Why Weight Is Almost Destiny

WASHINGTON -- After a 10-year study, researchers have explained what every dieter knows: It's hard to keep the weight off.

A team of scientists at New York's Rockefeller University discovered that a dieter's metabolism actually slows down after the pounds have been shed, so that doing the same amount of exercise at the new weight burns fewer calories. The body resists the change, trying to get back to its original weight.

This finding explains why even those who continue to eat moderately and exercise are likely to creep back up the scale.

The researchers found that after losing 10 percent of their body weight, newly slimmer patients expended 15 percent less energy than expected for someone of similar size and body composition.

That means people who lose that amount of weight and then stay within the average daily intake of 2,500 calories will nevertheless end up with a daily "positive energy balance" of about 375 extra calories -- the daily equivalent of a generous slice of lemon meringue pie or a heaping cup of potato salad.

They neither have their cake nor eat it, but they gain weight nonetheless.

The system also appears to work in the other direction: After a quick weight gain, the metabolism speeds up to make muscle activity burn more calories, quickly bringing the body back to its normal weight.

Scientists have known for some time that rats have a similar metabolic mechanism for keeping their weight steady, and that metabolism slows in obese people who have lost weight, making it easier for them to gain the weight back again.

But the new results, published in Wednesday's issue of the New England Journal of Medicine, show that the same regulatory system keeps weight stable in people of all body types, from skinny to obese.

"Whatever direction you go, up or down, losing weight or gaining weight, the body tends to resist that change," said Phillip Gorden, director of the National Institute of Diabetes and Digestive and Kidney Diseases, which helped fund the study.

How the body determines its "set point" is not known, but other research suggests it may depend either on total fat stores or on the size of individual fat cells.

Last year, Rockefeller University researchers discovered a gene in mice and humans that may produce a substance by which fat cells signal the brain and perhaps other organs.

The authors of the new study theorize that the body is able to adjust its energy expenditure by varying how efficiently skeletal muscles burn up calories.

The study does not mean that weight is destiny, researchers said.

An editorial in the journal by William I. Bennett of Cambridge Hospital suggests that the set point can shift gradually over time in response to external factors: For example, eating a high-fat diet tends to raise the set point, while regular exercise tends to lower it.

Bennett argued that most adults tend to gain weight over the years simply because they become more inactive, shifting their set point upward.

By the same reasoning, it may be possible to lower the set point gradually over the long term through a combination of diet and exercise.

Rudolph Leibel and the Rockefeller team concluded that even though they had shown that eating less and exercising more are less effective after weight loss, they remain the best available tools for combating the health problems associated with obesity.

"The beneficial effect of even a modest weight loss ... justifies persistent efforts at weight reduction and maintenance of a reduced body weight," the Rockefeller researchers reported.

"The message we don't want to get across is, 'That's the way God made me and that's immutable,'" said Susan Yanovski, director of NIDDK's Obesity and Eating Disorders Program. "Not everybody can be a size ten. But everybody can be healthier."

Leibel said that a deeper understanding of the body's regulatory mechanism could lead to better treatment strategies: "We might be able to alter the adjustments that are made" by the body, helping dieters to maintain a lower body weight.