Eight people sealed inside Biosphere II for two years experienced excess drops in metabolism and declines in physical activity because of their low-calorie, low-fat diet, a study has found.

"To cope with the limited resources available, these Biospherians had to reduce their energy expenditure," said Eric Ravussin, an obesity specialist with the National Institutes of Health. "They felt tired."Biosphere II is a huge, glass-enclosed structure designed to show that humans can survive in a self-sustaining environment, producing their own food, regulating their atmosphere and recycling their wastes.

The experiment was not entirely successful. Food production fell short of initial expectations, and Biosphere's residents were forced to subsist on 1,700 calories a day for the first six months.

During that period, they lost an average of 15 percent to 16 percent of their body weight, a substantial loss, Ravussin said. They were not malnourished because their diets contained all vital nutrients, but they simply did not have enough calories.

When they left Biosphere II, they ate heartily and regained an average of 24 pounds within months, Ravussin said.

Ravussin reported his findings Monday at the Seventh International Congress on Obesity, which is meeting through Thursday.

He began the study about halfway through the two-year stay of the initial eight volunteers to enter Biosphere II. Roy Walford, a volunteer who was also a doctor, asked Ravussin for help examining the Biospherians' metabolism.

Ravussin sent equipment that allowed Walford and the others to measure their metabolic rate before they left the enclosure. Ravussin tested them again last July.

"There's never been a study in such a well-controlled situation," he said.

As the Biospherians' weight fell, their metabolic rates fell, too - as expected - because they had less body mass to sustain. But Ravussin found that their metabolic rate fell 6 percent more than what would be expected from the weight loss.

Some researchers have argued that people who lose weight tend to regain it more easily because of this kind of excess drop in metabolism - what researchers call a "metabolic adaptation."

Ravussin said, however, that he doesn't believe the findings support that hypothesis.

"I think there is a metabolic adaptation when you go below the weight of a very lean person," he said, but not in overweight people trying to return to a normal weight.

Obesity specialist James Hill of the University of Colorado said that Ravussin's findings were interesting, but that they weren't definitive because he hadn't measured the Biospherians before they began the experiment.

Ravussin has remedied that in an ongoing study with five people who are now inside Biosphere II for one year. This time, he was able to test them before they went in.