Along with lots of love, one of the things every mother gives her baby is a dose of chemical contamination. Unfortunately, even in very high doses, love is not able to prevent the transfer of chemicals from mother to child.
A recent study measured levels of various contaminants in the placentas and breastmilk of Danish and Finnish women. The major chemicals found included p,p’-DDE, beta-HCH, hexachlorobenzene (HCB), endosulfan-I, dieldrin, oxychlordane, cis-heptachlor epoxide and p,p’-DDT. What the heck are these things? Most are pesticides or industrial chemicals, and they can pass through the placenta to the fetus, or enter babies through breastmilk. Due to the higher fat content of milk, contaminant levels are higher in milk than the placenta, but, exposure to the developing fetus is likely to be more critical to physical development. Although the levels of many of these contaminants have declined since most developed countries restricted their use decades ago, they are persistent and remain in the environment, and our bodies, for long periods of time (Shen et al. 2007).
These persistent organic pollutants (POPs) include some of the most well known, and most toxic, environmental contaminants, such as PCBs and dioxin. They accumulate in the fatty tissue of animals and humans, and biomagnify in the food chain, so that the higher up an animal is on the food chain, the higher the contaminant levels are (Tanabe 2002). Here's a pop quiz: who is on the top of the food chain? Did you answer a top predator? Humans? Yes, and yes. But really, a nursing infant is even higher on the food chain than its mother.
There are several hundred POPs, and most humans are exposed to mixtures of them (Lee et al. 2007b). The U.S. Centers for Disease Control and Prevention periodically tests a representative sample of U.S. residents of POPs, and has found widespread contamination (Patterson et al. 2009). POPs are fat-soluble, and tend to move through the environment together. Therefore, it may be difficult to delineate the separate effects they may have on health (Codru et al. 2007).
The studies on this page consider multiple POPs together; see the pages on PCBs and dioxin for information on studies on those contaminants in particular.
Since POPs contaminate virtually all people, even if they increase the risk of diabetes a small amount, these pollutants might have a large effect on the overall population (Porta 2006).
A groundbreaking 2006 study found "striking" relationships between six POPs and diabetes in U.S. adults exposed to normal levels of POPs. The higher the levels of these POPs, the higher the prevalence of diabetes. In the highest exposure group, the risk of diabetes was 37.7 times higher than in the people with the lowest levels of exposure. That is a lot-- far higher than any other study I have ever seen. The POPs included the dioxins HpCDD and OCDD, DDE, PCB-153, oxychlordane, and trans-nonachlor, with the latter three showing the most significant relationships. (Oxychlordane and trans-nonachlor result from the use of the organochlorine pesticide chlordane) (Lee et al. 2006).
Surprisingly, this study found that obesity did not increase the risk of type 2 diabetes if those people had very low levels of POPs in their bodies. In an editorial in The Lancet, Porta (2006) writes, "This finding would imply that virtually all the risk of diabetes conferred by obesity is attributable to persistent organic pollutants, and that obesity is only a vehicle for such chemicals. This possibility is shocking."
The dataset used in this study, the National Health and Nutrition Examination Survey (NHANES) 1999–2002, includes both type 1 as well as type 2 diabetes, and does not distinguish between them. The authors believe that most of the subjects had type 2 diabetes, however, because most of the subjects were over 40. Yet they also point out that, "POPs may be involved in the pathogenesis [development] of type 1 diabetes as well as type 2 diabetes" (Lee et al. 2006).
In a further expansion of this study, the authors used the same dataset but analyzed 19 POPs, divided into five groups, to see which were most strongly associated with diabetes. They found that individually, most POPs were associated with diabetes, and PCBs and organochlorine pesticides were most strongly associated. Among the groups, only the dioxin-like PCBs and organochlorine pesticides were associated with diabetes (Lee et al. 2007c).
