They used to call this the "set point" theory. What it means is that the body has a weight to which it naturally gravitates. Gain weight and your metabolism will speed up in an attempt to return to the set point. Lose weight, and metabolism slows. Theoretically, it's as hard to gain weight as it is to lose, but not nearly as fun.

In a study last year at Rockefeller University in New York researchers found that in volunteers who gained weight, metabolism was speeded up by 10 to 15 percent, and in those who lost weight, metabolism was 10 to 15 percent slower than normal. The volunteers, both female and male, ranged in age from their 20s to their 40s, but the effect on metabolism was independent of age and sex.

The researchers also found that the way the body adjusts its metabolism is by making muscles more or less efficient in burning calories. Their findings mean that a 140-pound woman, for example, who has lost ten pounds to achieve that weight will burn about 10 to 15 percent fewer calories when she exercises than a woman who maintains that weight effortlessly. Conversely, if the woman has gained ten pounds, her muscles will burn about 10 to 15 percent more calories than expected when she exercises.

Hmmm. It didn't say what type of exercise.

Dr. Jules Hirsch, physician in chief at Rockefeller and the senior author of the study, said the findings showed that obesity, rather than being an eating disorder, is "an eating order." Obese people, he said, eat to maintain the weight that puts their energy metabolism precisely on target for their height and body composition.

The researchers suggest that the best way to help dieters in the future might be to understand what makes the muscles more or less efficient with weight gain or loss, rather than focusing on diets and psychological counseling.

Most people find it easier to gain weight than to lose it. Although it is easy to eat just an extra 200 or so calories a day, resulting in added pounds, it is much harder to subtract 200 or so calories. Deprivation hurts. We rebell against negative energy balance.

The researchers also studied what the body does to change its metabolism. They found that about 65 to 70 percent of calories burned each day are used to keep up the routine body functions the pumping heart, the working kidneys, the metabolizing liver. About 10 to 15 percent are spent eating and assimilating food. And the rest, about 15 to 25 percent, are spent by the muscles' exercising.

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