Chubby? Blame Those Genes

It has long been clear that people's weight is determined by a balance of heredity and life-style. But which exerts the heavier effect? Two reports in last week's New England Journal of Medicine tip the scales firmly toward genetic makeup.

In one investigation, researchers from the U.S. and Sweden analyzed weight and height records from the Swedish Adoption/Twin Study of Aging. Reviewing data on 247 identical and 426 fraternal pairs of twins, the team found that siblings end up with similar body weights whether or not they are raised in different families, and that they are much more likely to grow up looking like their natural parents than their adoptive ones. "If both biologic parents are fat, about 80% of their kids are going to be fat," says Dr. Albert Stunkard of the University of Pennsylvania.

In a separate study, Canadian researchers fed twelve pairs of identical twins 1,000 calories above their normal daily intake for 84 days out of a 100- day period. Weight gains ranged from 4 kg to 13 kg (9 lbs. to 29 lbs.). But the difference in the amount gained was much less between twins than between subjects who were not siblings. Concludes Claude Bouchard, a professor of exercise physics at Quebec's Laval University: "It seems genes have something to do with the amount you gain when you are overfed." Some sets of twins transformed the extra calories into mostly fat, while others converted them into lean muscle.

Some unsuccessful dieters may find vindication in these findings. "The results take obesity out of being a moral problem -- that obese people have a lack of willpower -- and put it more in the realm of metabolism," observes Dr. Theodore VanItallie of Columbia University's College of Physicians and Surgeons. But this evidence could also lead to despair. If people are born to be fat, are attempts to slim down doomed? No, say weight specialists. Low-fat diets and exercise can help offset heredity. People may inherit a propensity to obesity, but it need not be their destiny.