that vitamin D supplements in infancy and early childhood may be protective against later type 1 diabetes.

Can Vitamin D Levels in Young Adulthood Affect the Later Risk of Type 1 Diabetes?

Trials

I do not know of any trials investigating this issue.

Observational Studies

Two longitudinal studies of U.S. military personnel have looked at this question, and say "yes:"
  • One found that those with lower vitamin D levels had a higher risk of later developing insulin-dependent diabetes than those with higher levels (Gorham et al. 2012).
  • Another found that for non-Hispanic whites, those with the lowest vitamin D levels were more likely to later develop type 1 diabetes (there was no association in blacks or Hispanics) (Munger et al. 2013).

Are Vitamin D Levels Associated With Newly Diagnosed Type 1 Diabetes? 

Many studies say "yes:"
  • In U.S. children, vitamin D deficiency and insufficiency are common in those newly diagnosed with type 1 diabetes, especially those with ketoacidosis (Al-Zubeidi et al. 2016).
  • Italian children had lower vitamin D levels at the time of type 1 diabetes diagnosis than children hospitalized for other diseases (Franchi et al. 2014).
  • Children in northern India had lower vitamin D levels at the time of type 1 diagnosis than healthy controls (Daga et al. 2012).
  • Young Swedish adults had lower vitamin D levels at the time of type 1 diagnosis than controls; 54% percent of those newly diagnosed had insufficient levels of vitamin D, especially men (Littorin et al. 2006).
  • Children and adolescents (both newly diagnosed and those with established type 1) in China had lower vitamin D levels than controls  (Liu et al. 2018).
Others say "no:"
  • In people aged 5-65 from Florida, vitamin D levels did not differ among those newly diagnosed with type 1 diabetes and those without diabetes, although both groups had low levels (Bierschenk et al. 2009).
  • In Danish children newly diagnosed with type 1, vitamin D levels were not associated with autoantibody levels (Thorsen et al. 2018).

Can Adequate Vitamin D Levels Help Extend the Honeymoon Period?

Many people who are first diagnosed with type 1 diabetes have a "honeymoon" period, in which insulin production is at least partly restored. A few case studies have shown that vitamin D may help extend the honeymoon:
  • Two young women who developed type 1 diabetes (antibody positive) had "clinical remission" of their diabetes for 4 years (!) with treatment with vitamin D and sitagliptin (Januvia, a drug used for type 2 diabetes) (Although only one of the patients did not use ANY insulin) (Pinheiro et al. 2016).
  • A man who had developed adult-onset type 1 diabetes (LADA) and was treated with vitamin D, sitagliptin, and metformin (a drug used for type 2 diabetes). He had very deficient vitamin D levels. After 8 weeks of supplements, his fasting blood sugar became normal and his HbA1c declined to normal levels as well, which has continued for 2 years. He never took insulin. His GAD autoantibody levels also declined to normal (Rapti et al. 2016).
  • An 8 year old child took high-dose vitamin D and omega-3 fatty acids (discussed on the Nutrition page), and showed near-normal blood sugar levels 1.5 years after diagnosis, only taking 1.5-2 units of insulin per day (Cadario et al. 2017); at least one other person also had a similar result (Cadario et al. 2018).
  • In a newly diagnosed 14 year old, high doses of vitamin D and omega 3 fatty acids helped preserve and even increase beta cell function (Baidal et al. 2016).

Do People with Type 1 Diabetes Have Low Vitamin D Levels?

Yes, a number of observational studies have found that people with type 1 diabetes have low vitamin D levels as compared to people without diabetes:

Meta-Analyses
  • A meta-analysis of 23 studies found that both children and adults with type 1 diabetes have lower vitamin D levels than those without diabetes (Feng et al. 2015).
  • A meta-analysis of 10 studies of children found that those with type 1 diabetes have lower vitamin D levels than those without diabetes (Liu et al. 2015).
  • A meta-analysis of 12 studies found that children with type 1 diabetes have lower levels of vitamin D than those without diabetes, but that the association was not significant in people 14 and older (Shen et al. 2016).
Individual Observational Studies
  • Swiss children and adolescents with type 1 diabetes had high levels of vitamin D deficiency (60% overall, rising to 84% in winter) (Janner et al. 2010).
  • In the U.S., people with type 1 diabetes were more vitamin D deficient than healthy controls (Devaraj et al. 2011).
  • U.S. youth with type 1 diabetes had a high rate of vitamin D insufficiency (The et al. 2013).
  • Floridians with type 1 diabetes had lower vitamin D-binding protein levels, a molecule important in the actions of vitamin D (Blanton et al. (2011).
  • Middle Eastern children with type 1 diabetes had lower vitamin D levels than those without diabetes (Talaat et al. 2016).
  • In Australia, children and adolescents with type 1 diabetes had lower vitamin D levels than those without diabetes (Greer et al. 2013).
  • Korean youth with type 1 diabetes had lower levels of vitamin D than those without diabetes (Bae et al. 2018); genetic differences may play a role in these associations (Nam et al. 2019).
Even in areas with abundant sunshine, a large percentage of people with type 1 diabetes (and often those without diabetes as well) have low vitamin D levels:
  • In Saudia Arabia, fully 100% of adults with type 1 diabetes tested were deficient in vitamin D, a higher percentage than those without diabetes (although deficiency levels were high in healthy controls as well-- 78% of control children) (Al-Dahgri et al. 2014).
  • In Qatar, 91% of children with type 1 diabetes were vitamin D deficient, as were 85% of children without diabetes (Bener et al. 2009).
  • Fully 84% of Saudi Arabian children with type 1 diabetes were vitamin D deficient, as compared to 59% of children without diabetes (Bin-Abbas et a. 2011).
  • In Kuwait, 99% of children with type 1 diabetes were deficient or insufficient in vitamin D, compared to 92% of controls (Rasoul et al. 2016).
  • In Egypt, 84% of children with type 1 had deficient or insufficient vitamin D levels (Ahmed et al. 2019).
  • In Israel, vitamin D levels were deficient or insufficient more than 80% of youth with type 1 diabetes, especially in those who were religious and dressed more heavily (Brody et al. 2016).
  • In northern India, over 90% of children with type 1 diabetes were deficient in vitamin D (Daga et al. 2012).
  • In Florida, all groups studied had low vitamin D levels, despite living in the "sunshine state" (Bierschenk et al. 2009).
Other factors may combine to increase risk as well. Overweight and obese children with both celiac disease and type 1 diabetes had lower vitamin D levels than controls (Setty-Shah et al. 2014). Genetic background also may affect the risk; people with certain vitamin D receptor (VDR) genes have an increased risk of type 1 diabetes (Penna-Martinez and Badenhoop 2017; Rasoul et al. 2019Sahin et al. 2017) as well as other autoimmune diseases (Gallone et al. 2017).

Can Adequate Vitamin D Levels Facilitate Blood Glucose Control and Reduce the Risk of Complications in People with Type 1 Diabetes?

Perhaps! 