Another study that used the NHANES dataset found three additional chemicals to be associated with diagnosed diabetes: another type of dioxin (HxCDD), PCB-126, and DDT. PCB-126 and DDT were also associated with undiagnosed diabetes. This study included people over 20, but excluded those who had developed diabetes before age 30 and were only taking insulin, thus focusing on type 2 (Everett et al. 2007). An additional study by one of the same authors also used the NHANES dataset and included people over 20 who had diabetes or undiagnosed diabetes (hemoglobin A1c levels over 6.5% (HA1c is a measure of long-term blood glucose levels), and people with "pre-diabetes," defined as somewhat elevated HA1c levels (5.7-6.4%). This study analyzed levels of organochlorine pesticides, including DDT, DDE, beta-hexachlorocyclohexane, oxychlordane, trans-nonachlor, heptachlor epoxide, mirex, and dieldrin. It found that six of the eight pesticides were associated with diabetes (all except mirex and dieldrin), and that heptachlor epoxide and DDT were associated with pre-diabetes (Everett and Matheson 2010).
An interesting study adapted the techniques used in genome-wide association studies, and instead conducted an "environment-wide association study" to consider 266 separate environmental factors and diabetes using the NHANES dataset. It found that the factors most associated with diabetes include heptachlor epoxide and PCBs (especially PCB 170). The effects of these factors are comparable to those found in genetic studies (Patel et al. 2010).
A study found that higher levels of hexachlorobenzene (HCB), some PCBs, and DDE (but not mirex) were associated with diabetes in adult Native Americans of the Mohawk Nation. The most elevated results were found for HCB. This study did not distinguish between type 1 and 2, but since the participants were over 30 and most did not use insulin, they presumably had type 2 (Codru et al. 2007). Another study also found that the levels of HCB were significantly associated with diabetes, in Swedish women over 50 (who presumably had type 2, although the study did not distinguish between them). This study did not find associations between diabetes and various other POPs. They pointed out that diabetes may affect the levels of POPs in individuals, since they and others have found that recent weight change can influence POP levels (Glynn et al. 2003).
A Belgian study found that people with diabetes had significantly higher levels of multiple dioxins and PCBs in their bodies than people without diabetes (Fierens et al. 2003). A study of Mexican Americans found that diabetes was associated with higher levels of various organochlorine pesticides (Cox et al. 2007). A Finnish study found that exposure to oxychlordane, trans-nonachlor, PCB-153 and DDE were associated with diabetes (Airaksinen et al. 2011).
A study has looked at the associations between diabetes and POPs in Swedish fishermen and their wives, who eat a lot of fatty fish from the Baltic Sea, a major source of POP exposure in Sweden. The researchers measured levels of PCB-153 and DDE as markers of POPs, because they seem to correlate well with PCB and dioxin (TCDD) levels, and found that both PCB-153 and DDE were associated with diabetes prevalence. The subjects were older and presumably had type 2 diabetes, but two subjects with diabetes took insulin only, and may have had type 1 (Rylander et al. 2005).
A small study from Korea found strong associations between a number of organochlorine pesticides and type 2 diabetes in people exposed to low, background levels of these substances. Since Asians tend to develop diabetes at a lower body mass index and younger age than people elsewhere, the findings may suggest that Asians are more susceptible to the effects of organochlorine pesticides. The authors suggest that this susceptibility might help to explain the current epidemic of type 2 diabetes in Asia (Son et al. 2010).
A study in New York state found an increase in the rate of hospitalization for diabetes among people residing in the ZIP codes containing toxic waste sites, especially those with waste sites containing POPs. The study also found that the rates of diabetes diagnosis were 36% higher among the Hudson River residents than those of clean sites, despite their healthier lifestyle; the Hudson River is contaminated with PCBs. This study included people 25-74 years old, and included all types of diabetes (90-95% of people with diabetes have type 2). While proximity to hazardous waste sites is a crude measure of exposure, residence near such sites may have constituted a risk to these populations in the past (Kouznetsova et al. 2007).
