Brown Fat is Not Just for Bears

Most of us have learned that bears use brown adipose tissue ("brown fat") to assist them in hibernation during the winter, and that other animals use it to regulate body weight and adaptive thermoregulation (control of body temperature). What is less well known is that humans also take advantage of their own version of brown adipose tissue. How it functions in humans may not only have implications for thermoregulation, but for a targeted strategy to combat obesity. The ratio of "white fat" ("bad" fat) to brown fat ("good" fat) may also be important.

The April 9 issue of the New England Journal of Medicine carried an article entitled "Cold-Activated Brown Adipose Tissue in Healthy Men" (N Engl J Med 2009;360:1500-8) authored by WD van Marken Lichtenbelt and colleagues. They noted that it has been held, until recently, that the presence of brown adipose tissue was thought to be relevant only in small mammals and infants, without much physiological significance in adult humans. So, they performed a systematic examination of the presence, distribution and activity of brown fat in lean and obese men during exposure to cold temperature. They studied both lean and obese men under thermooneutral conditions (22 degrees Centigrade or 71.6 degrees Fahrenheit) and during cold exposure (16 degrees C or 60.8 degrees F). They sought brown adipose tissue activity using special body scans, and then measured body composition and energy expenditures with specialized techniques.

Here's what they found. Brown adipose tissue activity was seen in 23 of 24 subjects during cold exposure, but not under thermoneutral conditions. The activity of the brown fat was significantly lower in the overweight or obese subjects than in the lean subjects. The amount of brown fat correlated in a positive fashion with resting metabolic rate and in a negative fashion with body mass index and overall percentage of body fat. The authors concluded that while many young men apparently carry brown adipose tissue, the activity of that tissue is reduced in men who are overweight or obese. In other words, if it is possible to the brown fat could be stimulated to consume energy by causing adaptive thermogenesis (generation of heat), this might provide an insight into an approach to combating obesity.

In another study in the same issue of the journal, entitled "Identification and Importance of Brown Adipose Tissue in Adult Humans," (N Engl J Med 2009;360:1509-17) by AM Cypress and colleagues, it was concluded that defined regions of functionally active brown adipose tissue are present in adult humans, are more frequent in women than in men, and are present in an amount that is inversely correlated with body mass index, particularly in older people.

The theoretical benefit of maintaining a certain amount of brown fat and causing it to be activated in humans for the control of obesity is tantalizing. Brown fat is not "storage fat" - that role is played by white fat. Brown adipose tissue, when activated, serves predominately to maintain core body temperature, in a process that consumes free fatty acids and glucose from the bloodstream (and thus, from the body). In a physiologically inefficient (or so it seems) process, brown fat releases energy in the form of heat. Precisely how this activity would be modulated and regulated in such a fashion as to be beneficial from a metabolism and weight control perspective, are to be determined, but the bears know something.

Tags: adipose tissue, fat, brown fat, wilderness medicine, outdoor medicine, healthline

Labels: brown adipose tissue, brown fat

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