Trials: Type 1 Diabetes Complications and Control

Reviews of Trials
  • A review of 7 randomized controlled trials of vitamin D supplementation in people newly diagnosed with type 1 found that certain types of vitamin D supplements (alphacalcidole and cholecalciferol, but not calcitriol) had beneficial effects on beta cell function (Gregoriou et al. 2017).
Individual Trials: Beta Cell Function and Blood Glucose Control

Most individual trials of vitamin D supplementation and beta cell function or blood glucose control in people with type 1 diabetes found positive results:
  • In the U.S., a randomized trial found that children recently diagnosed with type 1 diabetes who received an umbilical cord infusion, vitamin D, and omega 3 fatty acids, had a lower decline in beta cell function, but the results were not statistically significant (Haller et al. 2013).
  • In Germany, a randomized, controlled, double-blind crossover trial found that people with type 1 diabetes who took 4000 IU vitamin D per day for 3 months had a lower insulin requirement, better blood sugar control (lower HbA1c), and in six (of 39) people, even a rising c-peptide (a marker of beta cell function). Men also had better immune system function (Bogdanou et al. 2017).
  • In the U.K., a trial found that children with type 1 diabetes and vitamin D deficiency or insufficiency who took vitamin D for 3 months had lower HbA1c levels (Giri et al. 2017).
  • In Poland, an intervention trial found that children newly diagnosed with type 1 diabetes who took 3,000 IU vitamin D for a year had better preservation of beta cells and a lower HbA1c (Panjiyar et al. 2018).
  • In Brazil, a randomized, controlled, double-blind trial found that people newly diagnosed with type 1 diabetes who took 2000 IU vitamin D per day for 18 months had less beta cell destruction (Gabbay et al. 2012).
  • In Brazil, a trial found that people with type 1 diabetes who took vitamin D supplements for 12 weeks had lower glycemic variability and insulin dose (Felício et al. 2018).
  • In India, a randomized, controlled, double-blind trial found that children who took 60,000 IU vitamin D once per month for 6 months had higher beta cell function (c-peptide) than in children who did not receive supplements, although HbA1c was not different (Sharma et al. 2017).
  • In India, a 6-month long intervention trial found that children recently diagnosed who took 2,000 IU vitamin D per day for 6 months had a lower decline in beta cell function, but the results were not statistically significant (Mishra et al. 2016).
  • In Saudi Arabia, a controlled trial found that people with type 1 diabetes and vitamin D deficiency who took 4000 IU of vitamin D per day for 12 weeks had better blood sugar control if their vitamin D levels were higher after treatment than if their vitamin D levels did not rise as much from the supplements (Aljabri et al. 2010).
  • In Iran, a trial found that children with type 1 diabetes and vitamin D deficiency who took 300,000 IU of vitamin D had better blood glucose control (lower HbA1c) after 3 months (Mohammadian et al. 2015).
  • In Iran, a trial found that youth with type 1 diabetes who took 50,000 IU vitamin D biweekly for 3 months had lower HbA1c levels (Ordooei et al. 2017).
  • In Egypt, a trial found that children with type 1 diabetes and vitamin D deficiency who took vitamin D supplements for 3 months had lower HbA1c levels (Hafez et al. 2017) and lower LDL cholesterol levels (Hafez et al. 2019).
  • In Egypt, a trial found that children with type 1 diabetes took vitamin D supplements for 3 months had lower HbA1c levels (Ahmed et al. 2019).
However, not all trials found positive results:
  • In the U.K., a trial found that children with type 1 diabetes and vitamin D deficiency who were treated once with high-dose vitamin D did not show improved HbA1c after treatment, although at baseline, HbA1c levels were associated with vitamin D levels (Perchard et al. 2017).
  • In the U.S., a randomized trial found that adolescents with type 1 diabetes and vitamin D deficiency who took high-dose vitamin D for 6 months did not have lower HbA1c levels (Shih et al. 2016).
  • In the U.S., a trial found that children with diabetes and vitamin D deficiency who took vitamin D supplements did not have lower HbA1c levels. (Interestingly, there was improvement for children with type 2 diabetes) (Nwosu and Maranda, 2014).
Individual Trials: Immune System Function and Inflammation

Most trials on vitamin D supplementation on the immune system in people with type 1 diabetes found positive results:
  • In Italy, a trial found that youth with type 1 diabetes and vitamin D deficiency who took vitamin D supplements for a year had reduced reactivity of immune cells against beta cells, and no decline in c-peptide (Federico et al. 2014).
  • In Austria, a double-blind, randomized, controlled trial found that youth with new-onset type 1 diabetes who took vitamin D supplements for a year had improved immune system function (Treiber et al. 2015).
  • In Austria, a double-blind, controlled trial found that adults with type 1 diabetes who took vitamin D for 3 months had enhanced levels of immune cells that are thought to be protective against autoimmunity (c-peptide did not change) (Bock et al. 2011).
  • In Canada, a trial found that adolescents with type 1 diabetes and vitamin D deficiency who took vitamin D supplements had lower levels of inflammation and increased vascular function (although blood pressure and HbA1c did not change) (Deda et al. 2018).
Individual Trials: Diabetes Complications

Most trials found positive results for kidney-related complications; I don't know of trials for other complications:
  • In Denmark, a double-blind, randomized, controlled crossover trial found that adults with type 1 diabetes and kidney disease who took vitamin D analog (paricalcitol) had improvement in some kidney function markers (Joergensen et al. 2015).
  • In Brazil, a trial of people with type 1 diabetes who took high doses of vitamin D daily had a reduced prevalence of kidney disease and improvements in kidney disease stage in many participants (Felício et al. 2017).
  • In China, a trial of  people with type 1 diabetes who took vitamin D (calcitriol) supplements for 6 months had reduced inflammation and improved markers of kidney disease (Mao et al. 2014).
Observational Studies: Type 1 Diabetes Complications and Control

Reviews of Observational Studies
  • A review found there is a correlation between vitamin D deficiency and diabetic ketoacidosis (Iqbal et al. 2019).
Individual Observational Studies: Beta Cell Function, Blood Glucose Control, and Inflammation

Most observational studies on beta cell function, blood glucose control, and HbA1c have found that people with type 1 diabetes who have higher vitamin D levels have better outcomes, although not all findings reached the level of statistical significance:
  • In U.S. youth with type 1 diabetes, lower vitamin D levels were associated with higher HbA1c levels, however, the relationship was not quite statistically significant (Al Sawah et al. 2016).
  • In U.S. people with type 1 diabetes, lower vitamin D levels were associated with higher levels of inflammation and with microvascular complications (Devaraj et al. 2011).
  • In Polish adolescents with type 1 diabetes, lower vitamin D levels were associated with higher HbA1c levels (Wierzbicka et al. 2016).
  • In Italian youth with type 1 diabetes, lower vitamin D levels were associated with higher HbA1c levels (Savasito et al. 2016).
  • In Middle Eastern children with type 1 diabetes, lower vitamin D levels were associated with higher HbA1c levels, autoantibody levels, and markers of inflammation (Talaat et al. 2016).
  • In Egyptian adults with type 1 (and type 2) diabetes, lower vitamin D levels were associated with more beta cell dysfunction (Omar et al. 2018).
Individual Observational Studies: Insulin Resistance

Most observational studies have found that low vitamin D levels are linked to higher insulin resistance in people with type 1 diabetes:
  • In U.S. youth with type 1 diabetes, lower vitamin D levels were associated with higher insulin resistance (The et al. 2013).
  • In Polish adults with type 1 diabetes, lower vitamin D levels are associated with higher insulin resistance (Kamiński et al. 2019).
Individual Observational Studies: Kidney Disease

Most observational studies have found that people with type 1 diabetes and low vitamin D levels have a higher risk of kidney disease or related markers:
  • The Diabetes Complications and Control Trial (DCCT), a long-term study from the U.S. and Canada, found that low vitamin D levels were associated with an increased risk of microalbuminuria (a sign of kidney disease) in people with type 1 diabetes (de Boer et al. 2012).
  • A prospective European study of people with type 1 diabetes found that low vitamin D levels were associated with a higher risk of macroalbuminuria (also a sign of kidney disease) (Engelen et al. 2015).
  • In Brazilians with type 1 diabetes, lower vitamin D levels were associated with a higher risk of, and more severe, kidney disease (Felício et al. 2016).
But not all studies found associations between vitamin D levels and kidney disease markers:
  • A long-term Danish study found that severely deficient people with type 1 diabetes had a higher risk of all-cause mortality, but not kidney complications (Joergensen et al. 2011).
  • In U.S. children with type 1 diabetes, lower vitamin D levels were not associated with albuminuria (a sign of kidney disease) (Nandi-Munshi et al. 2017).
Individual Observational Studies: Retinopathy

