The fear that diets ultimately fail because the dieter's metabolism becomes abnormally sluggish appears to be unfounded, researchers report in today's Journal of the American Medical Assn.
But, they said, long-term weight loss is still very difficult to maintain for reasons that are poorly understood. The findings are similar to those reported recently by a USC researcher.
Experts previously assumed that a drop in weight triggered a permanent, corresponding drop in resting metabolic rate, the amount of energy the body requires at rest to maintain basic physiological functions such as heart rate and respiration. A permanently lowered resting metabolic rate would mean that a dieter would not burn calories as rapidly and might regain weight or have trouble shedding more pounds.
However, the new studies at the University of Pennsylvania show that when overweight women who also exercised were taken off a very-low-calorie diet, their metabolism bounced back to a level that is normal for their new, lower body weight.
The findings are important for individuals attempting medically supervised dieting. Some experts feared that very-low-calorie diets--such as liquid diets--put the body into a starvation mode, slowing metabolism to a rate that thwarted significant weight loss.
Theoretically at least, dieting can work, but researchers still don't understand why it is so hard for many people to permanently keep the weight off, said Thomas A. Wadden, associate professor of psychology at the University of Pennsylvania School of Medicine.
"Statistics tell us that it is extremely difficult to keep weight off; people will still have to work very hard at this despite our favorable metabolic findings," Wadden said.
Studies have shown that extreme dieting triggers a plunge in metabolic rate. But how the change in metabolic rate affects long-term weight control has been the subject of much research and debate.
"We have long known that when people diet, metabolic rate falls sharply. And the more you cut back your calories, the more your metabolic rate falls," Wadden said. "But does it ever come back up or does it stay down? That is what we were trying to assess."
The study followed 18 women weighing an average of 216 pounds for 48 weeks. Nine of the women received a conventional diet of 1,200 calories a day for 48 weeks. The other nine were placed on the very-low-caloried liquid-formula diet Optifast for 16 weeks, followed by a conventional reducing diet. All the women increased their physical activity by walking.
After five weeks, the resting metabolic rate dropped in both groups, most significantly in the very-low-calorie dieters. But metabolic rate rose again in both groups. After 48 weeks, metabolic rates of both groups were about 9% lower than at the start of the study and weight had been reduced by 16% to 19%.
"After people lost 40 or 50 pounds, their metabolic rate is lower than it was before. That reduction in metabolic rate has to be anticipated; it takes less fuel to keep a lighter body going than a heavier body," Wadden said.
"What they don't have is a long-term suppression of the metabolic rate due to the effects of dieting. Over the long term, the metabolic rate increases to a level that is appropriate for your new, reduced body weight."
The results mirror a similar study performed at USC by Dr. Ken Fujioka, now a San Diego-based researcher.
In a study of obese women placed on a 650-calorie diet for 10 weeks, Fujioka found that the dieters' metabolic rates fell in proportion to their weight loss. Wadden's finding that even diets of 420 calories do not trigger long-term metabolic suppression is significant, Fujioka said.
"The question was, how many calories can you go down and not permanently lower metabolic rate? I'm surprised at that number (420 calories)," he said.
Because the metabolic rate was lower than before dieting, however, the dieters still needed to consume fewer calories to maintain their weight loss, Wadden said. Generally, for each 25 pounds lost, a dieter should cut back 100 calories a day.
Wadden also noted that the women in the study also exercised--a factor that might be very important in boosting metabolic rate.
"It's important to exercise while you're dieting," he said. "The exercise preserves your muscle tissue, and the more muscle tissue you have, the higher your metabolic rate. Moreover, we know that exercise is the single known predictor of long-term weight loss."
In a related study published earlier this year, Stanford researchers found that men who dieted but did not exercise experienced a suppressed metabolic rate. But overweight men who lost weight through exercise, but did not diet, experienced no reduction in metabolic rates.
"We've always held that it would be better for overweight people to increase exercise than to rely totally on extremely low-calorie diets," said Peter Wood of Stanford. "Exercise is less likely to have this permanent effect on the resting metabolic rate, which makes it easier for people to maintain their weight in the future."
But, Wadden said, researchers have few other clues on how to maintain long-term weight loss.
"I'm afraid we can't say you can lose weight and never regain it."