Wed, 16 Jan. 2013 - 12:37 a.m. MT
Credit: ARA Staff - American Running Association
Researchers at the Dana-Farber Cancer Institute and Harvard Medical School have identified a new hormone that breaks down white fat cells in mice and humans. White fat, in contrast to brown fat, is the undesirable form of adipose tissue that accumulates around our thighs, hips, and waist. Brown fat is a healthy kind of fat that regulates body heat.
Brown fat is physiologically desirable. While white fat cells are essentially inert storehouses for fat, brown fat cells are metabolically active. They use oxygen and require energy. They burn calories. And it may be that the new hormone (called irisin) and exercise partially determine how much brown fat each of us contains.
We’ve known for a while that muscle cells influence biological processes elsewhere in the body. The whole of scientific literature suggests that muscle cells communicate biochemically with body fat. Because exercise involves muscle cells, it benefits a variety of organ systems in mammals, and some of the best-recognized effects on muscle are mediated by a “transcriptional co-activator” known as PGC1-alpha. Researchers published online in January delivered the news that PGC1-alpha expression in muscle stimulates an increase in irisin. The results were reported in the journal Nature, and state:
“Irisin acts on white adipose cells in culture and in vivo to stimulate a broad program of healthy brown-fat-like development. Irisin is induced with exercise in mice and humans, and mildly increased irisin levels in the blood cause an increase in energy expenditure in mice with no changes in movement or food intake. This results in improvements in obesity and glucose homeostasis. Irisin could be therapeutic for human metabolic disease and other disorders that are improved with exercise.”
We’ve also known that PGC1-alpha is produced in abundance in muscles during and after exercise. Mice bred to produce large amounts of PGC1-alpha in their muscles are typically resistant to age-related obesity and diabetes, much as people who regularly exercise are. The biological mechanisms by which PGC1-alpha jump-starts such beneficial effects had been unknown. For the new study, though, the researchers used advanced algorithms to determine that increases in PCG1-alpha in muscles caused a subsequent bump in the expression of a protein known as Fndc5. That protein had long interested biologists, but they hadn’t been able to pinpoint what it did.
The researchers realized that one thing the protein did was break apart and form the newly-identified irisin hormone. The hormone is produced in response to exercise, and seems to turn white fat brown. This is a startling discovery with divergent implications. The first is another reason to see exercise as an extremely powerful modality in the fight against obesity worldwide. The second is that with the discovery of irisin, we may one day be able to separate the good effects of exercise from exercise itself. This is remarkable and perhaps a little troubling.
Still, we won’t be discarding our running shoes for hormone supplements any time soon. While irisin appears to have a critical impact on metabolism, it does not appear to play any discernible role in the effects that exercise has on the heart or the brain, and irisin does not induce muscle strength gains. But down the road, there could be a powerful role for irisin in treatment of people who because of disease or disability cannot exercise. At a cellular level, lessening some people’s susceptibility to obesity, diabetes, and other health problems can’t be bad. With 10 days of treatment, the mice had better control of blood sugar and insulin levels—in effect, preventing the onset of diabetes—and lost a small amount of weight.
Follow-up experiments with muscle cells from human volunteers who’d completed a controlled, weeks-long jogging program found that they had much higher levels of irisin in their cells than before the exercise program began. In essence, irisin appears to be one of the more important missing links in our understanding of how exercise improves health.
Nature, published online Jan. 11, 2012, http://www.nature.com/nature/journal/vaop/ncurrent/full/nature10777.html
Harvard Gazette, Jan. 11, 2012, “Dana-Farber Cancer Institute Communications” by Richard Saltus,http://news.harvard.edu/gazette/?p=99352&utm_source=SilverpopMailing&utm_medium=email&utm_campaign=01.17.12%2520%281%29&utm_content=#.TxVzieP9r7w.email
The New York Times, Jan. 11, 2012, “Exercise Hormone May Fight Obesity and Diabetes,” by Gretchen Reynolds, http://well.blogs.nytimes.com/2012/01/11/exercise-hormone-helps-keep-us-healthy/?pagemode=print
Time, Apr. 8, 2009, “Brown Fat: A Fat That Helps You Lose Weight” by Alice Park
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