WASHINGTON — Three new studies show that most adults have unexpectedly large and active deposits of a calorie-burning type of fat that biologists once thought disappeared after infancy.
The persistence of brown fat suggests a potential new strategy to fight obesity, which is epidemic in the United States and increasing rapidly in the developing world. In addition to eating less and exercising more, people may one day be able to stimulate their bodies to get rid of stored energy -- in the form of ordinary fat -- purely as heat.
"It is, in a sense, the discovery of a new organ," said Dr. Sven Enerback, a researcher at the University of Gothenburg in Sweden and the lead author of one of three studies appearing today in the New England Journal of Medicine.
"This is a tissue whose sole physiological purpose is to expend energy," said Dr. Francesco Celi, a metabolism researcher at the National Institutes of Health, whose commentary accompanies the studies. "That makes it an ideal target" for drugs or other measures designed to make it more active.
At the moment, however, the only safe way of activating brown fat is to stay chilly, right at the verge of shivering, for prolonged periods. That reproduces the conditions that led to the evolution of brown fat -- namely, life-threatening cold in babies and small furry animals that can't put on clothes to keep themselves warm.
Although the new research poses a difficult question -- which would you rather be, thin or warm? -- the goal is to find a more comfortable way exploit this ancient adaptation.
The three studies add to the emerging view that brown fat is involved in the body's complicated energy balance and may play a role in diseases such as Type 2 diabetes that can arise when that balance is thrown off.
For example, leaner people have more detectable brown fat than overweight people. Brown fat also appears to be more active in women than men, even though obesity is more prevalent in women. Studies show that stimulating the production of brown fat in mice -- a species in which it is naturally plentiful -- makes them resistant to gaining weight or to developing diabetes when fed a high-calorie diet.
In humans, it is unclear whether brown fat is more a cause of leanness or a result of it.
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Energy lost as heat
