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. In fact, a randomized trial found that sunlight exposure during summer was generally not sufficient to raise vitamin D levels to the recommended amount. Supplementation was more effective (Lee et al. 2020). 

The Summary

Links Between Vitamin D and Diabetes

There are many many hundreds of studies on vitamin D in relation to diabetes (over 900 that I have found). 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.

And finally there is published guidance for using vitamin D supplementation to prolong the "honeymoon" phase right after type 1 diagnosis, based on findings from clinical trials (Nwosu et al. 2022). 

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):

For more on vitamin D deficiency, see the section below, An Epidemic of Vitamin D Deficiency.

About the Studies on this Page

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; Argano et al. 2023; Daskalopoulou et al. 2022; Harinarayan 2014; Mathieu et al. 2005; Papandreou and Hamid, 2015; Wu et al. 2023). 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 and autoimmunity (Chakhtoura and Azar, 2013; He et al. 2022; Rak and Bronkowska, 2018; Sîrbe et al. 2022). A review on vitamin D and beta cells finds that supplementation can help protect beta cells (Yu et al. 2022).

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).

A meta-analysis of 16 studies on vitamin D and type 1 diabetes found that people with the lowest vitamin D levels has the highest  increased risk of type 1 (Hou et al. 2020).

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). A review of trials did find that people with low levels of vitamin D benefited in a number of ways from vitamin D supplementation in relation to diabetes and obesity-related outcomes (Zou et al. 2021). 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). People with insulin resistance or type 2 diabetes should get their vitamin D levels checked, and bring their levels up to ensure they are not deficient; supplementation could be beneficial in these cases (but not necessarily if people are already vitamin D sufficient) (Gröber and Holick, 2019).

Reviews of Vitamin D and Diabetes Complications

A review of vitamin D and type 1 diabetes finds that vitamin D (and omega 3 supplementation) could "provide potential benefits, mainly when done early in the diagnosis, since it reduces the need for insulin and the risk of complications generated by the disease." (Bastos et al. 2022).

A review of vitamin D and diabetic retinopathy finds that "Vitamin D demonstrates a protective role on the development and progression of diabetic retinopathy by reducing blood sugar, hypertension and atherosclerosis, but randomized studies are still needed to establish the direct causal relationship" (Totolici et al. 2022).

A review finds that vitamin D is linked to lower long-term blood glucose leves (HbA1c) in people with diabetes (Akhter et al. 2024).

An Epidemic of Vitamin D Deficiency

One surprising thing that most of the studies on this page 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; Díaz-Rizzolo et al. 2022), 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). 

A study of infants with an increased risk of type 1 diabetes from Germany, Poland, Belgium, England, and Sweden found that despite supplementation, almost a third of infants were vitamin D insufficient, and half of those did not resolve by the next visit (especially if in winter). They suggest increasing treatment guidelines (Jacobs et al. 2024).

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).

There are SO MANY STUDIES on this topic that I will use collapsible text so you can more easily find the topic you are interested in.

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 intervention trials investigating this issue.

Observational Studies

Meta-Analyses

Meta-analyses all conclude different things regarding if a mother's vitamin D levels during pregnancy affect her children's risk of type 1 diabetes:

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: 

Some say "maybe" or "it depends:"

Other observational studies say "not really:"

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 intervention 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

Individual Observational Studies

Some observational studies say "yes:"

While others say "no:"

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. Note that low vitamin D levels in infancy are also linked to a higher risk of Celiac disease autoimmunity in children genetically at risk of type 1 diabetes in the international TEDDY study (Aronsson et al. 2021).

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

Trials

I do not know of any trials investigating this issue.

However, re autoimmunity, the large, randomized, double-blind, placebo-controlled trial VITAL (Vitamin D and omega 3 trial) found that vitamin D and omega-3 polyunsaturated fatty acid (PUFA) co-supplementation can reduce the incidence of autoimmune diseases in adults over age 50 (Hahn et al. 2022; Infante et al. 2022).  

Observational Studies

Two longitudinal studies of U.S. military personnel have looked at this question, and say "yes:"

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

Many studies say "yes:"

Others say "no:"

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 U.S. double-blind, randomized, controlled trial in newly diagnosed people with type 1 diabetes found that vitamin D2 supplements (ergocalciferol) reduced inflammation and reduced the rate of HbA1c decline, but did not improve other factors (Nwoso et al. 2021). Further analysis from this study found that vitamin D2 slowed the decrease in beta cell decline as well, however (Nwosu et al. 2024).

A few case studies have shown that vitamin D may help extend the honeymoon:

In adults newly diagnosed with type 1 diabetes (also known as LADA, Latent Autoimmune Diabetes in Adults), those who took vitamin D supplements as part of a randomized trial had better beta cell function over a year follow-up (Zhang et al. 2020).

