Brown fat, white fat…isn’t all adipose tissue the same?
Previously thought of as the domain of infants and hibernating bears, brown adipose tissue has been identified in the adult human body as well.
Two papers that appeared in The New England Journal of Medicine in 2009 reported that adult humans have brown fat, found in small blobs. These blobs showed on PET/CT scans when the people scanned were in cool surroundings, with room temperatures at 61–66°F. Blobs of brown fat show up on PET scans because these scans identify areas in the body where cells are more actively using glucose (1–2).
Generally, more brown fat is found on thin people than on heavier people, more on younger people than older people and more brown fat on persons with higher metabolic rates than in those with sluggish metabolisms. Women frequently have more brown fat than do men.
Brown fat is actually brown in color, owing to the presence of mitochondria. The mitochondria contain iron, giving the tissue a reddish brown color.
That’s right. Fat that contains mitochondria, those cellular powerhouses. Mitochondria supply most of a cells energy needs in the form of ATP.
Earlier this year, 2012, a study published in The Journal of Clinical Investigation showed that brown fat burns other fats (i.e., intracellular triglycerides) in volunteers exposed to lowered temperatures (3).
If that news wasn’t enticing enough, another study, also from 2012, showed that another type of brown fat can be made from white fat, by exercising (4). The mechanism is being teased out as follows:
Muscle is known to communicates with fat during physical activity. PCG-1α (transcriptional co-activator PPAR-γ co-activator-1 α) is a substance produced in muscles during and after exercise and known to influence biological processes elsewhere in the body. Mice bred to produce abnormally high amounts of PCG-1α were shown to avoid age-related obesity and diabetes, similar to people who exercise regularly.
To determine how PCG-1α worked in communication between muscle and fat, researchers looked for a protein whose expression was upregulated with PCG-1α. This search led them to FNDC5, which showed increased release with elevated PCG-1α.
FNDC5 was discovered to break into several components, one of which is ‘irisin’ (named for the messenger goddess from Greek mythology, Iris). Irisin moves from muscle through the bloodstream to fat cells, where it emits a biochemical signal that stimulates white adipose tissue to change to brown fat.
If that’s not cool enough, this change of white fat to brown fat appears to happen in particular to the deep abdominal fat surrounding organs. We are learning that fat around the waistband is a particularly strong indicator of poor outcomes for diabetes and other obesity-related concerns (5).
The scientists next took white fat cells from mice and treated the cells with irisin, which caused the cells to increase respiration, indicating they were burning more fat. Furthermore, when mice that were fed a high-fat chow were treated with FNDC5, their glucose tolerance improved (4).
As for irisin studies in humans, volunteers who completed a several weeks-long jogging program were found to have much more irisin in their muscle cells than they did prior to that study.
In case you, like me, are ready to run out and look for some irisin, hold the phone. While researchers do see weight loss in mice when their brown fat is activated, there is a concern that in humans, activating brown fat might simply increase appetite. Better, at least for now, to run out and get some jogging shoes.
There is however, an experiment you could try. Mice predisposed to obesity were put into a cold room at 41°F for a week, during which their brown fat was activated and they lost 14 percent of their body weight.
Suddenly, though summer is just beginning, I find myself happily anticipating winter. Going to spend some quality time outdoors. In the cold. Or maybe turn down the thermostat and save money, burn less hydrocarbon and lose some fat.
References
- Wouter, D. et al. (2009) N Engl J Med 360, 1500-08
- Cypess, A.M. et al. (2009) N Engl J Med 360, 1509-17.
- Ouellet, V. et al. (2012) J. Clin. Invest. 122(2), 545-52.
Boström P, Wu J, Jedrychowski MP, Korde A, Ye L, Lo JC, Rasbach KA, Boström EA, Choi JH, Long JZ, Kajimura S, Zingaretti MC, Vind BF, Tu H, Cinti S, Højlund K, Gygi SP, & Spiegelman BM (2012). A PGC1-α-dependent myokine that drives brown-fat-like development of white fat and thermogenesis. Nature, 481 (7382), 463-8 PMID: 22237023- Mucio, D.M. (2010) N Engl J Med 363, 291-293.
Kari Kenefick
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