Approximately one-third of all adults in the United States are obese and another third (including me) are overweight. The expansion of the American waistline is a major medical problem since excessive weight gain and fat accumulation are associated with a number of serious health issues ranging from diabetes to heart disease. In my case, I have tried to justify my girth as essential insulation for the new ski season. My argument fails for two major reasons. First, while fat has strong insulating properties, so do quality ski clothes. But second, and perhaps more interestingly, the fat around my waist (and other places), referred to as "white fat," may not be the best kind of fat to battle cold temperatures. Recent research has revealed that the second kind of fatty tissue, referred to as "brown fat," may not only have superior properties in fighting the cold but might also provide significant health benefits.
Brown fat has been a topic of great interest to doctors for centuries. It is found in human babies (around the shoulder blades) and in animals that hibernate. It has also recently been found present to varying degrees in some adult humans. While referred to as fat, it is better considered as a kind of fatty tissue that looks distinctly different from the yellowish-white jelly-like material with which we are all familiar. A key property of brown fat is that it has "uncoupled" its cells' energy factories, or mitochondria, so that they produce heat rather than drive other bodily functions such as muscle contraction and digestion. In fact, brown fat is thought to produce 300 times more heat than any of the body's other tissues. It is rather like revving a car engine while the transmission is in neutral - a great deal of gas is burnt, but it is converted into heat rather than vehicle motion. In the case of hibernating animals, brown fat provides an inner furnace that keeps the animal warm as it sleeps through the winter cold. In the case of babies, it provides essential warmth as they enter the world wet and naked. And in the case of adult humans, it can convert excess calories into heat rather than white fat.
Just a few years ago, a gene was discovered that controlled the generation of brown fat in the body. This gene induced the expression of an "uncoupling protein" that triggers heat production in the mitochondria. But what turns on this pathway? Recent experiments have indicated that chilling of the body results in activation of brown fat, an increase in the rate of metabolism and the burning of the body's white fat and other energy sources such as blood sugars. Consequently, individuals with more brown fat can tolerate significantly more cold before they begin to shiver (another of the body's mechanisms to produce heat). It is therefore not surprising that people living in very cold climates, especially those who work outside, tend to have the largest stores of brown fat. Scientists have also found that hard exercise can stimulate the production of a certain type of brown fat. Exciting new research has revealed that active muscles produce a hormone called irisin which can turn white fat cells into brown fat cells. Irisin also makes the body more sensitive to glucose and may play a role in preventing the development of diabetes, a disease associated with excessive weight gain and lack of exercise.
The demonstration of brown fat in humans has excited scientists around the world, and the implications of this discovery are many. It appears that brown fat in overweight and obese individuals is either absent or not properly activated; instead they convert excess calories into white fat rather than burning it off through the action of brown fat. If brown fat is activated by cold temperatures, it may be that overweight people are so well insulated that they don't feel the cold as much as lighter people and therefore do not activate their brown fat reserves - a vicious cycle. Therefore, the search is on for drugs that regulate brown fat production and potentially control the obesity epidemic. Irisin is a leading candidate in this area, and it might not be long before I start taking a daily irisin supplement pill. Until then, like many Americans, I will have to rely on dieting and more visits to the gym.
David L. "Woody" Woodland, Ph.D. is the Chief Scientific Officer of Silverthorne-based Keystone Symposia on Molecular and Cellular Biology, a nonprofit dedicated to accelerating life science discovery by convening internationally renowned research conferences in Summit County and worldwide. Woody can be reached at 970-262-1230 ext. 131 or firstname.lastname@example.org.