Claimed link between phthalates exposure and obesity is 'hard to swallow'

Friday, 27 April, 2007

Experts who have reviewed a paper claiming a link between phthalates exposure and obesity say that the study should be taken with a large grain of salt.

"Based on our review, conducted by member company toxicologists and statisticians, this study is hard to swallow - pun intended," said Marian Stanley, manager of the Phthalate Esters Panel, a unit of the American Chemistry Council.

"The authors themselves advise that there are 'several important limitations' to their findings " seven, to be exact. Perhaps the most important flaw is the selective way they presented their results. They reported results on their studies of males only. The results of their study of females are not shown because, they concede, the 'dataset gave dissimilar results'!"

The paper, published in Environmental Health Perspectives, claimed that phthalate exposure correlated with "abdominal obesity and insulin resistance in adult US males". Its principal author is Richard Stahlhut of The University of Rochester School of Medicine.

The paper does not claim to show that phthalates actually contributed to obesity, and states that additional study would be need to establish such a link. But the paper was publicised by the university and gained much media attention.

The Phthalate Esters Panel said the data was drawn from single examinations of thousands of men, women and children in a government health and nutrition survey. The authors concede that obesity is a syndrome gathered over time, and a single-point snapshot cannot reliably predict the impact of anything.

They also acknowledge that obesity has many causes, and trying to single out one cause is difficult. The panel scientists also criticise the study for bending or breaking some of the basic rules of statistical analysis. For example, they grouped phthalate exposure data, because no association with any single phthalate could be found.

Grouping data is done when the amount of data is limited, but in this case, they were drawing on a mountain of data from the government study. Grouping it has the effect of reducing the level of information, not improving it. The authors also applied linear regression analysis to non-linear data, which exaggerates the differences in the data, and in the judgment of the panel's experts "invalidates the analysis and thus the claims in the paper's conclusions".

"The Phthalate Esters Panel takes all science seriously, and we had this paper reviewed carefully by experts in the field," said Stanley. "It is our reviewers' conclusion that the authors performed their statistical analysis poorly and then over-interpreted it, and excluded data that disagreed with their conclusions. It is fair to ask whether this paper even deserved to be published."

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