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What is brown fat?

Dr. David L. Katz, MD, MPH
Preventive Medicine
Brown fat is hot, because it may help keep us warm, burn calories, and help keep us thin. But that may just be a lot of hot air. That was my impression when brown fat started heating up in 2009. In the April 9 issue of the New England Journal of Medicine that year, three articles and an editorial highlighted the potential and apparently overlooked importance of brown fat in human weight regulation.

The New York Times suggested that these findings might offer a "cool way to lose weight" -- namely, by using some yet-to-be-discovered wonder drug to reset the human thermostat. Or, in the interim, turning down the thermostat in our homes.

The heat has been turned up rather than down on the topic with the advent of the two new brown fat studies. One, in mice, purports to establish the existence of a new hormone, irisin, which is integral to converting garden-variety white fat into its hottie counterpart, brown fat.

Irisin exists in humans as well as in rodents. If irisin does in people what it does in mice, and if we can develop irisin to give people, it might cause them to burn more calories without exercising.

The second study demonstrated in six healthy men that cold can induce brown fat to burn white fat. If the body gets even colder, shivering ensues -- and the muscle activity of shivering helps restore a normal temperature. By making the participants cold up to but not past the point of shivering, metabolic rate was reportedly increased by some 80% -- resulting in the expenditure of an extra 250 calories or so over three hours. A brisk walk for one hour would do the same.

One opportunity to which this research points is weight management by toughing out the cold. But being cold is uncomfortable. If people were willing to be uncomfortable to control weight, a whole lot more of us would exercise!

The second opportunity is to increase the generation of calorie-burning brown fat by exercising. But this is really just another way of saying if you exercise more, you are apt to weigh less -- and almost certain to be healthier.

Which brings us to the last great opportunity: a new wonder drug -- irisin, or something like it, in a capsule or syringe. An effort to use a drug to rework the fundamentals of human metabolism seems to me an enterprise fraught with peril. I contend that we will win or lose the war of weight control with our feet and our forks -- not pharmacotherapy.

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