Prospective studies follow people over time, measuring exposures early, and then track to see if people develop diabetes at different rates over time depending on their exposures. These studies are important to show if chemicals lead to diabetes development, to rule out the possibility that diabetes itself somehow causes people to have higher chemical loads.
One prospective study used a dataset that measured POP exposures in US residents before disease development, and again almost 20 years later when some participants had developed type 2 diabetes. They found that a number of POPs were associated with increased diabetes risk, with the highest risk at somewhat low doses. These POPs included trans-nonachlor, oxychlordane, mirex, highly chlorinated PCBs, and PBB 153 (Lee et al. 2010). Another study from Sweden also found that various POPs (PCBs and trans-nonachlor, but not dioxin) substantially increased the later risk of type 2 diabetes in the elderly (Lee et al. 2011).
A study of Swedish women found PCB-153 and DDE to be associated with type 2 diabetes (Rignell-Hydbom et al. 2007). Following these women over time, the researchers found that women with the highest levels of DDE showed an increased risk of developing type 2 diabetes, in the cases where diabetes was diagnosed more that six years after the original measurements were done. This finding shows that DDE exposure can be a risk factor for developing type 2 diabetes, supporting the idea that some POPs can affect the development of diabetes (Rignell-Hydbom et al. 2009).
DDE exposure (but not PCBs or PBDEs) was also found to be associated with type 2 diabetes in a long-term study from fish consumers in the Great Lakes region (fish is one of the main sources of POP exposure). These authors also found that higher exposures to PBDEs and DDE combined were associated with an increased risk of diabetes, suggesting interactive effects from different contaminant exposures (Turyk et al. 2009b). This group of fish consumers was followed for over 10 years, and POP levels were measured annually. The levels of POPs in people did not differ over time based on whether or not they had diabetes. This is an important finding, because it supports the idea that POPs might lead to diabetes, and not the other way around (that diabetes affects the levels of POPs in the body) (Turyk et al. 2009a). Dr. Turyk has measured levels of GAD autoantibodies (one of the markers of type 1 diabetes) in the people in this study, and found them to be low. Therefore the participants almost certainly have type 2, not type 1 diabetes (pers. comm. 2011).
A prospective study that measured various POP levels and later development of diabetes in US nurses found that HCB exposure was associated with type 2 diabetes. These authors also combined their data with those from other prospective studies in a meta-analysis, and found that both HCB and total PCB levels were associated with diabetes (Wu et al. 2012).
It is possible that diabetes causes people to have higher levels of POPs, not vice versa. Yet Lee et al. (2006) point out a number of reasons why they think it is more likely that the POPs lead to diabetes instead of the other way around. For example, the idea that dioxin could cause diabetes is consistent with the known biology of these pollutants (see the dioxin page for more information on dioxin and diabetes). As for the idea that diabetes changes the way the body processes POPs, one study has found that people with diabetes eliminate dioxin at the same rate as people without diabetes (see Michalek et al. 2003), implying that diabetes does not affect POP levels.
In an animal study, researchers curious whether long term DDT exposure could cause cancer fed DDT to two species of monkeys for 130 months, beginning in 1969. The surviving monkeys were analyzed in 1994. While the researchers were not looking for diabetes, two of the 24 exposed (and none of the unexposed) monkeys developed diabetes (high blood glucose). Two also developed hypoglycemia (low blood glucose), implying that DDT affected glucose metabolism or insulin production. (Two of the exposed monkeys did develop malignant cancer, and three benign tumors, while none of the unexposed developed any tumors or cancer. There were a number of other health effects in the exposed group as well, from fatty changes in the liver to neurotoxic and estrogenic effects). Although all the treated monkeys received the same dose of DDT, the levels in the bodies varied substantially. This finding could be due to differences in the amount of body fat, metabolism, absorption, secretion, or fluctuating levels over time in the same individual (an effect also seen in humans) (Takayama et al. 1999).
