Exposure to a class of chemicals commonly found in soap and plastics could be fueling the obesity epidemic by contributing to abdominal obesity and insulin resistance in men, a new study suggests.

The chemicals, known as phthalates, have already been implicated in male reproductive problems including low sperm counts and low testosterone levels. However, it's too soon to know whether they are actually causing these health problems, cautioned the researchers and others.

"It's premature for folks to be alarmed," said study author Dr. Richard Stahlhut, a resident in preventive medicine at the University of Rochester School of Medicine & Dentistry in New York.. "What is more alarming is the reason we are doing studies like this. Another study showed that testosterone levels had dropped about 22 percent in men, and that sperm counts had dropped to levels that are considered subfertile or infertile."

"It's an important observation that chemical exposures could be contributing to obesity and diabetes in the general population," added Dr. Ted Schettler, science director for the Science and Environmental Health Network. "This is one more example of a family of chemicals that may be contributing to this problem, but this study has obvious limits that the authors acknowledge in great detail."

The study was published in the March 14 online edition of Environmental Health Perspectives.

Phthalates have been widely used for more than half a century in everything from paint to time-released medicines, but only recently have they become a topic of concern. Animal studies show that phthalates decrease testosterone levels while human studies have found that phthalates are associated with poor sperm quality in men.

This study follows up on other studies that correlated abnormal sperm counts and low testosterone levels with phthalates. Men with low testosterone levels develop abdominal obesity and insulin resistance, so these authors speculated that phthalates might be behind the depressed testosterone levels.

"That's the missing link, testosterone as a [possible] link between phthalates and obesity," Stahlhut said.

Stahlhut and his team analyzed urine, blood samples and other data collected for the National Health and Nutritional Examination Survey (NHANES), a large government survey, from 1999 to 2002.

Of the adult men available, 1,451 had data on phthalate exposures, obesity and waist circumference. Of these, 651 also had data on fasting glucose and insulin levels needed to calculate insulin resistance.

According to the analysis, more than 75 percent of the U.S. population has measurable levels of several phthalates detectable in their urine.

Men with the highest levels of phthalates in their urine had more belly fat and insulin resistance, even after adjusting for other factors.

One drawback of the data, and therefore of the study, is that no information on hormone levels was available, nor was there any long-term data.

In any event, phthalates are unlikely to be the whole story. The chemicals have been shown, in animal studies, to have an effect on thyroid hormone, which could also be a pathway to increased obesity.

"This is just part of the search for answers," Stahlhut said. "The thing we're certain of is not that phthalates are doing this, but that phthalates require very careful scrutiny. I'm certain that the declines in testosterone and sperm production require urgent investigation, and I'm certain that phthalates are on the list of chemicals that could be part of the issue."

"It's a complex, multi-factorial problem," Schettler added. "What the authors are suggesting is that a chemical exposure may be one among many factors. The study is certainly hypothesis-generating. It clearly makes a case that this potential link ought to be studied in more detail in more systematic ways."