Some studies have found links between retinopathy and vitamin D levels in people with type 1 diabetes:
  • In Japanese youth with type 1 diabetes, vitamin D deficiency is associated with retinopathy (Shimo et al. 2014).
  • In Australian youth with type 1 diabetes, vitamin D deficiency is associated with retinopathy (Kaur et al. 2011).
But many have not:
  • The long-term study Danish study cited above also found that severe vitamin D deficiency was not associated with microvascular eye complications (Joergensen et al. 2011).
  • The prospective European study cited above also found that vitamin D levels were not associated with the risk of retinopathy (Engelen et al. 2015).
Individual Observational Studies: Cardiovascular Disease and Related Measures

Some studies have found links between vitamin D levels and cardiovascular-related outcomes in people with type 1 diabetes:
  • In Spanish children with type 1 diabetes, vitamin D deficiency was associated with higher triglyceride levels (Zambrana-Calví et al. 2016).
  • In Spanish people with type 1 diabetes, lower vitamin D levels was associated with arterial stiffness (Llauradó et al. 2015).
  • In Saudi Arabian children with type 1 diabetes, vitamin D deficiency was associated with high triglycerides (and not HbA1c) (Al Shaikh and Al Zahrani 2015).
But many have not:
  • The DCCT study cited above also found that vitamin D levels were not associated with hypertension (de Boer et al. 2012).
  • The prospective European study cited above also found that vitamin D levels were not associated with cardiovascular disease (Engelen et al. 2015).
  • In Spanish adults with type 1 diabetes, vitamin D levels were not associated with coronary artery disease (Serra-Planas et al. 2015).
  • In Italian adults with type 1 diabetes, vitamin D levels were not associated with non-alcoholic fatty liver disease (NAFLD) (Cipponeri et al. 2019).

Laboratory Studies: Type 1 Diabetes

Animal studies can help validate (or dispute) the results of human studies, and also reveal the mechanisms involved. There are a few different animals used for studies of type 1 diabetes, including: non-obese diabetic (NOD) mice, which spontaneously develop diabetes similar to type 1; or mice/rats given the chemical streptozotocin (STZ), which kills beta cells and induces beta-cell deficient diabetes.

In NOD mice, life-long, high doses of vitamin D can prevent the development of diabetes (Takiishi et al. 2014). In mice given STZ, vitamin D reduced the incidence of diabetes, enhanced insulin secretion, reduced pancreatic inflammation, and decreased beta cell death (Wang et al. 2016). Vitamin D also improved diabetic kidney disease (Deng et al. 2016). In rats given STZ, vitamin D improved HbA1c numbers, increased insulin levels, and improved liver markers (Derakhshanian et al. 2019). STZ-induced rats also showed better cognitive function when given vitamin D (Calgaroto et al. 2014), and better lipid levels, which may help prevent cardiovascular complications (Calgaroto et al. 2015). STZ-induced rats given vitamin D had lower inflammation, less liver damage, and lower insulin resistance (Liu et al. 2016). Also in STZ-induced rats who received islet transplantation, vitamin D and omega 3 fatty acids led to lower blood sugar levels and improved inflammation (Gurol et al. 2016).

Exposure During Development

When female mice were deficient in vitamin D while pregnant, the offspring developed higher blood glucose levels, fewer beta cells, and less insulin secretion, as well as a variety of other changes to the pancreas later in life (Maia-Ceciliano et al. 2016).

In Vitro Studies on Cells

Vitamin D can protect beta cells (He et al. 2019). Beta cells exposed to vitamin D can show both increased and decreased insulin secretion, depending on the amount of glucose they were exposed to, and how long they were exposed to vitamin D (Jeddi et al. 2015).

Geographic and Historical Patterns

One reason vitamin D was suspected of being protective against type 1 diabetes is that countries with high incidence of type 1 tend to be found near the polar regions (e.g., Finland, Sweden), where there is a lack of sunlight during long stretches of the year (see the incidence page). In northern Finland, there are only 2 hours of sun each day during December. In a study of 51 regions around the world, Mohr et al. (2008) found that lower levels of ultraviolet (UV) radiation (a source of vitamin D in humans) was associated with higher incidence of type 1 diabetes.

Yet differences in vitamin D status do not explain all of the differences in type 1 diabetes incidence. For example, type 1 diabetes rates are six times higher than those in a neighboring section of Russia, despite similar genetic background (and similar latitudes). Vitamin D levels have been found to be essentially the same in both regions, in children and in pregnant women. More people in Russia, in fact, were deficient in vitamin D than in Finland. A number of Scandinavian researchers point out that vitamin D supplementation has long been common in these countries (Viskari et al. 2006). And yet vitamin D deficiency is still common in these countries, perhaps because the recommended supplementation dose has decreased over time.

Using historical data, Mohr et al. (2010) showed that in Finland, the incidence of type 1 diabetes rose gradually after the recommended daily dose of vitamin D was reduced from 4500 IU to 2000 IU, in 1964. In 1992, the recommended daily dose was lowered again, to 400 IU. This change was followed by a steep rise in type 1 diabetes incidence. And then, in 2003, vitamin D intake increased again, and by 2006 the incidence of type 1 diabetes had plateaued (Mäkinen et al. 2014). Is this an interesting coincidence or a causal correlation? That remains to be seen.

People who develop type 1 diabetes tend to be born in certain seasons. A Danish study found that the association between type 1 and season of birth in males disappeared during the years that margarine was fortified with vitamin D. This finding implies that in utero exposure to vitamin D may reduce the seasonality of diabetes diagnosis in males (Jacobsen et al. 2016c). However, the same authors also found that supplementing margarine with vitamin D in that country did not affect the overall risk of type 1 diabetes in children (Jacobsen et al. 2015).

Type 2 Diabetes and Insulin Resistance

Can Adequate Vitamin D Levels Help Delay or Prevent the Onset of Type 2 Diabetes?

Trials

Meta-Analyses of Trials

Most meta-analyses of randomized controlled trials have found that vitamin D supplementation does not help prevent type 2 diabetes:
  • A meta-analysis of 11 randomized controlled trials found no effect of vitamin D on diabetes-related outcomes, although In two trials among patients with baseline glucose intolerance, vitamin D supplementation improved insulin resistance (Mitri et al. 2011).
  • A systematic review and meta-analysis of 35 randomized controlled trials found that vitamin D supplementation had no effect on type 2 diabetes development, insulin resistance, insulin secretion, or HbA1c (Seida et al. 2014).
Individual Trials

