NEW YORK (Reuters Health) - A new study ties exposure to additives found in everything from shampoo to pesticides to an increased risk of obesity and insulin resistance in men.

The higher the level of "phthalates" a man had in his urine, the bigger his waist size and the lower his sensitivity to the key blood sugar regulating hormone, researchers found.

Given that excess belly fat and insulin resistance also are linked to low testosterone levels, and that phthalates have been shown to interfere with or block testosterone function, the findings provide indirect support to the idea that phthalates are behind the recent decline in sperm counts and testosterone levels seen in many industrialized nations, Dr. Richard W. Stahlhut told Reuters Health in an interview.

"I am certain that the decline in testosterone and sperm counts requires urgent investigation," said Stahlhut of the University of Rochester School of Medicine and Dentistry, New York.

"Phthalates are on the list of candidate chemicals that we need to be looking at," he added.

Seventy-five percent of the US population has measurable levels of phthalates in their bodies, Stahlhut and his team note in the journal Environmental Health Perspectives. He and his colleagues looked at levels of six different phthalate metabolites among men participating in the National Health and Nutrition Examination Survey for 1999-2002.

Four of the metabolites were tied to increased waist circumference, while three were linked to greater insulin resistance. The relationship remained after the researchers adjusted for a number of other factors, including activity levels, smoking, and race.

It will be necessary to study the issue over time in order to confirm the relationship identified in the current study, the investigators say.

"Since phthalates are rapidly metabolized, unlike PCBs and other persistent organic contaminants, such confirmation could prompt effective actions to reduce phthalate exposure in the population," they conclude.