Overexposed - The link between toxic chemicals and diabetes

Are hormone-disrupting chemicals contributing to the diabetes epidemic? The debate dates back to the 1950s and now Australian researchers are helping fill the evidence gap. Melinda Ham reports.

YOU might not immediately make the link between preparing meals with tinned foods and type 2 diabetes but a growing body of research is raising concerns that man-made chemicals, such as those commonly found in food containers, might be contributing to the global growth of the disease.

Last month UK science-based charity Chemtrust released a review of 240 studies concluding that exposure to hormone-disrupting chemicals commonly found in food and manufacturing products such as phthalates, bisphenol A (BPA) and some pesticides can adversely affect glucose regulation and may be under-recognised risk factors for both obesity and diabetes.1

With the predicted prevalence of type 2 diabetes expected to hit around 400 million over the next 20 years, researchers in Australia, the US and Europe are looking beyond the conventional risk factors to understand what else might be contributing to such exponential growth and the latest UK review appears to be adding fuel to the fire.

Australian experts say traditional risk factors such as genetic susceptibility and an ‘obesogenic’ lifestyle explain the majority of cases, the cause for some diagnoses remain illusive. 

But the evidence for environmental risk factors is anything but conclusive and there is little longitudinal data on which to build a case.

In the face of the mounting concerns one Australian researcher is taking steps to address this important lack of data. 

Dr Dianna Magliano, a senior epidemiologist and researcher at Baker IDI Heart and Diabetes Institute in Melbourne is in the midst of a world-first study investigating the links between type 2 diabetes and human exposure to BPA, a chemical used since the 1950s as an additive in PVC plastic products (type 3 and 6) and used in the resin lining of canned food and drinks. 

BPA is a commonly used chemical also found in some medical devices, dental fillings and sealants and thermal paper in cash register receipts. Traces are also present in drinking water and household dust. 

“BPA leaches out, especially when a container is heated, microwaved and scrubbed,” Magliano says. 

“More and more is also coming out in the scientific press about how BPA is an endocrine [hormonal] disrupter and can lead to insulin resistance,” she tells MO. 

In one animal BPA model, exposure among female mice led to long-term effects on glucose metabolism, which were passed onto their offspring in vitro, especially among males who, compared to controls, developed decreased glucose tolerance and increased insulin resistance at six months of age.2 

BPA has already raised enough concern to be banned in Australia in baby toys, bottles and food containers. In 2010, Food Standards Australia and New Zealand (FSANZ) announced the industry would voluntarily phase-out BPA in plastics, but BPA continues to be used for canned food and drinks. 

“Manufacturers I’ve spoken to say they need more evidence before they will find an alternative,” Magliano says.

The results of the first large scale study on the links between humans, BPA and diabetes was published in the Journal of the American Medical Association in 2008.

The joint American-British project, led by Dr Iain Laing and his team analysed population data from 1455 adults from the US National Health and Nutrition Survey 2003/2004. 

“We found that higher urinary concentrations of BPA were associated with an increased prevalence of cardiovascular disease, diabetes, and liver-enzyme abnormalities,” Dr Laing concluded.3

But the lack of longitudinal data continues to hamper the issue.

Dr Magliano’s project is a collaboration between Baker IDI, Monash University, the CSIRO, the National Research Centre for Environmental Toxicology at University of Queensland and an environmental epidemiology group from the University of Exeter in the UK.

The study focused on collecting urine samples from adults who have developed type 2 diabetes who participated in the 11,000 strong nationwide Australian Diabetes, Obesity and Lifestyle (AusDiab) Study. 

AusDiab is one of the few studies worldwide to have stored urine samples, dietary information and other data from the study participants, ideal data for the assessment of BPA exposure. 

Subjects in the national longitudinal study had their urine collected and tested in 2000 and 2005 by which time 225 participants had developed type 2 diabetes.

In coming months researchers will test participants’ urine a third time and they anticipate a further 125 cases of diabetes will have developed, bringing the total to 350 over 11 years. 

It’s at this point the team will determine the degree to which the participants with diabetes have had long term exposure to BPA, compared to controls. Dr Magliano hopes to be able announce her initial findings by the end of 2012. 

In her grant proposal, she pointed out the global significance of her research.

