How does brown fat differ from the other fat in your body?

Dr. Jack Merendino, MD
Endocrinology Diabetes & Metabolism
In humans, “brown” fat can’t be distinguished from the rest of our body fat based on how it looks. “Brown” fat looks white just like so-called “white” fat. But brown fat has a very different purpose than white fat. White fat, which constitutes the vast majority of fat in our body, serves to store energy. We draw on those fat stores for energy during periods of fasting. Brown fat serves to generate heat. The energy stores in brown fat are not used to provide fuel for other cells in body, but rather to generate heat to raise the body temperature. Humans have a relatively small amount of brown fat, but certain animals, such as small rodents that are often exposed to cold, have a high percentage of brown fat. In these animals, it appears that white fat may be converted to brown fat when there is the need for more heat generation. Researchers are trying to understand how this conversion takes place. One possible implication of this could be in the treatment of obesity. If a higher percentage of the calories we ingest were used for heat generation rather than for fat storage, we might have less of a tendency to gain weight. On the other hand, this might result in an increase in body temperature which might cause other problems. This is an area of intense research, but it is not yet clear whether it will prove useful in practical terms in the future.
Dr. Michael Roizen, MD
Internal Medicine
Brown fat is usually found on the back of your neck and around you arteries (and has absolutely nothing to do with how much chocolate you eat). This increases in outdoor workers during cold weather to protect them from the weather; it insulates our vital organs. Though you have a fairly small percentage of brown fat as an adult, about a third of fat in babies is brown fat, and it's used primarily to keep them warm.

What makes brown fat different? Brown fat is alive. It has nerve fibers, like any organ, and it also has leptin receptors. When the level of this appetite-suppressing hormone goes up, it turns on energy consumption in the brown fat and burns it. This is important because it shows that the right leptin levels can signal you to immediately get rid of this fat. And it's also symbolic of the inherent goodness of body fat—when it's found in the right amounts.
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