However, many individual trials have found that vitamin D supplementation has beneficial effects on people with pre-diabetes or before type 2 diabetes onset:
  • In the U.S., a randomized controlled trial found that overweight/obese African Americans with pre-diabetes or early diabetes who took 4000 IU vitamin D per day for 12 weeks had improved insulin secretion and insulin resistance, but not glucose levels (Harris et al. 2012).
  • In the U.S., a randomized controlled trial found that obese adolescents who took 4000 IU vitamin D per day for 6 months had lower insulin resistance (Belenchia et al. 2013).
  • In Canada, a double-blind, randomized controlled trial found that people with pre-diabetes or newly diagnosed type 2 diabetes who took 5,000 IU vitamin D per day for 6 months had lower insulin resistance and higher beta cell function (Lemieux et al. 2019).
  • In Brazil, a double-blind, randomized controlled trial found that post-menopausal women who took 1000 IU vitamin D per day for 9 months had lower insulin resistance, glucose, and triglyceride levels, and a lower risk of developing metabolic syndrome (Ferreira et al. 2019).
  • In Austria, a double-blind, randomized controlled trial found that pre-menopausal women who took 20,000 IU vitamin D per week for 24 weeks had lower insulin resistance (Trummer et al. 2019).
  • In Australia, a randomized trial found that middle aged adults at risk of type 2 diabetes who took vitamin D and calcium supplements for 6 months did not have improved insulin secretion or beta cell function, but in those who had pre-diabetes, the supplements reduced insulin resistance (Gagnon et al. 2014).
  • In Australia, a randomized controlled found that overweight/obese people of Asian decent who took vitamin D supplements for 16 weeks had improved fasting glucose levels (Scott et al. 2019).
  • In Israel, a double blind, randomized controlled trial found that men without diabetes who were deficient or insufficient in vitamin D who were given 100,000 IU vitamin D bimonthly for a year maintained the same level of insulin resistance and insulin levels as compared to increased levels in those who took the placebo (Tepper et al. 2016).
  • In Turkey, a trial found that overweight or obese pre-menopausal women with vitamin D deficiency who took vitamin D supplements for 6 months had lower insulin resistance and LDL cholesterol levels (Imga et al. 2019).
  • In Japan, a double-blind, randomized controlled trial found that adults without diabetes who took 420 IU of vitamin D for 1 year had improved fasting glucose levels and lower insulin resistance (Sun et al. 2016).
  • In Iran, a double-blind, randomized controlled trial found that adults with prediabetes and low vitamin D levels who took high doses of vitamin D for 6 months had lower insulin resistance and a slower rate of progression to type 2 diabetes, but did not have lower glucose levels (Niroomand et al. 2019).
Although many other trials found no or few beneficial effects:
  • In the U.S., a large randomized controlled trial found that people with prediabetes who took 4000 IU/day for 2.5 years did not have a statistically significant lower risk of type 2 diabetes (although the number of people who developed type 2 was lower) (Pittas et al. 2019).
  • In the U.S., a triple-blind, randomized controlled trial found that while at the beginning of the study, low vitamin D levels were associated with higher insulin resistance, at the end of the trial, healthy children who took various doses of vitamin D for 12 weeks had no improvement in glucose, insulin, or insulin resistance (Ferira et al. 2016).
  • In the U.S., a trial of obese, mostly Hispanic adolescents found that correcting vitamin D deficiency improved oxidative stress, but did not affect insulin resistance (Grunwald et al. 2017).
  • In Canada, a double-blind, randomized controlled trial found that people with pre-diabetes and low vitamin D levels who took 28,000 IU/week of vitamin D for 24 weeks did not have improved glucose tolerance, insulin resistance, or other diabetes-related measures, but did have lower LDL cholesterol levels (Moreira-Lucas et al. 2017).
  • In the UK, a double-blind, randomized controlled trial found that people with pre-diabetes and low vitamin D levels who took 3,000 IU/day of vitamin D for 26 weeks did not have improved insulin resistance, blood sugar control, or beta cell function (Wallace et al. 2019).
  • In Norway, a randomized controlled trial of found that people with pre-diabetes who took 20,000 IU/week for 5 years did not have improved glucose, insulin secretion, insulin resistance, or blood pressure levels, and did not have a lower risk of developing type 2 diabetes (Jorde et al. 2016; Sollid et al. 2014).
  • In Sweden, a randomized controlled trial found that people with pre-diabetes who took 30,000 IU/week vitamin D for 8 weeks did not have improved insulin secretion, insulin resistance, or glucose levels (Wagner et al. 2016).
Meta-Analyses of Observational Studies on Vitamin D and Type 2 Diabetes and Insulin Resistance

Meta-analyses of observational studies find that low vitamin D levels are associated with an increased risk of type 2 diabetes:
  • A meta-analysis of 8 observational studies found that people with lower vitamin D levels had a higher risk of type 2 diabetes (Mitri et al. 2011). 
  • A meta-analysis of 21 long-term prospective studies found that people with lower vitamin D levels had a higher risk of type 2 diabetes (Song et al. 2013). 
  • A meta-analysis of 11 studies found that people with type 2 diabetes had lower vitamin D levels than those without diabetes (Shen et al. 2016).
Individual Observational Studies

Most observational studies have found associations between low vitamin D levels and insulin resistance, metabolic syndrome, or the development of type 2 in people without diabetes:
  • In U.S. sedentary, obese or overweight, black and white post-menopausal women without diabetes, low vitamin D levels were associated with higher fasting glucose levels, insulin resistance, abdominal fat, and triglyceride levels (Sorkin et al. 2014). These authors found that there is a certain level of vitamin D in the blood below which these effects occur (about 26 μg/L); vitamin D levels above this amount were associated with normal blood sugar control.
  • In U.S. adults, lower vitamin D levels were associated with higher HbA1c levels (Kositsawat et al. 2010).
  • In U.S. youth, low vitamin D levels were associated with higher insulin resistance (Bacha et al. 2019; Fu et al. 2019).
  • In Mexican youth, low vitamin D levels were associated with higher insulin resistance (Denova-Gutiérrez et a. 2019).
  • In British adults without diabetes, low vitamin D levels were longitudinally associated with the later development of increased insulin resistance, high blood sugar, and metabolic syndrome over 10 years (Forouhi et al. 2008).
  • In northern Finland, low vitamin D levels were associated with an increased risk of metabolic syndrome (Mutt et al. 2019).
  • In elderly Australian men, low vitamin D levels were associated with higher glucose levels and a higher risk of type 2 diabetes (Carrivick et al. 2019).
  • In Greek children, vitamin D insufficiency was associated with insulin resistance, independent of obesity (Moschonis et al. 2018).
  • In Turkish adults with and without diabetes, low vitamin D levels were associated with increased insulin resistance (Nur-Eke et al. 2019).
  • In obese Sri Lankan children, lower vitamin D levels were associated with increased insulin resistance (Adikaram et al. 2019).
  • In Japanese adults, low vitamin D levels were longitudinally associated with an increased risk of developing type 2 diabetes over 5 years, especially in those with prediabetes (Akter et al. 2019).
  • In Chinese adults, low vitamin D levels were longitudinally associated with an increased risk of developing prediabetes, type 2 diabetes, and insulin resistance over 4 years (Gao et al. 2018).
  • In northeast Chinese middle-aged men, low vitamin D levels were associated with an increased risk of metabolic syndrome (Weldegiorgis et al. 2019).
  • In Chinese postmenopausal women, low vitamin D levels were associated with an increased risk of metabolic syndrome (Huang et al. 2019).
However, not all individual observational studies have found associations:
  • In U.S. children without diabetes, vitamin D levels were not associated with insulin resistance, glucose levels, or beta cell function (Rajakumar et al. 2012).
  • In Hong Kong adults, vitamin D levels were not retrospectively associated with the later development of type 2 diabetes (Leung et al. 2019).
  • In Iranian women, vitamin D levels were not associated with metabolic syndrome (Mehri et al. 2019).

Can Adequate Vitamin D Levels Facilitate Blood Glucose Control and Reduce the Risk of Complications in People with Type 2 Diabetes?

Trials

Meta-Analyses of Trials: Blood Glucose Control, Inflammation, Insulin Resistance

Most reviews of trials found beneficial effects of vitamin D supplementation in people with type 2 diabetes:
Meta-Analyses of Trials: Diabetes Complications
  • Numerous reviews and meta-analyses of trials of people with diabetes and kidney disease found that vitamin D supplementation is safe and effective at reducing markers of kidney damage (Chokhandre et al. 2015; Hu et al. 2019Wang et al. 2019; Zhao et al. 2014).
  • A meta-analysis of 26 randomized controlled trials of vitamin D supplementation in people with diabetes found that supplements lowered systolic (but not diastolic) blood pressure (Jafari et al. 2018).
  • A review of trials (and other studies) of vitamin D and cardiovascular disease found that vitamin D deficiency may be a risk factor for cardiovascular disease, but more large, controlled trials are needed (Mandarino et al. 2015).
  • A review proposes that the vitamin D receptor is involved in the development of non-alcoholic fatty liver disease (NAFLD) (Cimini et al. 2019).
Individual Trials: Blood Glucose Control, Insulin Resistance