“This project will provide first time prospective evidence of whether BPA is associated with an increased risk of incident diabetes, obesity and cardiovascular disease, and will provide vital data for public policy on the use of BPA.” 

While the publication of Dr Magliano’s research may be the beginning of a public discussion on this hidden cause of the growing type 2 diabetes epidemic, Professor Paul Zimmet, director emeritus at Baker IDI, maintains the evidence is not yet compelling.

“Because there are so many potential variants, it is difficult to determine the potential cause and just isolate chemical pollutants,” Professor Zimmet says.

Given that chemicals appear to be only a relatively small contributing factor to the development of type 2 diabetes there is still a certain degree of scepticism in the scientific world about the role they play, he says. 

Despite Professor Zimmet’s caution, the BPA story continues to divide researchers, regulators and scientists and the US FDA is currently “in consultations” over a range of health concerns associated with BPA according to a story published in late March in The Lancet.4

Following World War II, there was an explosion in the commercial production of thousands of synthetic chemicals used in industry, agriculture and manufacturing, despite a of lack research into their full, long or short term impact on humans and the environment. 

Some of the more deadly combinations known as Persistent Organic Pollutants (POPs) including polychlorinated biphenyls (PCBs), polybrominated biphenyl (PBBs) and dichlorodiphenyltrichloroethane (DDT) have since been outlawed in most western countries. 

To further enforce this globally, the Stockholm Convention on Persistent Organic Pollutants (POPs) was adopted in 2001 and came into force in 2004. 

It requires all signatories to eliminate and/or reduce all POPs that have devastating effects on humans and wildlife, which can affect all mammals in vitro and travel through the food chain. 

But even after their banning and restricting, POPs’ effects live on in many ways, stored over time in fatty human and animal tissues, after consumption of vegetables, fruit, plants and meat sprayed with POPs. 

And, a recent Finnish study published in Diabetic Care has linked exposure to POPs with an increased prevalence of type 2 diabetes.2 The researchers analysed serum samples taken from almost 2000 subjects born in Helsinki between 1934–1944, before the global manufacture and emission of POPs reached their peak. 

Among the participants, those with the highest exposure to chemicals such as PCBs and oxychlordane, especially if they were overweight, had a risk of type 2 diabetes up to more than twice those participants with lower levels of exposure. 

It would follow then that farmers and agriculture workers are at an acutely high risk. A study of more than 33,000 American licensed pesticide applicators, part of the Agricultural Health Study, provided researchers information about lifetime exposure to chemicals and their medical history. 

In the results from the National Institute of Environmental Health Scie­n­­ces (NIEHS) and the National Cancer Institute published in the American Journal of Epidemiology 1 the researchers discovered that exposure to seven pesticides – aldrin, chlordane, dichlorvos, heptachlor, trichlorfon, alachlor, and cyanazine – greatly increased the workers’ chances of developing diabetes and that the incidence of diabetes also was heightened with cumulative days of exposure.

Back in Australia, even though it is likely that our farmers have been exposed to POPs and the general population to other POPs and BPA, there’s been a shortage of research on the link, but thanks to Dr Magliano and the team at the Baker IDI, we could have some solid data by the end of 2012.

References 

1. Review of the Science Linking Chemical Exposure to the Human Risk of Obesity and Diabetes (March 2004). www.chemtrust.org.uk

2. Alonso-Magdalena P, et al. Bisphenol A Exposure during Pregnancy Disrupts Glucose Homeostasis in Mothers and Adult Male offspring. Environmental Health Perspect 2010; 118:1243-50.

3. Laing IA, et al. Association of Urinary bisphenol A concentration with medical disorders and laboratory abnormalities in dults. JAMA 2008; 300: 1303-10.

4. Siva, N. Controversy continues over safety of bisphenol A.  The Lancet. 2012;379; 1186.

5. Airaksinen R, et al. Association Between Type 2 Diabetes and Exposure to Persistent Organic Pollutants, Diabetes Care 2011, doi: 10.2337/dc10-2303.

6. Montgomery MP et al. Incident Diabetes and Pesticide Exposure among licensed Pesticide Applicators: Agricultural Health Study, 1993-2003. Am J  Epidermiol. 2008; 167[10]: 1235-1246.