Dieters often fail to keep off their hard-lost weight because their bodies become more efficient, burning up calories sparingly, a study shows.
The work confirms the long-held suspicion that the body's own metabolism conspires against a successful diet.It suggests that people inexorably gain back lost weight because their bodies need fewer calories, so they get stored as fat. They add pounds even though they seem to be eating and exercising sensibly.
"We think the body is resisting the new lower body weight," said Dr. Rudolph L. Liebel of Rockefeller University, who directed the study.
His work, being published in Thursday's issue of the New England Journal of Medicine, was based on painstaking analysis of the food consumption, energy expenditure and weight of volunteers, 18 of them obese and 23 people who had never been overweight.
They found that the body reacts identically to weight change - whether they gain weight or lose it - in both fat people and thin.
However, when the overweight people lose pounds, their bodies need considerably less energy to meet their needs than do those who have always been at the lower size. They even need less energy to exercise.
When they lost 10 percent of their body weight, the volunteers' bodies compensated by burning up 15 percent fewer calories than would be expected. When they increased their weight by 10 percent, they used up 15 percent more calories than would be expected.
In an accompanying editorial, Dr. William Ira Bennet of Cambridge Hospital said the study shows that "the body has a complex, highly sophisticated system for regulating its fat stores."
Liebel said his work suggests that the body has a built-in weight target that it tries to hold.