Tuesday July 17 2012
Do phthalates in nail varnish really raise diabetes risk?
“TOWIE girls watch out! Women who use nail varnish and hairspray ‘may have higher risk of diabetes’,” is the headline in the Daily Mail. The Mail’s focus on high-maintenance The Only Way Is Essex girls being at risk of diabetes seems more a desperate attempt to give the story a celebrity spin than a considered view of the link between phthalates and diabetes.
Phthalates are chemicals that are used in a wide range of products, such as packaging, cosmetics, perfumes, nail polishes, flooring and industrial products. A previous study found that at least three-quarters of the US population have detectable levels of phthalates in their urine. It has been proposed that phthalates may affect the way the body stores fat and interfere with glucose metabolism. This, in turn, could lead to an increased risk of developing type 2 diabetes.
The headline is based on the results of a study that found that women with higher levels of phthalates in their urine were more likely to report having diabetes. However, as both urine levels of phthalates and diabetes were assessed at the same time it is not possible to draw a firm conclusion about the nature of the link between them.
Phthalates are found in some diabetes drugs as well as medical equipment used in the diagnosis and treatment of diabetes. This could contribute to the increased levels of phthalates in people with diabetes.
The key word in the Daily Mail headline is “may”. Further well-designed studies are required to establish whether there is a link between phthalates and diabetes.
Where did the story come from?
The study was carried out by researchers from Brigham and Women’s Hospital and Harvard Medical School, the University of Rochester School of Medicine and Dentistry, the University of Michigan School of Public Health, and Harvard School of Public Health. It was funded by the American Diabetes Association and the US National Institute of Environmental Health Sciences. The study was published in the peer-reviewed journal, Environmental Health Perspectives.
This story was covered by the Daily Mail. Although the headline was attention-grabbing and arguably misleading, the research was well described in the story, and the need for further studies discussed.
What kind of research was this?
This was a cross-sectional study. The researchers looked at whether, in women, there was an association between the concentration of a chemical called phthalate and its breakdown products in urine, and having diabetes.
Cross-sectional studies only look at one point in time and, therefore, can only find associations. The order of events also cannot be determined. A cohort study would be better able to address this question as, by following a group of people over time, it could establish which event came first. For example, whether the increase in phthalate concentrations occurred before diabetes developed.
However, if a cohort study was carried out it could still be possible that another unmeasured factor was responsible for the association seen. A randomised controlled trial (RCT) would be required to determine causation, though this would not be performed for ethical reasons. Putting people at potential risk of developing diabetes could place them at risk of life-threatening complications.
What did the research involve?
The researchers looked at the relationship between the concentration of phthalate breakdown products in urine and self-reported diabetes in 2,350 women aged between 20 and 80 years old who participated in the National Health and Nutrition Examination Survey 2001-2008. The researchers adjusted for some potential confounders that could be related to both phthalate levels and diabetes, including:
- the levels of creatinine in urine
- sociodemographic factors (including age, race or ethnicity, education and poverty)
- behavioural and dietary factors (including fasting time, physical activity, smoking status, total caloric and total fat intake)
- body size (body mass index and waist circumference)
The researchers then performed a secondary analysis to see whether in women who had not yet developed diabetes there was a relationship between phthalate breakdown products in urine and fasting blood glucose, insulin resistance and levels of glycosylated haemoglobin (a longer-term indication of blood glucose control).
What were the basic results?
The researchers found that women with higher levels of phthalate breakdown products in their urine had increased odds of having diabetes. Women who had levels of specific phthalate breakdown products in the top 25% had almost twice the odds of having diabetes compared with women with levels in the bottom 25%. The researchers also found associations between some particular phthalate levels, but not others, and markers of diabetes risk (fasting plasma glucose levels and insulin resistance) in women without diabetes.
How did the researchers interpret the results?
The researchers state that “Urinary levels of several phthalates were associated with prevalent diabetes. Future prospective studies are needed to further explore these associations to determine whether phthalate exposure can alter glucose metabolism, and increase the risk of insulin resistance and diabetes”.
In this cross-sectional study, women with higher levels of phthalate breakdown products in their urine were more likely to report having diabetes. However, this study has many limitations, restricting the applicability of this finding. These include:
The study design
Cross-sectional studies only look at one point in time and, therefore, can only find associations. As both phthalates and diabetes were assessed at the same time, the order of events also cannot be determined. The researchers point out that due to the cross-sectional study design, reverse causation cannot be ruled out. That is, that people with diabetes may have elevated levels of phthalates due to the fact that these chemicals are present in certain medications and medical devices that may be used in the treatment of their diabetes.
Measurement of phthalate levels
Phthalate exposure was estimated from only one measurement, and the authors state that phthalate levels at one point in time are only modestly predictive of levels over weeks and months.
Determination of diabetes
Diabetes was only assessed by self-report, and no distinction was made between type 1 and type 2 diabetes. (The authors’ theory is that phthalates may cause the development of type 2 diabetes.) The authors also state that another report found that approximately 30% of diabetes cases are undiagnosed; therefore, the study may not have reliably identified all women with diabetes.
Exclusion of confounders
Although the researchers adjusted for many potential confounders that could influence both phthalate levels and diabetes risk, they cannot exclude the possibility that another factor could be responsible for the association seen.
In conclusion, further well designed studies are required to determine whether there actually is a link between phthalate exposure and the development of diabetes.
Analysis by NHS Choices. Follow Behind the Headlines on twitter.