Middle-aged men may be able to lay a hefty share of the blame for their expanding bellies on their parents, study findings suggest.

The study, involving about 8,000 male twins followed for 20 years, found that genes appeared to account for more than 50 percent of the extra padding the men packed on between young adulthood and middle-age. Environment, which would include lifestyle factors like diet and exercise, was responsible for the rest of the weight gain.

The finding that genes play a role in a person’s weight is not new; it’s believed that genes explain why some people can stay thin on a steady diet of doughnuts, while others wage the battle of the bulge their whole lives.

The strength of this study, said lead author Dr. James C. Romeis, is that it followed men over time and focused on “adult-onset” weight problems.

Still, the fact that it found genes were the biggest predictor of weight gain doesn’t mean a man should feel resigned to one day sport his father’s spare tire, according to Romeis, a professor of health services research at Saint Louis University School of Public Health.

Rather, he said in an interview with Reuters Health, people with a genetic vulnerability to weight gain probably “need to work a little harder” than others to maintain the figure of their youth.

“Genes are not destiny,” said Romeis, noting that today’s obesity epidemic did not arise from changes in people’s genetic vulnerability, but in their lifestyles.

For their study, published in the journal Twin Research, Romeis and his colleagues analyzed data from a study of male twins who served in the military during the Vietnam War. At enlistment in the late 1960s, the men were mostly normal-weight, as obesity typically precludes military service. Less than 20 percent of the young men were overweight.

Twenty years later, however, only 46 percent were of normal weight, while the rest were overweight or obese. The researchers determined that genes appeared responsible for just over half of the men’s change in body mass index over the years.

The role of genes, according to Romeis, has gotten little attention in the discussion over what to do about the expanding U.S. waistline. He said that people with a familial tendency toward weight problems should take heart when they are having difficulty controlling their own weight, since it’s not an easy task to “work against biology.”

“You’re not just ‘weak-willed’,” Romeis said.

According to the researcher, it’s also possible, though not yet proven, that genes play as much a role in a person’s ability to lose weight as they do in weight gain - which would make preventing the weight gain in the first place particularly important.

SOURCE: Twin Research, December 2004.