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Forget high fructose corn syrup, couch-potatoism, and gluttony, which have been variously blamed for the obesity epidemic among Americans, the problem may be fetal exposure to the chemicals in plastic bottles, reported the February 24th issue of the Economist Magazine. Dubbed “obesegons” by researcher, Bruce Blumberg, the theory is speculative and controversial, but it received an airing at a meeting of the American Association for the Advancement of Science.

And herein lies the dilemma for journalists: given that only 28 percent of the U.S. population knows enough about science to understand a news story like the one in the Economist (according to another paper presented at the AAAS meeting), should the magazine be more circumspect about reporting information that is far from proven and when the level of debate is intense?

Take the following statement in the Economist piece:

“Other synthetic hormones and endocrine disruptors common in the modern world seem to have a similar impact and not just in the womb. A study of Japanese women has suggested a link between obesity and adult exposure to bisphenol-A, a component of plastic bottles. Frederick vom Saal of the University of Missouri has investigated the impact of early exposure to this compound. His work on laboratory animals showed that fetal exposure to bisphenol-A led to obesity and cancer.”

Vom Saal was the first scientist to posit risk from low-dose exposure to bisphenol-A (BPA), a charge that was controversial, as other labs couldn’t replicate his findings on prostate size in mice. More problematically, a review of the available evidence up to 2002 by the Harvard Center for Risk Analysis found that the evidence for such low-dose effects is weak. An update to this research by the Gradient Corporation, which was published in the June 2006 issue of the Risk Sciences Bulletin came to similar conclusions, and criticized Vom Saal for not critically evaluating the low-dose effect studies he has cited in two summaries of the evidence.

Vom Saal cites the Japanese study, (Positive Relationship between Androgen and the Endocrine Disruptor, Bisphenol A, in Normal Women and Women with Ovarian Disfunction,” Endocrine Journal 2004, 51, Takeuchi et al) in one of these summaries, a commentary for the August 2005 issue of Environmental Health Perspectives, which calls for a new risk assessment for bisphenol-A.

“The literature we reviewed shows that the rate of leaching from commonly used BPA-containing products (the lining of tin cans and polycarbonate food and beverage containers) is high enough to result in adverse effects in laboratory animals (Raloff 1999). These recently published findings indicate that the accepted migration limit [recently set by the European Union (ECB 2003)] of 30 ppb BPA from polycarbonate or resins into food and beverages is not sufficiently protective of human health. The case-control study reporting that ovarian disease in Japanese women is related to blood levels of BPA provides a first confirmation of this prediction in adult humans (Takeuchi et al. 2004).”

But Takeuchi is not a case-controlled study; it is a cross sectional study. A case-controlled study typically tests the relationship between two occurrences to see if they are correlated by comparing/contrasting people with a certain trait (the cases), against similar people who do not exhibit the same trait (the controls). Cross sectional studies take a snapshot of a population, by measuring, say, the occurrence of a disease and exposure to a chemical at the same time – and thus, they cannot show causality (the disease may have predated the exposure). A case-controlled study is therefore more rigorous than a cross-sectional study, which can only point to correlations, and can’t address the problem confounding factors.

These facts led Joseph Politich, a research associate professor at Boston University School of Medicine to write a response accusing Vom Saal of “misrepresenting” the Takeuchi study: “it does not demonstrate that BPA is specifically associated with ovarian disease,” Politich writes. He also notes that

“a number of recent studies have reported that several of the ELISA kits available for measurement of serum BPA [the analytic method used by Takeuchi et al. (2004)] overestimate BPA concentrations and exhibit considerable cross-reactivity, calling into question the validity of results generated by such methods (Fukata and Mori 2004; Fukata et al. 2003; Kawaguchi et al. 2003). Furthermore, it is well known that BPA is metabolized and eliminated rapidly (Volkel et al. 2002), so serum levels provide only a snapshot of BPA exposure within the last day. It is not meaningful to correlate an acute exposure (serum BPA at one time-point) with a chronic disease that took years to develop. Chronic exposure to BPA would have to be demonstrated and not assumed.”

Vom Saal then replied to Politich, although not on a point-by-point basis. But – back to the Economist, and its reporting that the Takeuchi study seems to show a link between obesity and BPA. Does it? It all depends on how reliable the measurements of serum BPA were, which at this point, is unknowable. The overstimation of serum levels may account for the correlation. But let’s assume what we can’t, which is that the serum levels were accurate: The study did find a correlation between body mass index and serum BPA for obese women, but it also found a correlation with non-obese women suffering from polycystic ovary syndrome. And correlation doesn’t mean causation. Moreover, there were only 13 “obese” women in this study; the vast majority of other studies that have looked at BPA and bodyweight found no correlation.

Should the Economist have reported the Takeuchi study without noting these acute limitations? No. Are these limitations so great that the study shouldn't have been reported at all? Probably, as people are going to remember the story as saying "plastic bottles make you fat." At the very least Vom Saal, revolutionary though he might be, should be counterbalanced by the weight of current evidence, and by actually checking up on what he claims.