A double-blind placebo-controlled intervention using glutamic acid decarboxylase (GAD)-alum, vitamin D and ibuprofen in recent onset type I diabetes found that increasing vitamin D levels may be associated with C-peptide preservation (better beta cell function) (although neither ibuprofen nor the overall combination was effective) (Ludvigsson et al. 2020). Additional drugs to prevent type 1 are discussed on the Pharmaceuticals page.

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

Individual Observational Studies

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:

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. 2019; Sahin 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

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:

However, not all trials found positive results:

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:

Individual Trials: Diabetes Complications

Most trials found positive results for kidney-related complications; I don't know of trials for other complications:

Observational Studies: Type 1 Diabetes Complications and Control

Reviews of Observational Studies

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:

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:

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:

But not all studies found associations between vitamin D levels and kidney disease markers:

Individual Observational Studies: Retinopathy

Some studies have found links between retinopathy and vitamin D levels in people with type 1 diabetes:

But many have not:

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:

But many have not:

Vitamin D and the Gut

In adults, a randomized, double-blind study found that vitamin D supplementation had positive effects on gut microbiota and inflammation (Charoenngam et al. 2020). 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 and gut inflammation is related to diabetes.

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. Even dogs in the U.S. also appear to develop diabetes more often in the winter and in northern areas of the country (Qiu et al. 2022).

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.

Genetics certainly play a role. A meta-analysis of 39 studies on genetic background in vitamin D-related genes and type 1 diabetes found that there was no overall association, but in Africans and Americans, vitamin D-related genetic background was associated with type 1 risk (Zhai et al. 2020). A large European study that looked at genetically-determined vitamin D levels were unlikely to have a large effect on type 1 diabetes development (Manousaki et al. 2021).

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).

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). In STZ-induced mice, vitamin D helped prevent diabetic nephropathy (Nakhoul et al. 2020). Vitamin D deficiency contributes to the rapid development and progression of kidney disease in rats with type 1 diabetes (Souza et al. 2023).

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). Rats deficient in vitamin D in early life developed changes to the gut microbiota and developed impaired glucose tolerance an adults, as well as in their offspring (Liu et al. 2023).

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).

Type 2 Diabetes, Obesity, and Insulin Resistance


Vitamin D levels are also linked to type 2 diabetes and insulin resistance.

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, children and adolescents with obesity have high rates of vitamin D deficiency (Zakharova et al. 2019). A study of people with obesity 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). Some studies have found that vitamin D supplementation helps with weight loss (Aliashrafi et al. 2019), and others do not (Chou et al. 2021). There are plenty more studies on vitamin D and obesity, but I'm not going to get into them here. Studies on type 2 and insulin resistance are below.


Are Prenatal Levels of Vitamin D Linked to Later Type 2 Diabetes?

One Danish study has examined this question and did not find a link (Keller et al. 2021).

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

Trials

Meta-Analyses of Trials

Some meta-analyses of randomized controlled trials have found that vitamin D supplementation does not help prevent type 2 diabetes:

However, some reviews of trials found that vitamin D supplementation does help prevent type 2 diabetes:

And, importantly, a review notes that most studies suggest health benefits when vitamin D supplementation is provided to vitamin D deficient populations and little benefit when given to people that are vitamin D sufficient/insufficient. For example, one trial on type 2 diabetes found no benefit overall, but if you look at just the prediabetic adults who were vitamin D deficient and received vitamin D, they had a 62% reduced risk of developing type 2 diabetes (Charoenngam et al. 2019).

An editorial discusses why some of the conflicting findings exist, and suggests that differences in study design can explain some of the differing findings, and that "The evidence for vitamin D in favor of diabetes prevention is accumulating" (Pittas and Bulk 2020).

Individual Trials

Many individual trials have found that vitamin D supplementation has beneficial effects on people with pre-diabetes or before type 2 diabetes onset:

Although many other trials found no or few beneficial effects:

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:

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:

However, not all individual observational studies have found associations:

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

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:

Although some did not:

Individual Trials: Diabetes Complications

Many individual trials found beneficial effects of vitamin D supplementation on diabetes complications and metabolic outcomes:

Although some did not:

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:

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. For example in Qatar, people with type 2 diabetes who did not take vitamin D supplements and who had lower vitamin D levels had a higher risk of various diabetes complications (Butler et al. 2020). In the U.S., those with higher vitamin D levels had a lower risk of mortality from diabetes (Wan et al. 2020).

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:

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:

Although not all:

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:

Laboratory Studies: Type 2 Diabetes and Obesity

Vitamin D reduces insulin resistance in rats with type 2 diabetes (Amin et al. 2018). In mice, vitamin D deficiency induces insulin resistance, while supplementation improves insulin sensitivity, but only in lean mice (Mutt et al. 2019). Vitamin D also improves beta cell function and reduces insulin resistance in rats with diabetes (Fathi et al. 2022).