Many individual trials found at least some beneficial effects of vitamin D supplementation on insulin resistance or blood glucose control:
  • In Massachusetts, a retrospective study found that children with type 2 diabetes and vitamin D deficiency who took vitamin D supplements for 3 months had lower HbA1c and lower BMI (in children with type 1, there was no change) (Nwosu and Maranda, 2014).
  • In Australia, a randomized controlled trial found that people with new-onset type 2 diabetes who took high-dose vitamin D for 3-6 months had improved glucose levels at 3 months but not at 6 months (Elkassaby et al. 2014).
  • In Iran, a randomized controlled trial found that adults with type 2 diabetes and vitamin D insufficient who took both vitamin D and calcium (but neither alone) had improved blood sugar control (HbA1c levels) and beta cell function, and lower insulin levels and insulin resistance (Tabesh et al. 2014).
  • In Iran, a randomized controlled trial found that people with type 2 diabetes who consumed a daily consumption yogurt drink enriched with vitamin D had improved blood glucose control (Nikyooeh et al. 2011) and glycemic markers (Jafari et al. 2016).
  • In Iran, a double-blind, randomized controlled trial found that people with type 2 diabetes and coronary artery disease who took high doses of vitamin D for 6 months had lower insulin resistance, lower blood glucose levels, and lower insulin levels (Farrokhian et al. 2017).
  • In Iran, a double-blind, randomized controlled trial found that people with type 2 diabetes and vitamin D deficiency who took high doses of vitamin D for 8 weeks had improved blood glucose (HbA1c) and HDL cholesterol levels (Safarpour et al. 2018).
  • In Iran, a randomized, controlled trial found that people with type 2 diabetes and vitamin D insufficiency who took high doses of calcium and vitamin D (either together or alone) for 8 weeks had lower inflammation levels (Tabesh et al. 2014).
  • In Saudi Arabia, a double-blind, randomized controlled trial found that people with type 2 diabetes who took 5000 IU vitamin D per day for 12 weeks had improved beta cell function, but no change in HbA1c or insulin resistance (Al-Sofiani et al. 2015).
  • In Saudi Arabia, a prospective observational study found that people with diabetes who took vitamin D supplements for 9 months had lowered HbA1c levels (Buhary et al. 2017).
Although some did not:
  • In the Netherlands, a double-blind, randomized, controlled trial found that people with well-controlled type 2 diabetes who took high-dose vitamin D for 6 months did not have improved blood sugar control or insulin resistance. Even those with low vitamin D levels or higher HbA1c levels (over 7%) did not show improvement (Krul-Poel et al. 2015).
  • In Norway, a double-blind, randomized, controlled trial found that people with type 2 diabetes who took high doses of vitamin D for 6 months did not have improved insulin resistance or insulin secretion (Gulseth et al. 2017).
Individual Trials: Diabetes Complications

Many individual trials found beneficial effects of vitamin D supplementation on diabetes complications and metabolic outcomes:
  • In Brazil, a trial found that people with type 2 diabetes who took 4000 IU vitamin D per day for 8 weeks had improved cholesterol levels, insulin and glucose levels, and more (Fagundes et al. 2019). 
  • In Mexico, a double-blind, randomized controlled trial found that women with type 2 diabetes who took vitamin D supplements had improved triglyceride levels (Muñoz-Aguirre et al. 2015).
  • In Pakistan, a prospective study of people with type 2 diabetes who took high dose vitamin D had less painful neuropathy (Alam et al. 2017Basit et al. 2016), as well as in Iran (Ghadiri-Anari et al. 2019).
  • In Iran, a double-blind, randomized controlled trial found that people with a diabetic foot ulcer who took high doses of vitamin D for 12 weeks had better wound healing (and better blood glucose and cholesterol levels) (Razzaghi et al. 2017). 
  • In Iran, a double-blind, randomized controlled trial found that people with diabetes and nephropathy with low vitamin D levels who took high doses of vitamin D for 8 weeks had improved cholesterol levels (Barzegari et al. 2019).
  • In Iran, a double-blind, randomized controlled trial found that people with type 2 diabetes and coronary heart disease who took vitamin D, K, and calcium supplements for 12 weeks had improved insulin resistance and cholesterol levels (Asemi et al. 2016).
  • In Iran, a double-blind, randomized controlled trial found that people with type 2 diabetes and kidney disease who took high doses of vitamin D had improved outcomes (Esfandiari et al. 2019).
  • In Iran, a double-blind, randomized controlled trial found that people with type 2 diabetes with vitamin D deficiency and coronary heart disease who took high-dose vitamin D and omega 3 fatty acids for 6 months had improved markers of cardiovascular risk, plus lower blood glucose and insulin levels, lower insulin resistance, and improved cholesterol levels (Talari et al. 2019).
  • In Iran, a double-blind, randomized controlled trial found that people with type 2 diabetes and mild to moderate depression who took 4000 IU vitamin D per day for 3 months had improved depressive symptoms, as well as improved HbA1c, insulin, and triglyceride levels (Omidian et al. 2019).
  • In Egypt, a randomized trial found that people with type 2 diabetes getting kidney dialysis who took vitamin D supplements had better fasting blood sugar, lower HbA1c, better beta cell function-- but interestingly, also more insulin resistance (Ibrahim et al. 2015).
  • In India, a trial found that people with type 2 diabetes and vitamin D deficiency who took high doses of vitamin D for 8 weeks had improved vascular function (Anandabaskar et al. 2017).
Although some did not:
  • In the U.S, double-blind, randomized controlled trials found that people with well-controlled type 2 diabetes who took 4000 IU vitamin D per day for 48 weeks had no improvement in HbA1c or insulin secretion (Angellotti et al. 2018a), nor in cardiovascular markers (Angellotti et al. 2018b).
  • In Italy, a double-blind, randomized controlled trial found that people with type 2 diabetes and non-alcoholic fatty liver disease (NAFLD) who took 2000 IU vitamin D per day for 24 weeks had no improvements (Barchetta et al. 2016).
Observational Studies: Diabetes Control and Complications

Meta-Analyses of Observational Studies: Diabetes Control and Complications

Most reviews of observational studies on type 2 diabetes control and complications found that low vitamin D levels were associated with worse health parameters:
  • A review of vitamin D and diabetes complications found that, "An increasing body of literature suggests a possible pathogenetic role of vitamin D in the long-term complications of diabetes and vitamin D deficiency may also exacerbate symptoms of painful diabetic peripheral neuropathy. It remains unknown if supplementation of vitamin D to normal or non-deficient levels alters pathogenetic processes related to diabetic microvascular complications." (Alam et al. 2016).
  • A meta-analysis of 6 studies found that in people with type 2 diabetes, vitamin D deficiency was associated with a higher risk of peripheral neuropathy (Lv et al. 2015).
  • A meta-analysis of 14 studies found that in people with type 2 diabetes, vitamin D deficiency was associated with a higher risk of retinopathy (Zhang et al. 2017).
  • A review of animal and clinical studies found that vitamin D may be protective against diabetic kidney disease (Guan et al. 2014).
  • A systematic review and meta-analysis of 7 studies found that in people with diabetes, severe vitamin D deficiency was associated with an increased risk of foot ulcers (Dai et al. 2019).
  • Genetic risk might also play a role in these associations. People with diabetes who had certain vitamin D-related genes were more susceptible to kidney disease than those without those genes (Liu et al. 2014).
Individual Observational Studies

Most individual observational studies on type 2 diabetes control and complications found that low vitamin D levels were associated with worse health parameters.

