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Studies Find A Way Adult Bodies May Fight Obesity

In a second study, the Swedish team led by Enerback kept five volunteers in a cool room for two hours and then gave them PET scans while intermittently putting one foot of each subject in an ice-water bath. Brown fat deposits lighted up each time. In three, the tissue was then biopsied and proved to be brown fat.

In the third study, C. Ronald Kahn and Aaron M. Cypess of the Joslin Diabetes Center in Boston examined PET scans performed on 1,972 patients for various reasons over a three-year period. What appeared to be active brown fat was visible in 7.5 percent of the women and 3.1 percent of the men.

That was a much lower prevalence of detectable brown fat found in either of the other two studies. There are a number of possible reasons.

The Boston researchers did not attempt to induce activity in the brown fat by cold exposure (although PET scans done on days when it was cold outside were more likely to light up). The study also included many elderly, overweight and diabetic patients -- all conditions in which brown fat activity is less likely.

Other studies and extrapolation from animal experiments suggest that brown fat could be responsible for as much as one-fifth of the energy we burn at rest. Fully stimulated, the tissue might be able to burn off 10 pounds in a year.

Research published last summer by Kahn and his colleague at the Joslin Center, Yu-Hua Tseng, showed that when mice are injected with a growth factor called BMP7, they develop more brown fat. Kahn said he thinks ways will be found to increase brown fat in humans "a lot, at least in activity if not in amount."

Enerback, the Swedish scientist, agreed that the search for a drug that stimulates brown fat "is definitely something we should pursue." He added, however, that "I don't think there will ever be a pill that substitutes for a fundamental change in lifestyle in the treatment of obesity."

The latest findings highlight once again the extent to which obesity is a consequence of Homo sapiens carrying into an era of abundance, leisure and warmth the physiology that evolved in a world marked by barely enough food, constant physical activity and dangerous cold.

"We are living in a very comfortable time," said Wouter D. van Marken Lichtenbelt, a physiologist at Maastricht University who led the Dutch study. "But we did not evolve in such a time."

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