Frustrated dieters have always suspected it. They lose weight, but then - even though they’re eating less and exercising more - the extra pounds creep back on. They feel as if their bodies are out to get them.
And they’re right.
A study in today’s issue of the New England Journal of Medicine provides evidence to back the long-held theory that the human body resists dieting and tries to maintain a stable weight by burning fewer calories when weight is lost.
“This study shows there are biological mechanisms that regulate weight loss. This is contrary to the notion people have that obesity is a disorder of willpower,” said the study’s lead author, Dr. Rudolph Leibel, an assistant professor at Rockefeller University.
The study provides evidence for the theory that body metabolism has a “set point” that determines how quickly the body burns calories. That theory provides one explanation of why weight is so hard to keep off, and it may help scientists design effective exercise regimens or even weight-loss drugs.
In the study, Leibel, Dr. Michael Rosenbaum and Dr. Jules Hirsch of Rockefeller analyzed the food intake, energy output and weight of 41 volunteers, of whom 18 were obese and 23 were not.
Because the study required a controlled environment, the volunteers spent from four months to two years living as inpatients in the university’s labs. They subsisted on a bland liquid formula of corn oil, carbohydrate and protein.
The study showed that moderately obese people who lost 10 percent of their weight burned calories more sparingly than before, using roughly 15 percent less energy than at their previous weight.
Most of the reduction came in calories burned during physical activity. A 130-pound person who had always weighed 130 would burn more calories walking a mile than a 130-pound person who had recently trimmed down from 150.
The study is the latest published finding in the scientific effort to understand the complex weave of metabolism, genetics, environmental factors and behaviors that affect human weight. The study, which took eight years, was praised by other scientists as “elegant” because research on eating behavior and energy expenditure is notoriously hard to conduct in humans.
The author of an editorial accompanying the study, Dr. William Ira Bennett of Cambridge Hospital, said the new research sheds light on one piece of the puzzle of obesity.
“If the body is trying to defend a given level of fat, there are so many different ways the body can resist weight loss,” he said Wednesday. “Every dieter knows their own body is a formidable opponent.”
Scientists compare the set point to a thermostat. The new study shows those thermostats might be set differently in obese and non-obese people, but work the same way in both populations.”The message to the obese is
there’s really nothing so different about your metabolism,” said James O. Hill, an professor of pediatrics at the University of Colorado Health Science Center.
Hill said people should not walk away from this news thinking that it is impossible to lose weight. They should, however, realize that they’ll have to either take in fewer or burn more calories long after they lose weight.