Individual Observational Studies: Blood Glucose Control, Insulin Resistance, Beta Cell Function, and HbA1c

Most individual observational studies on blood glucose control, insulin resistance, HbA1c, or beta cell function find higher vitamin D levels are associated with better outcomes:
  • In non-obese Iranians with type 2 diabetes, low vitamin D levels were associated with higher fasting blood glucose levels (but not insulin resistance, HbA1c, or insulin levels) (Haidari et al. 2016).
  • In Egyptians with type 2 diabetes, low vitamin D levels were associated with a higher HbA1c (Ali et al. 2019Saif-Elnasr et al. 2017).
  • In adults with type 2 diabetes from the Gaza Strip, low vitamin D levels were associated with a higher HbA1c (Yassin et al. 2019).
  • In Saudi Arabians with type 2 diabetes, low vitamin D levels were associated with a higher HbA1c (Darraj et al. 2019).
  • In Malaysians with type 2 diabetes and chronic kidney disease, lower vitamin D levels were associated with a higher HbA1c (Lim et al. 2018).
  • In Chinese women newly diagnosed with type 2 diabetes, low vitamin D levels were associated with insulin resistance and worse beta cell function (there was no difference in men) (Gao et al. 2015).
Individual Observational Studies: Neuropathy, Retinopathy, Kidney Disease

Most individual observational studies on neuropathy (nerve disease), retinopathy (eye disease), an nephropathy (kidney disease) find better outcomes associated with higher vitamin D levels:
  • In a small study of Indian adults with type 2 diabetes, vitamin D deficiency was associated with neuropathy, retinopathy, and kidney disease (Bajaj et al. 2014).
  • In elderly Chinese people with type 2 diabetes, low vitamin D levels were associated with neuropathy, but there was no association between vitamin D levels and neuropathy in younger people with type 2 diabetes (Niu et al. 2019).
  • In Europeans with type 2 diabetes, lower vitamin D levels were associated with painful neuropathy (Shillo et al. 2018).
  • In Chinese adults with type 2 diabetes, lower vitamin D levels were associated with retinopathy (Wan et al. 2019).
  • In Turkish adults with type 2 diabetes, vitamin D deficiency was associated with kidney disease, as well as with more severe disease (Usluogullari et al. 2015).
  • In Romanians with type 2 diabetes and end-stage renal disease, low vitamin D levels are associated with a higher risk of mortality (Schiller et al. 2015).
  • In Chinese adults with type 2 diabetes, lower vitamin D receptor expression was associated with more severe albuminuria (a sign of kidney disease) (Yi et al. 2016). 
  • In Chinese adults with type 2 diabetes, low vitamin D levels were associated with an increased risk of kidney disease as well as with markers of kidney disease (Xie et al. 2019).
Although not all:
  • In Indian adults with type 2 diabetes, vitamin D deficiency was not associated with retinopathy (Reddy et al. 2015), nor was it in a study from the UK (Alam et al. 2016).
Individual Observational Studies: Cardiovascular and Other Complications

Most individual observational studies on cardiovascular and other diabetes complications find better outcomes associated with higher vitamin D levels:
  • In U.S. adolescents with type 2 diabetes, low vitamin D levels were associated with higher arterial stiffness (a risk factor for cardiovascular disease) (Jha et al. 2015).
  • In Swedish men with type 2 diabetes, low vitamin D levels were associated with a higher risk of mortality (but not in women) (Jennersjö et al. 2015).
  • In Chinese adults with type 2 diabetes, vitamin D deficiency is associated with a higher risk of peripheral arterial disease (Yuan et al. 2019) and periodontitis (Wang et al. 2019).
  • In Chinese adults with type 2 diabetes, low vitamin D levels are associated with an increased risk of cognitive impairment (Rui-Hua et al. 2019).
  • In Bahranians with diabetes, those with lower vitamin D levels had impaired wound healing (Smart et al. 2019).
  • In Indians with type 2 diabetes and foot infections, vitamin D deficiency was associated with higher levels of inflammation (Tiwari et al. 2014).
  • In Indians with type 2 diabetes, low vitamin D levels were associated with periodontal disease (Joseph et al. 2015).
  • In Egyptians with type 2 diabetes, low vitamin D levels were associated with non-alcoholic fatty liver disease (NAFLD) (Hosny et al. 2019).

Obesity

Obesity is sometimes associated with vitamin D deficiency, probably because vitamin D can be deposited in fat stores and then not be available to the body (Holick 2004). For example, obese children and adolescents have high rates of vitamin D deficiency (Zakharova et al. 2019). A study of obese people found that those who were metabolically healthy had higher vitamin D levels than those who were metabolically unhealthy (Estghamati et al. 2014). A systematic review and meta-analysis of randomized controlled trials on vitamin D and weight loss found that vitamin D was associated with lower BMI and waist circumference, but not with weight loss per se (Perna 2019). There are plenty more studies on vitamin D and obesity, but I'm not going to get into them here.

Laboratory Studies: Type 2 Diabetes and Obesity

Vitamin D reduces insulin resistance in rats with type 2 diabetes (Amin et al. 2018).

Long-term ultra-violet radiation (which produces vitamin D in the skin) suppresses weight gain, glucose intolerance, and insulin resistance in mice fed a high-fat diet. However, many of these benefits did not appear with vitamin D supplementation-- perhaps other mechanisms are at work (Geldenhuys et al. 2014).

Exposure During Development

Prenatal vitamin D levels influenced the development of long-term fatty tissue damage in mice (Belenchia et al. 2017).

Gestational Diabetes and Vitamin D in Pregnant Women

Can Vitamin D Sufficiency Help Prevent Gestational Diabetes?

Trials

Meta-Analyses of Trials

According to meta-analyses of trials, it is not yet known whether vitamin D supplementation could help prevent gestational diabetes:
  • A systematic review and meta-analysis of 25 randomized controlled trials found that vitamin D supplementation during pregnancy reduced fasting blood glucose and insulin resistance, did not affect beta cell function or HbA1c, and had unclear effects on preventing gestational diabetes (Zhang et al. 2018).
  • A systematic review and meta-analysis of 13 randomized controlled trials on gestational diabetes and other outcomes found that there was no association between vitamin D supplementation and gestational diabetes (there were other benefits however) (Pérez-López et al. 2015).
Individual Trials

Some individual trials of vitamin D supplementation in pregnant women have interesting findings:
  • In Iran, a randomized trial found that women with low vitamin D levels who took 50,000 IU vitamin D every two weeks had a lower incidence of gestational diabetes than who took 400 IU per day (Mojibian et al. 2015).
  • In Iran, a randomized controlled trial found that women at risk of gestational diabetes who took 5000 IU vitamin D per day for the 1st and 2nd trimesters had a lower risk of gestational diabetes (Shahgheibi et al. 2016).
  • In obese European women, a randomized controlled trial found that women who took 1600 IU vitamin D per day starting in mid-pregnancy had lower fasting glucose levels at 35-37 weeks but did not have a lower risk of developing gestational diabetes (Corcoy et al. 2019).
Observational Studies

Meta-Analyses of Observational Studies
  • A systematic review and meta-analysis of 87 observational studies found that low vitamin D levels were associated with an increased risk of gestational diabetes, as well as a higher HbA1c, fasting blood glucose levels, and insulin resistance (Zhang et al. 2018).
  • A systematic review and meta-analysis of 29 observational studies found that vitamin D insufficiency was associated with gestational diabetes (Hu et al. 2018).
  • A systematic review and meta-analysis of 26 observational studies found that vitamin D deficiency was associated with an increased risk of gestational diabetes (Amraei et al. 2018).
  • Two systematic reviews and meta-analyses of 20 observational studies found that vitamin D deficiency was associated with an increased risk of gestational diabetes (Lu et al. 2016Zhang et al. 2015).
Individual Observational Studies

While most individual observational studies have found a link between low vitamin D levels and an increased risk of gestational diabetes (e.g., Al-Ajlan et al. 2018Arnold et al. 2015; Ede et al. 2019Haidari et al. 2016; Rajput et al. 2019Wen et al. 2017; Xia et al. 2019Xu et al. 2018), not all have (e.g., Lee et al. 2017; Makgoba et al. 2011). Some have found an association between low vitamin D levels and higher fasting blood glucose, but not a statistically significant level of gestational diabetes development (Al-Shaikh et al. 2016; Sahin et al. 2019). Others have found general diabetes-related benefits of higher vitamin D levels during pregnancy (Benaim et al. 2019; McLeod et al. 2012Walsh et al. 2018).

