Phthalates, a class of chemicals used in some plastic food packaging and soaps, have been implicated in higher belly fat in men.

Phthalates are used to make plastic flexible, and are found in plastic tubes, some children's toys, cosmetics, shampoos, soaps, lotions, lubricants, paint, pesticides, and other plastics.

The chemicals have been implicated in reproductive problems in men such as low sperm counts and low testosterone levels, and subtle changes in the reproductive organs of baby boys.

In a new study to be published in the journal Environmental Health Perspectives, Dr. Richard Stahlhut of the University of Rochester Medical Center and his team looked at the connection between phthalates and testosterone.

The researchers wanted to test the idea that phthalates may be linked to obesity, since low testosterone levels appear to cause abdominal obesity and pre-diabetes in men.

"Substantial declines in testosterone levels and sperm quality have been observed in the United States and other countries over the last several decades … and it urgently requires explanation," Stahlhut said.

"While we can't say yet that phthalates are a definite cause, I am certain they are on the list of chemicals that demands careful study."

Exposure could contribute to insulin resistance: study

The team analyzed urine and blood samples from a national cross-section of men in the U.S. from 1999 to 2002. They looked at data on phthalate exposure, obesity, waist circumference, fasting glucose and insulin levels for 651 men.

Men with the highest levels of phthalates in their urine had more belly fat and insulin resistance, after adjusting for other factors that could affect the results. In insulin resistance, the body is less able to use insulin to control blood sugar levels, a condition that can lead to Type 2 diabetes.

"If confirmed by longitudinal studies, our findings would suggest that exposure to these phthalates may contribute to the population burden of obesity, insulin resistance, and related clinical disorders," the study's authors concluded.

More than 75 per cent of the U.S. population had measurable levels of several phthalates in their urine, the researchers found.

Researchers do not know what combinations of common low-dose exposures to phthalates may be contributing to reproductive problems, Stahlhut said, but long-term data on hormone levels is needed to investigate the possible link.

Phthalates are among the 69 known contaminants that Statistics Canada is testing 5,000 Canadians for as part of a comprehensive survey announced last year.