An obscure substance in the body called brown fat seems to make much of the difference between being fat and being skinny, according to scientists.
Brown fat is a poorly understood tissue that seems to act the opposite of white fat, the ordinary stuff of bulging waistlines. Brown fat burns energy rather than conserving it, said endocrinologist Bradford Lowell, at Beth Israel Hospital in Boston.
Based on experiments in genetically engineered mice, Dr. Lowell and his colleagues found that loss of brown fat leads to gross obesity, plus disorders such as diabetes that are commonly associated with obesity. A report on the experiments appeared yesterday in the journal Nature.
Whether brown fat plays the same role in humans has yet to be learned. But Dr. Lowell said some rare cancer cases in humans, in which tumors stimulate release of the hormones adrenalin and noradrenalin, show that brown fat can be stimulated; the patients "end up skinny and have a lot of brown fat."
Pharmaceutical companies are also looking for drugs that stimulate brown fat activity, seeking new ways to control body weight, Dr. Lowell said.
Normally, in both animals and humans, there is far more white fat than brown fat, Dr. Lowell said. Until recently, in fact, there was hot debate whether adult humans retain any brown fat at all. It was obvious that humans have brown fat at birth, and recent evidence shows it persists into adulthood.
"White fat is just fat, and people who are obese have a lot of it. But brown fat is quite different, and opposite in its function," Dr. Lowell said yesterday.
Brown fat, which is difficult to recognize in the body because it occurs in small amounts and is hard to distinguish from other tissues, "has the function to spend an enormous amount of calories through a process called uncoupled mitochondrial respiration," he added. The extra energy is dissipated as heat.
The term mitochondrial refers to the tiny sausage-shaped bodies, mitochondria, which serve as potent energy factories inside living cells. The cells even look brownish because they are crowded with energy-churning mitochondria.
Usually they burn only as much fuel as is needed, but in brown fat cells they work overtime, "a tremendous way to waste a lot of calories," Dr. Lowell said.
Brown fat cells, Dr. Lowell explained, "can spend way in excess of what is needed" in normal metabolism. Their task, apparently, is to help keep the energy supply in balance, and perhaps prevent obesity.
To uncover what brown fat does, Dr. Lowell and his co-workers put a new gene into mice that selectively kills brown fat cells. As the brown fat cells disappeared, the mice ballooned into gross obesity.