Other factors may also play a role. For example, in the U.S., a prospective study found that women who took vitamin D before pregnancy had a lower risk of developing gestational diabetes (Bao et al. 2018). In Canada, the risk of gestational diabetes is higher in women who have low vitamin D levels and who also smoke during pregnancy (Dodds et al. 2016). Genetic variations may also affect the relationship between vitamin D levels and gestational diabetes (Shi et al. 2016; Zhu et al. 2019), as does obesity (Valkama et al. 2018).

Do Vitamin D Levels Affect Blood Glucose Control in Women With Gestational Diabetes?

Trials

Almost all trials have found benefits of vitamin D supplementation in women with gestational diabetes.

Meta-Analyses of Trials
  • A systematic review and meta-analysis of 5 randomized controlled trials found that women with gestational diabetes who took vitamin D supplements had lower fasting glucose levels and improvements in cholesterol levels (Jahanjoo et al. 2018).
  • A systematic review and meta-analysis of 5 randomized controlled trials found that women with gestational diabetes who took of vitamin D supplements had lower fasting glucose, insulin, and HbA1c levels (Ojo et al. 2019).
Individual Trials
  • In Iran, a double-blind, randomized controlled trial found that women with gestational diabetes who took 1000 IU vitamin D per day and evening primrose oil for 6 weeks had lower glucose and lipid levels, and lower insulin resistance (Jamilian et al. 2016). 
  • In Iran, a double-blind, randomized controlled trial found that women with gestational diabetes who took magnesium-zinc-calcium-vitamin D supplements for 6 weeks had better blood glucose control and cholesterol levels (Karamali et al. 2018). 
  • In Iran, a double-blind, randomized controlled trial found that women with gestational diabetes who took vitamin D and probiotics had lower fasting glucose and insulin levels, and lower insulin resistance (Jamilian et al. 2018). 
  • In Iran, a randomized controlled trial found that women with gestational diabetes who took vitamin D and calcium supplements had lower fasting glucose and insulin levels, lower insulin resistance, and better cholesterol levels (Asemi et al. 2014).
  • In Iran, a double-blind, randomized controlled trial found that women with gestational diabetes who took vitamin D and omega 3 fatty acids (Jamilian et al. 2017), and that this combination lowered inflammation and oxidative stress, and improved pregnancy outcomes (Razavi et al. 2017).
  • In Iran, a double-blind, randomized controlled trial found that women with gestational diabetes who took vitamin D had lower fasting glucose and HbA1c levels (Yazdchi et al. 2016).
  • In China, a double-blind, randomized controlled trial found that women with gestational diabetes who took high doses (50,000 IU every 2 weeks) of vitamin D had lower insulin, insulin resistance, and cholesterol levels-- but not those who took 200 IU per day or even 2000 IU per day (Zhang et al. 2016).
  • In China, a double-blind, randomized controlled trial found that women with gestational diabetes who drank a yogurt drink enriched with vitamin D had lowered insulin resistance and better cholesterol levels (Li and Xing 2016).
Why are there so many vitamin D studies out of Iran, by the way? I have no idea. Anyhow.

Individual Observational Studies

Observational studies have also found that having adequate vitamin D levels during pregnancy is beneficial:
  • In Egyptian women with gestational diabetes, lower vitamin D levels were associated with higher fasting glucose, insulin, and HbA1c levels (El Lithy et al. 2014).

Women with Prior Gestational Diabetes

As gestational diabetes increases the risk of later type 2 (and actually type 1 also) diabetes, does vitamin D help prevent later diabetes development in women who have had gestational diabetes? Perhaps:

Trials
  • In Asian women with prior gestational diabetes (and therefore at high risk of developing diabetes in the future), a double-blind randomized controlled trial found that women who took 4000 IU vitamin D per day for six months had improved beta cell function and reduced blood glucose levels (HbA1c) (Yeow et al. 2015).
  • In Iran, a trial found that women with gestational diabetes who took high doses of vitamin D while pregnant did not have better glucose levels at 6-12 weeks after delivery (Valizadeh et al. 2016).
Observational Studies
  • In Swedish women of European, Asian, and Arab descent with prior gestational diabetes, low vitamin D levels were associated with beta cell dysfunction and insulin resistance 1-2 years after delivery, but not the later development of diabetes (Shaat et al. 2017).
  • In Hungarian women with prior gestational diabetes, low vitamin D levels were associated with higher insulin resistance 3 years after delivery (Tänczer et al. 2017).

Pregnant Women with Type 1 Diabetes

Observational studies generally show that having low vitamin D levels can increase some risks during pregnancy in women with type 1 diabetes:
  • In pregnant Danish women with type 1 diabetes, low vitamin D levels were associated with a higher risk of preterm delivery, although the difference was not statistically significant after adjusting for other factors. Women with low vitamin D levels did not have an increased risk of pre-eclampsia (Vestgaard et al. 2017).
  • In pregnant U.K. women with type 1 diabetes, cord blood vitamin D levels correlate with those of the mothers, and tend to be low throughout pregnancy, delivery, and post-delivery. Women with low vitamin D levels had higher HbA1c levels, and umbilical cord blood vitamin D levels were lower in obese women (Bennett et al. 2014Bennett et al. 2013aBennett et al. 2013b). (These all appear to be the same study, published 3 times in 3 places... hmmm) 

Vitamin D During Pregnancy and Effects on the Offspring

Low vitamin D levels in pregnant women may also affect the offspring (bringing us back to the topic at the top of this page, whether a mother's vitamin D levels can affect the risk of type 1 in the offspring).

Trials
  • In Iran, a double-blind, randomized controlled trial found that the offspring of women with gestational diabetes who took vitamin D and omega 3 fatty acids had lower rates of hospitalization and of jaundice (hyperbilirubinemia) (Razavi et al. 2017).
  • In Iran, a double-blind, randomized controlled trial found that the offspring of women with gestational diabetes who took high doses of vitamin D had lower rates of jaundice (Asemi et al. 2015).
  • In Iran, a double-blind, randomized controlled trial found that the offspring of women with gestational diabetes who took vitamin D and calcium supplements were less likely to be delivered by C-section, be hospitalized, or have high birth weight (macrosomia) or jaundice (Karamali et al. 2016). 
Meta-Analyses of Observational Studies
  • A systematic review and meta-analysis of data from 30 observational studies on offspring growth found an association between low maternal blood vitamin D levels and low birth weight, increased risk of small for gestational age birth, and elevated weight at 9 months, although low vitamin D levels were not associated with the offspring developing type 1 diabetes (see top of this page for more on type 1) (Santamaria et al. 2018).
  • A systematic review and meta-analysis of 5 randomized controlled trials found that the offspring of women with gestational diabetes who took vitamin D supplements had lower rates of jaundice (Jahanjoo et al. 2018).
Individual Observational Studies
  • In Brazilian women with gestational diabetes, low vitamin D levels were associated with hypoglycemia in infants and small for gestational age birth (Weinert et al. 2016), as well as low birth weight (Padilha et al. 2019).
  • In the U.K., lower maternal vitamin D levels were associated with lower offspring birth weight and with higher fat mass at 4 and 6 years of age (Crozier et al. 2012
  • In Spain, lower maternal vitamin D levels were associated with higher offspring weight at 1 year but not at 4 years (Morales et al. 2015). 
  • In the U.S., maternal vitamin D levels were associated with various measures of body composition in infants (Sauder et al. 2017).
  • In the U.S., vitamin D levels in umbilical cord blood were associated with insulin and beta cell function levels, implying that vitamin D may play a role in controlling fetal insulin secretion (Switkowski et al. 2019).
  • In Denmark, maternal exposure to extra vitamin D from food fortification may lower the offspring's risk of gestational diabetes in girls born in the spring (Keller et al. 2018).
Not all studies have found associations, however:
  • In Singapore, maternal vitamin D levels were not associated with birth outcomes, growth, or weight gain in the offspring in the first 2 years of life (Ong et al. 2016). 

Laboratory Studies: Gestational Diabetes

In guinea pigs, higher vitamin D levels before pregnancy were protective against gestational diabetes, however, vitamin D levels during pregnancy did not affect risk (Tabatabaei et al. 2014). In animals, maternal vitamin D deficiency during pregnancy leads to insulin resistance in the offspring (Zhang et al. 2014).

