Is Losing Weight a Losing Battle?

By Denise Grady

For those who have struggled to lose weight or keep it off, research into the origins of obesity has begun to offer an absolution of sorts. Last week two studies published in the New England Journal of Medicine showed that some fat people, rather than being slothful or gluttonous, have an inborn predisposition to gain weight. Reason: instead of burning off excess calories as others do, obese bodies are programmed to convert them into fat. Thus, while fat people may eat the same amount of food as thinner people do, they gain more weight. Moreover, their tendency to get fat is probably hereditary.

Researchers have long suspected that a metabolic problem underlies obesity, but until now the evidence has been ambiguous. The studies are the first to have focused on people before they became fat and monitored them as they gained weight. The major finding: certain measurements of metabolism, such as the rate at which the body consumes oxygen and produces carbon dioxide, can be used to predict who is likely to become obese. Says Dr. George Bray, an obesity specialist at the University of Southern California: "It's the predictive nature of this work that is so important."

A team from the National Institutes of Health found a link between low metabolism rates and excessive weight gain over a period of several years. Their subjects were 171 Pima Indians in Arizona, a tribe in which two-thirds of the women and half the men are obese. Though there were variations among individuals, researchers generally found that the slower the metabolism, the greater the weight gain. Yet after a gain of between 20 lbs. and 45 lbs., % metabolism rates changed, rising to a new level.

What does it mean? Some people, it appears, are programmed by their genes to store fat instead of burn it. Scientists speculate that this ability may be a vestige of early human history, when those who could live off their fat reserves were more likely to survive droughts and famines. The bodies of such individuals actively resist every effort to slim down. Below a certain weight, their metabolism slows in order to allow fat to accumulate, but their appetites remain undiminished. Once body weight rises to a certain point, the metabolism seems to speed up, so that they maintain that weight without gaining any more. But, researchers note, a sluggish metabolism does not explain all obesity. Overeating and lack of exercise also seem to play a role in getting fat.

The second study supported the idea that people inherit physiological traits that predispose them to obesity. A group led by Physiologist Susan B. Roberts of Tufts University studied 18 infants during their first year of life: six of their mothers were thin; twelve were overweight. Roberts and her colleagues measured how many calories the babies took in and how many they burned off. By three months of age, six of the babies with overweight mothers were generating 21% less energy than the rest. At one year, the six had become overweight, although they ate no more than the thinner babies.

Researchers, however, were unable to pin down the reason that some infants seemed to expend less energy than others. Their educated guess: babies who gained too much weight were less active than those who didn't. The obvious conclusion: even for infants, exercise is probably the best medicine for obesity. The prescription for adults is the same. The new findings may lift some of the blame for being fat from the obese, but not the responsibility. In certain people, the tendency to put on fat never wanes, and only a life of dieting and exercise can thwart it.

With reporting by Suzanne Wymelenberg/Boston