What about people exposed to higher levels of POPs? Because POPs migrate to polar regions of the globe and accumulate in animal fats, and because the Inuit have a diet high in marine mammals, the Inuit have very high levels of POPs in their bodies. A study of type 2 diabetes, which is becoming more common among the Inuit, and POPs found that POP levels were not associated with diabetes or insulin resistance. This finding is actually consistent with others of humans exposed to high levels of POPs-- associations have generally only been found among people exposed to lower levels of POPs (Jørgensen et al. 2008). The levels of PCBs and chlordanes found in the Inuit were 10-12 times higher than those found by Lee et al. (2006). The authors suggest that perhaps exposure above a certain level does not add to risk. If low levels of POPs can increase the risk of diabetes, and higher levels of exposure do not increase the risk further, then you might expect to find weak or no associations between exposure levels and diabetes in highly exposed populations (Son et al. 2010).
Some studies of more highly exposed populations, however, have still found some associations between diabetes and POPs. A study of a First Nation community in Northern Ontario found that diabetes was associated with exposure to DDE and some PCBs. The levels of exposure in this community approached the range of some Inuit communities (Philibert et al. 2009). A study from a polluted area of Eastern Slovakia found that people with higher levels of five POPs (including PCBs, DDE, DDT, HCB, and beta-HCH) had higher rates of both prediabetes and diabetes. Those with the highest exposures to all of these POPs combined had 3 times higher rates of prediabetes and 6 times higher rates of diabetes, as compared to those with the lowest exposure levels (Ukropec et al. 2010). And, a study of elderly Faroe Islanders found that those with type 2 diabetes or impaired fasting glucose had higher PCB levels and higher past intake of traditional foods (which are high in POPs), especially in childhood and adolescence. For people without diabetes, higher PCB levels were associated with lower insulin levels but higher glucose levels. The authors suggest that impaired insulin secretion is an important part of diabetes development associated with these contaminant exposures (Grandjean et al. 2011).
The study of the Inuit people also found that POPs may affect insulin secretion from beta cells (see the beta cell stress page), since those people with higher exposures had less insulin secretion (Jørgensen et al. 2008).
For information on the overlap between type 2 and type 1, see the types of diabetes page.
A number of studies have found associations between various POPs and increased insulin resistance. Another study by Lee et al. (2007a), for example, found that some POPs (oxychlordane and trans-nonachlor, and two nondioxin-like PCBs) were associated with increased insulin resistance in adults without diabetes. They conclude that background exposure to organochlorine pesticides (oxychlordane and trans-nonachlor result from the use of the organochlorine pesticide chlordane) and nondioxin-like PCBs may increase type 2 diabetes risk by increasing insulin resistance, since increased insulin resistance often precedes type 2 diabetes. POPs may interact with obesity to increase the risk of type 2 diabetes. They also suggest that chlordane may be the most important POP involved in the development of type 2 diabetes by influencing insulin resistance.
A study that followed people over time found that POP levels were associated with development of increased insulin resistance, higher body mass index (BMI) (a measure of obesity) and other cardiovascular risk factors 20 years later. The main culprits included DDE and persistent PCBs (Lee et al. 2011).
Two animal studies have shown experimentally that exposure to POPs, taken from farmed Atlantic salmon, can cause increased insulin resistance and obesity in rats and mice. In one, organochlorine pesticides and DDT inhibited insulin action in fat cells, and affected gene expression. Some types of PCBs also reduced insulin action in the fat cells, but not as strongly. Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) did not have an effect on insulin action in the fat cells (Ruzzin et al. 2010; Ibrahim et al. 2011). Another animal study found that lifelong exposure to low levels of POP mixtures affected the body weight of zebrafish. The chemicals affected the genes associated with weight control, and insulin signaling, showing that these chemicals appear to affect metabolism and lead to weight gain or obesity (Lyche et al. 2011). Cellular studies have found that some POPs affect certain aspects of fat cell function/dysfunction that may be involved in obesity and type 2 diabetes (Howell and Mangum 2011).