An Epidemic of Vitamin D Deficiency

One surprising thing that most of the above studies have in common is the finding that many people have insufficient levels of vitamin D in their bodies-- even people who live in regions with bountiful sunshine (e.g., in Florida (Bierschenk et al. 2009), the Mediterranean region (Karras et al. 2016), the Middle East (Singh et al. 2019), and India (Daga et al. 2012; Sharma et al. 2019). Ginde et al. (2009) found that based on a nation-wide sample, 77% of the U.S. population has insufficient vitamin D levels (including nearly all African Americans and almost all Mexican Americans). In addition, levels of vitamin D have declined in the U.S. population, comparing the periods 1988-1994 to 2001-2004. Even in the hot, sunny climate of Saudi Arabia, most people tested were vitamin D deficient (Al-Daghri et al. 2014). I just don't think sunshine is enough.
 
Mohr et al. (2008) suggest that children over age 1 who live more than 30 degrees from the equator (that includes almost all of the U.S.), take 1000-2000 IU vitamin D3 per day, especially during the winter, "to substantially reduce their risk of type 1 diabetes." The current U.S. recommendations are only 200 IU vitamin D per day, a level that is inadequate to address the growing epidemic of vitamin D deficiency (Ginde et al. 2009). Others recommend that children under 1 receive 1000 IU/day, older children 3000 IU/day, and young adults even more (Papadimitriou 2017). I don't know enough to make recommendations here; please contact your doctor. During pregnancy, the current recommendations of 200-400 IU per day are likely inadequate to maintain normal levels of vitamin D during pregnancy and lactation (Mulligan 2010). Parents of children at risk of developing type 1 might take note and supplement. And, during the sunnier months, parents might also try the "leave no child indoors" method of vitamin D "supplementation." Although, be aware that while the urban myth may argue that adequate vitamin D can be obtained from the sun, it appears that few US residents make adequate vitamin D even without sunscreen, at any time of the year (Godar et al. 2012).
 
Norris (2001) suggests that various public health initiatives may have acted together to reduce vitamin D exposure in children, including keeping babies out of the sun, using sunscreen on young children, and exclusive breastfeeding. Exclusively breastfed infants are at higher risk of vitamin D deficiency than are formula-fed infants (Kovacs 2008); each month of breastfeeding (without supplementation) increases the risk of vitamin D deficiency by 6% (Darmawikarta et al. 2016). This deficiency can be remedied however, with sufficient supplementation for pregnant and nursing mothers. During pregnancy, if the mother has sufficient levels of vitamin D (which may require supplementation), then the fetus should too. After birth, the mother may need up to 4000 IU per day to maintain sufficient levels in her and her exclusively breastfed baby (Kovacs 2008). 

Vitamin D and the Gut

In infants, vitamin D levels are associated with gut microbiota (Sordillo et al. 2017). Vitamin D levels also appear to affect the gut microbiota of mice (Jin et al. 2015). A review argues that the interactions between vitamin D and the gut microbiota may be critical factors in the increasing incidence of type 1 diabetes (Clark and Mach 2016). See the Diet and the Gut page for more information on how the gut might be related to diabetes.

Vitamin D and Environmental Chemicals

Unfortunately, some contaminants may be able to affect vitamin D levels, including air pollutants, heavy metals, >persistent organic pollutants (POPs), and endocrine disrupting chemicals such as BPA and phthalates> (for a free full text review on the role of air pollution and environmental chemicals in vitamin D deficiency, see Mousavi et al. 2019).

Human Studies

Longitudinal Studies

Environmental Chemical Levels Are Associated With Low Vitamin D Levels

vitamin D and chemicals

In U.S. pregnant women, higher levels of both phthalates and BPA were associated with an increased risk of vitamin D deficiency. In these graphs you can see as chemical levels increase, vitamin D levels decrease. 
In U.S., pregnant women, higher phthalate and BPA levels are associated with an increased risk of vitamin D deficiency (Johns et al. 2017).

Cross-Sectional Studies in Adults

In U.S. adults, higher levels of various POPs, including DDT and DDE were associated with lower levels of vitamin D, especially people in old age, who were white, or who had chronic diseases (Yang et al. 2012). In U.S. adolescents and adults, one type of perfluoroalkyl substance (PFAS) was associated with lower vitamin D levels, and another with higher levels (Etzel et al. 2019). In U.S. adults, higher phthalate and BPA levels (in women) are associated with lower vitamin D levels (Johns et al. 2016).

In Spanish pregnant women, higher levels of some PCBs were associated with lower vitamin D levels (Morales et al. 2013).

In Korean adults, a combination of vitamin D deficiency and arsenic exposure were associated with an increased risk of diabetes in adults (Lee and Kim 2013).

Cross-Sectional Studies in Children

Children exposed to higher levels of air pollution in Mexico City had lower vitamin D levels than those exposed to lower levels (Calderón-Garcidueñas et al. 2015). Similarly, higher air pollution was associated with lower vitamin D levels in Iranian adolescents (Feizabad et al. 2017). In Bangladeshi children, higher cadmium levels were associated with lower vitamin D levels (Malin Igra et al. 2019).

However, not all human studies have found a link between chemical exposures and vitamin D levels. In Mexican adolescents, lead and cadmium levels were not associated with vitamin D levels, and those with higher levels of molybdenum, thallium, uranium, and arsenic in fact had higher levels of vitamin D (Zamoiski et al. 2014).

Exposure During Development

In newborns, those whose mothers were exposed to higher levels of air pollutants during pregnancy (especially in the third trimester) had lower vitamin D levels than infants whose mothers were exposed to lower levels of air pollutants (Baïz et al. 2012).

Laboratory Studies

Laboratory studies have also examined the possibility that chemicals may influence vitamin D levels. For example, early life exposure to the pesticide permethrin caused low vitamin D levels in adult rats (Fedeli et al. 2013).

After vitamin D3 forms in the body, or is obtained via diet, it is then converted to other forms of vitamin D. In the liver, it is converted to 25-hydroxyvitamin D (25-D) and in the kidney to the more active form, 1,25 dihydroxycholecalciferol (1,25-D) (Norris 2001). Some chemicals may affect the conversion of one type of vitamin D to another. When researchers fed mother rats a mixture of PCBs similar to those found in human breastmilk, they found that the PCBs caused reductions in the levels of 1,25-D in the mothers at birth and weaning, and these reduction were dependent on PCB dosage. Levels of 1,25-D were also reduced in the offspring at higher exposure levels. Levels of 25-D were lower mothers at the time of delivery, and in the offspring at the highest dosage levels. In the mothers, levels of 1,25-D were reduced even at the lowest PCB treatment level, although this level was still higher than the PCB levels normally found in humans (Lilienthal et al. 2000). Rats given PCBs had lower vitamin D levels than unexposed controls (Alvarez-Lloret et al. 2009).
 
Another animal study found that POPs may disrupt vitamin D conversion processes in seals. It compared vitamin D levels in seals from the highly polluted Baltic Sea to seals from other less polluted waters. It found that while vitamin D3 levels in the liver were higher in the Baltic seals (probably due to sunlight or diet), levels of 1,25-D were lower. Both the contaminant levels and the hormone levels also varied by species. The authors suggest that these findings might explain the bone abnormalities found in gray seals in the Baltic Sea (Routti 2008).
 
Another study on bone toxicity examined the effects of dioxin, another POP, on vitamin D levels in mice. This study found that dioxin actually increased levels of 1,25-D in the mice, but also impaired bone mineralization (Nishimura et al. 2009). How exactly contaminants can affect vitamin D processes in animals, whether the same effects occur in humans, and whether these processes could be a factor in diabetes development all deserve further study.

References

To see or download the references cited on this page, as well as many others on this topic, see the collection Vitamin D and diabetes/obesity in PubMed.