Waistline worry: common chemicals might boost obesity.

A family of chemicals implicated in testosterone declines may also be contributing to recent spikes in obesity and diabetes, according to a new study.

Phthalates show up in a wide range of manufactured items, from cosmetics to vinyl flooring to medical devices and drug coatings. With people's extensive exposure to phthalates, the chemicals' breakdown products, or metabolites, appear in the urine of more than 75 percent of the U.S. population.

Previous research had shown that phthalates decrease testosterone concentrations and harm reproductive development in male animals (SN: 4/3/99, p. 213). Effects have also been found in people. Exposure to phthalates in the womb has been linked to genital changes in male infants (SN: 6/4/05, p. 355), while a study in adult men found an association between the chemicals and sperm abnormalities (SN: 5/31/03, p. 339).

In men, low testosterone can lead to abdominal obesity and insulin resistance--conditions that are precursors of heart disease and type 2 diabetes, notes Richard W. Stahlhut, a research physician at the University of Rochester (N.Y.) School of Medicine and Dentistry. "If phthalates are affecting sperm counts and testosterone levels, then you would expect these guys would get abdominal obesity and insulin resistance," he says.

Stahlhut and his colleagues examined data from the Centers for Disease Control and Preventions National Health and Nutrition Examination Survey, which collects physical-exam and survey data on a large, representative sample of the U.S. population. The researchers considered measurements of phthalate metabolites in urine and waist circumference for 1,451 adult men. For 651 of these men, the researchers also had the data available to calculate insulin resistance.

In the statistical analysis, the team controlled for other factors, such as age, race, total fat and calorie intake, and activity level, that can influence obesity and insulin resistance.

The researchers found that the men with abdominal obesity, insulin resistance, or both were more likely than the other men to have high concentrations of phthalate metabolites in their urine. The team reports its results online and in an upcoming Environmental Health Perspectives.

"It's another piece of evidence that phthalates are a place we need to look when we try to get to ... potential chemical causes of obesity and insulin resistance," Stahlhut says.

However, he cautions that longer studies are necessary to confirm the findings. "What you'd like to do is measure some adults that seem to be heavy phthalate consumers and those that aren't, follow them over time, and see what develops," he says.

Thomas G. Travison, a biostatistician and epidemiologist at the New England Research Institutes in Watertown, Mass., agrees on the value of longer studies. Nevertheless, the work by Stahlhut's team "underlines the potential importance of these [chemical] effects that aren't often measured," he says.