A number of human studies have also found associations between in utero exposure to various POPs and higher BMI in humans later in life. Verhulst et al. (2009) found that prenatal exposure to DDE and PCBs is associated with increased BMI during the first three years of life. Smink et al. (2008) found that in utero exposure to higher levels of HCB was associated with a higher BMI at age 6. A study by Karmaus et al. (2009) found that in utero exposure to DDE, but not PCBs, is associated with increased weight and BMI in adult women.
Elobeid et al. (2010) found that people with higher levels of various POPs had higher BMI and waist circumference measurements. For example, individuals with higher DDT and octachlorodibenzo-p-dioxin (OCDD) levels had a higher BMI, and higher heptachlordibenzo-p-dioxin (HPCDD) levels had a larger waist circumference. Oxychlordane exposure was associated with a higher BMI in males but lower in females, perhaps due to hormonal differences. Dirinck et al. (2010) found that people with higher levels of beta-hexachlorocyclohexane (betaHCH) had a higher BMI and higher insulin resistance, although higher PCB levels were associated with lower BMI and lower insulin resistance.
Different POPs have also been associated with various components of metabolic syndrome (common in people with type 1 or 2 diabetes). Organochlorine pesticides were most strongly associated with metabolic syndrome, similar to the findings of type 2 diabetes and increased insulin resistance. Interestingly, in this study PCBs showed unusual dose-response relationships, like has been seen with endocrine disruptors in laboratory experiments. The authors suggest the hypothesis that lower doses of some PCBs may be more harmful than higher doses, and that this possibility is worth more investigation (Lee et al. 2007b). Park et al. (2010) found that the POP heptachlor epoxide was associated with metabolic syndrome in a small study of Koreans without diabetes.
A study from a contaminanted area in Taiwan found that higher PCDD/F exposure was associated with an increased prevalence of metabolic syndrome (Chang et al. 2010a). These authors also found that increasing PCDD/F levels were associated with increasing insulin resistance in people without diabetes (Chang et al. 2010b). And, perhaps most significantly, they found that people with the highest levels of exposure to both PCDD/Fs and mercury had 11 times the risk of insulin resistance than those with the lowest exposures. Insulin resistance increased with both mercury and PCDD/F exposure, but simultaneous exposure to both compounds may increase the risk of insulin resistance more than exposure to one or the other alone. This study also found that each component of metabolic syndrome was associated with both mercury and PCDD/F exposure levels, including an increased waist circumference. Higher PCDD/F exposures were also associated with defective beta cell function in people without diabetes, which supports the idea that PCDD/Fs are involved in the development of diabetes (Chang et al. 2010c).
A New York study found that people who live in areas near environmental sources of POPs have a higher rate of hospitalization for metabolic syndrome (Sergeev and Carpenter 2011).
Based on the above studies, some authors have hypothesized that POPs may contribute to the development of type 1 diabetes. Yet there are almost no human studies on this possibility (see the PCB page for a description of one study).
A second study from Sweden measured in utero exposures to PCB-153 and DDE, and compared the levels these contaminants in children who went on to develop type 1 diabetes and those who did not. (Sweden, like other Scandinavian countries, has very good medical research data, and this study included data from children born in 1970-1990, and followed until 2002). The authors found that POP exposure did not increase the risk of type 1 diabetes; in fact, mothers of children who developed type 1 had generally lower levels of exposure than mothers of children who did not develop type 1 (although the differences were not statistically significant). The authors suggest that their findings do not imply that POPs are protective against type 1. Instead, since in Sweden, higher POP levels are often due to higher fish consumption, it may be that the fatty acids found in the fish provide the possible protective effect (see the nutrition page for more on these fatty acids and type 1 diabetes) (Rignell-Hydbom et al. 2010).
Researchers at the University of Florida have been studying the effects of organochlorine pesticides on mice and their relation to autoimmunity. The researchers fed three organochlorine pesticides (o,p' DDT (a component of DDT), methoxychlor, and chlordecone) to genetically susceptible mice, and found that these compounds accelerated the appearance of the autoimmune disease lupus, with chlordecone having the most significant effect. O,p' DDT and methoxychlor produced effects even at very low doses, for methoxychlor, four times lower than the U.S. EPA's "No Observable Effect Level." In the case of chlordecone, levels of autoantibodies were dependent on the dose received (Sobel et al. 2005).
