It's a question that's been bandied about at least since Oprah Winfrey announced her drastic weight loss two years ago: Do extreme low-calorie diets, such as the one Winfrey was on, permanently lower a person's metabolic rate, making it even harder to keep the weight off?

A report in today's Journal of the American Medical Association says no.

"The concern prior to the study was that dieting may permanently lower metabolic rate," said Thomas Wadden, one of the report's authors. Many dieters feared that their bodies simply learned to scavenge more calories from food, making people regain weight even if they ate less than usual. Winfrey, for example, regained some of the weight she lost, and experts on her television talk show have expounded this theory. But Wadden said, "This study indicates that metabolic rate increases to an appropriate level over the long term."

The study followed 18 obese women over 48 weeks. Half were put on a regimen similar to the one followed by many people who try extreme low-calorie diets: 1,200 calories a day for the first week, then a 420-calories-a-day liquid diet for the next 16 weeks, and then back to 1,200 calories a day for the remainder of the time. The other nine women stayed on a 1,200-calorie-a-day diet for all 48 weeks.

The regimen also included increasing physical activity by walking.

The researchers measured the women's metabolic rate nine times, by a method called indirect calorimetry. They found that the rate of the women on the extreme regimen dropped an average of 8.3 percent by the end of the 48 weeks, which the researchers say was not significantly different from the 9.4 percent drop in metabolic rate found among the other women.

The weight of the women on the extreme regimen was 19.5 percent lower after 48 weeks, while that of the women on the less extreme regimen was 16.6 percent lower. The researchers attributed the lower metabolic rates to the fact that the women weighed less after dieting and therefore had less work to do to carry the weight.

The report's authors warn that extreme dietary regimens can have adverse effects on the heart and other complications and should be undertaken only under strict medical supervision. "We were testing {these diets} because those were the diets that were most suspect of permanently lowering metabolic rate," said Wadden, an obesity researcher at the University of Pennsylvania Medical School. "The study does not say that now it's okay to go out and diet aggressively."

He added that the study involved women at least 30 percent above their ideal weight and says nothing about slightly overweight people seeking to lose a few pounds.

C. Wayne Callaway, an obesity specialist at George Washington University Medical School, said he disagrees with the study's conclusions because the women on the extreme low-calorie diet began gaining weight as soon as they stopped the 420-calorie-a-day part of the program.

"The conclusion that metabolic rate doesn't go down is not proven by this paper," he said. "As soon as people go off the {extreme low-calorie regimen}, their weight starts to go up again."

Wadden responded that the women may have started gaining the weight back because they didn't stay on the diet and exercise plan.

"Unfortunately we can't say, 'great news,' that now people can lose weight and keep it off easily," he said. "It's going to be just as hard."