In mice with type 2 diabetes, vitamin D reduced body weight, fasting glucose, insulin levels, and insulin resistance, and improved cholesterol and triglycerides as well. It also reduced beta cell decline and pancreatic injury. In damaged beta cells, vitamin D reduced cell death, improved insulin secretion, and reduced inflammation and oxidative stress (Liu et al. 2023). Pretty impressive!

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


Vitamin D levels are also linked to gestational diabetes.

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, but that it could be beneficial:

Individual Trials

Some individual trials of vitamin D supplementation in pregnant women have interesting findings:

Observational Studies

Meta-Analyses of Observational Studies

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. 2018; Al-Shafei et al. 2020; Arnold et al. 2015; Ede et al. 2019; Haidari et al. 2016; Iqbal et al. 2020; Rajput et al. 2019; Shang and Zhao, 2022; Wen et al. 2017; Xia et al. 2019; Xu et al. 2018; Yue and Ying 2020), 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. 2012; Walsh 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). Perhaps most critical is the level of vitamin D in the body. A prospective Chinese study found that women who had vitamin D concentrations >50 nmol/L in their blood had a lower risk of gestational diabetes. Women who took supplements but still had lower levels in their blood did not have a decreased risk (Yin et al. 2019).

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

Individual Trials

Why are there so many vitamin D studies out of Iran, by the way? I have no idea. Anyhow.  Some of them do look questionable (see Asemi et al. 2024).

Individual Observational Studies

Observational studies have also found that having adequate vitamin D levels during pregnancy is beneficial:

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:

Reviews

Trials

Observational Studies

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:

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

Meta-Analyses of Observational Studies

Individual Observational Studies

Not all studies have found associations, however:

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).

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), perfluorinated compounds (PFAS), and other 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).

Adequate levels of vitamin D may also protect people from the harmful effects of these chemicals (e.g., arsenic: Burchiel et al. 2020).

Human Studies

Longitudinal Studies

In U.S. pregnant women, higher phthalate and BPA levels are associated with an increased risk of vitamin D deficiency (Johns et al. 2017). Some of these authors also found that women with low vitamin D levels in the first trimester had higher levels of lead and other metals later in pregnancy (Jukic et al. 2021).

However PFAS levels were mostly linked to higher vitamin D levels in pregnant African American women (Chang et al. 2021).

In Canadian pregnant women, metal levels were associated with vitamin D cross-sectionally. However, longitudinally, it looks like the vitamin D levels influenced the metal levels and not the other way around. Higher 1st trimester vitamin D levels were associated with lower metal levels later in pregnancy (Fisher et al. 2022).

Environmental Chemical Levels Are Associated With Low Vitamin D Levels

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. 

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 the UK, POP levels were generally not associated with vitamin D levels in women (but were linked to worse kidney function) (Brennan et al. 2023). In Spanish pregnant women, higher levels of some PCBs were associated with lower vitamin D levels (Morales et al. 2013).

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 Sweden, levels of PFHxS (a type of PFAS) were associated with vitamin D metabolites (Carlsson et al. 2023).

In U.S. adults, higher phthalate and BPA levels (in women) are associated with lower vitamin D levels (Johns et al. 2016). In the U.S., DEHP phthalate metabolite levels were associated with lower vitamin D levels (Mahmoud et al. 2024). In UK women, phthalate metabolites (MCPP, MiBP, and MBP) were associated with lower vitamin D levels (Brennan et al. 2024).

Higher air pollution levels are linked to lower vitamin D levels in the UK (Yang et al. 2021). In Korea, the annual average concentrations of PM10, NO2, and CO were associated with a lower vitamin D concentration. Females (not males) with obesity had a stronger association between different air pollutants and low vitamin D concentration (Kim et al. 2022). 

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). In Chinese women, those with higher cadmium levels had lower vitamin D levels (Chen et al. 2020).

In workers occupationally exposed to lead in India, those with higher exposures had lower vitamin D levels (Batra et al. 2020).

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 Californian pregnant women, a mixture of numerous chemicals was linked to lower vitamin D levels (Berger et al. 2023).

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).

In Turkish newborn babies' umbilical cord blood, BPA levels were not associated with vitamin D levels, although a lot of babies had low vitamin D levels, and most had BPA in their blood as well (Tabanli et al. 2023).

Laboratory Studies: Vitamin D and Environmental Chemicals

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). Numerous lines of evidence show that PFAS may interfere with vitamin D signaling (Di Nisio et al. 2020).

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). 

Ironically, the sunscreen ingredient octocrylene may disrupt normal vitamin D synthesis (Abdi et al. 2022).

Polystyrene nanoplastics cause metabolic changes in zebrafish, and the changes are alleviated by vitamin D (Li et al. 2023).

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.