A follow-up study found that chlordecone increased the rate of progression of the disease in mice that were genetically susceptible to lupus, but not in mice that were not susceptible to this disease, showing the importance of genetic background for this effect (Sobel et al. 2006).
In addition, autoimmune antibodies have been found to be higher in people who have higher levels of PCBs, DDE, and HCB in Slovakia (Cebecauer et al. 2009). These researchers found higher thyroid autoantibodies as well as impaired fasting glucose levels in people with higher exposures to these POPs (Langer et al. 2008). (Up to 25% of patients with type 1 diabetes have evidence of thyroid disease, the most common autoimmune disease associated with type 1 diabetes (Umpierrez et al. 2003)). Hexachlorobenzene (HCB) exposure has effects on the immune systems of animals and humans, and autoimmunity might be one of these effects (Michielsen et al. 1999).
While the levels of some other environmental contaminants has risen over the past few decades (e.g., see the historical trends page), the levels of POPs in the environment fell after most developed countries restricted their use in the 1970s-1980s (Tanabe 2002). To explain how the rates of diabetes could be rising while levels of POPs fell, Lee et al. 2007b propose a number of explanations:
Here's a new idea: perhaps contaminants can affect the blood glucose control or contribute to complications in those of us with diabetes. One study suggest that this may be the case. It found that a variety of POPs were associated with higher hemoglobin A1c (HA1c) levels, a measure of long-term glucose control, as well as a higher risk of peripheral neuropathy (nerve damage). Of the various POPs, organochlorine pesticides were the most strongly and most consistently associated with higher HA1c and neuropathy. If these findings are confirmed in other studies, the authors state, "new therapeutic approaches such as avoiding POPs or an increased excretion of POPs from the body can be developed for the management of type 2 diabetes" (Lee et al. 2008).
Many POPs are considered to be immunotoxicants, since they can affect the immune system (see the autoimmunity page) (Holladay 1999).
Many POPs are considered endocrine (hormone) disruptors, since they can affect the endocrine (hormone) system (Edwards and Myers 2007). Researchers have investigated whether chlordecone's ability to enhance autoimmunity in mice was due to its ability to act like the hormone estrogen. While the effects of chlordecone and the hormone estradiol (an estrogen) on mice showed some of the same effects, they were not identical. The differences showed that chlordecone is not acting only as an estrogen mimic in its effects on the immune system, but exactly how it acts is not known (Wang et al. 2007).
A number of studies show that exposure to POPs can alter gene expression in a diverse variety of genes (e.g., Adeeko et al. 2003; Vezina et al. 2004), which may be a mechanism linking POPs and diabetes (Kouznetsova et al. 2008). In one of the Slovakian studies mentioned above, researchers have found evidence that may show transgenerational endocrine disrupting effects in humans. Youth who lived in the polluted area but who had similar organochlorine levels to youth living in unpolluted areas, showed the same health effects as their parents, who were exposed to high levels. These health effects included impaired fasting glucose levels and the presence of thyroid antibodies (Langer et al. 2008). Low dose exposures to POPs have also been associated with changes in gene expression in healthy humans (Kim et al. 2010).
Various POPs have also been shown to influence type 1 diabetes risk factors, such as viruses and growth rates. They are present in foods, including cow's milk, breastmilk, and fish oil.
There is strong evidence that exposure to persistent organic pollutants, at levels commonly found in developed countries, can increase the risk of developing type 2 diabetes. The ability of some of these pollutants to also enhance autoimmunity implies that they could potentially contribute to the development of type 1 diabetes as well. This possibility should be studied. These studies should consider the effects of mixtures of POPs that humans are exposed to, and the possibility that the effects could be passed down from one generation to the next.