Jan. 21, 2005 -- Can't zip up your favorite jeans anymore? Your parents may be to blame.
A new study suggests that genes may explain about 50% of weight gain that starts in middle age.
Researchers studied sets of twins who served in the military during the Vietnam War. Some of the twins were identical and shared the same genes, and the others were nonidentical twins.
After 20 years of follow-up, the study showed that genes accounted for about half of the weight gain among the men. Environmental factors, such as diet and exercise, accounted for the other half.
Researchers say the findings may help explain why some people have such a hard time losing weight.
"We're not acknowledging the strength of genetic factors in our weight loss strategies," says James C. Romeis, PhD, professor of health services research at Saint Louis University School of Public Health, in a news release. "You've got this genetic thing working against you that helps to explain why you're so heavy and why you may fail at diets and weight loss programs."
Weight Gain May Be in the Genes
In the study, which appeared in a recent issue of the journal Twin Research, researchers studied nearly 8,000 middle-aged, middle-class male twins who enlisted in the U.S. military during the late 1960s.
At the time of enlistment in early adulthood, more than three-fourths of the men were considered normal weight, 17% were overweight, and 2.5 % were obese or severely obese.
Twenty years later, less than half of the men were still considered normal weight. Meanwhile, more than half had become overweight.
The study showed that about 50% of the weight differences among the men appeared to be caused by genetic factors. Other aspects of their lifestyle and environment accounted for the rest.
Romeis says the influence of these environmental factors increases with age and puts people with a genetic disposition toward weight gain at risk.
"While genetic vulnerability has probably not changed during the past few years, environments have, thus allowing for the genetic vulnerability to be expressed as what appears to be an alarming rate of increase," says Romeis.
Although genetics might explain why some men gain weight more easily and have a harder time losing weight, researchers say it doesn't excuse them from trying to maintain a healthy weight, because weight gain doesn't happen overnight.
"Weight gain appears to increase gradually," says Romeis. "For these guys, it's at about 30 years old. Your behavior changes at 30; you become more sedentary. At some point they tip into being overweight. Those who are overweight tip into becoming obese. It's slow, incremental change."
"Losing weight is going to be a lot of work for these guys," he says. "Treatments and public health interventions need to recognize the magnitude of genetic factors if short-term and long-term interventions are to be effective."