What is brown fat, and why is it good?Our white fat serves essentially as a fuel depot, storing calories to use when needed. Brown fat plays a more active role. It's loaded with mitochondria that burn those calories when we need energy. We lose almost all of our brown fat as we age. That's too bad, because the more we learn about it, the more promising it looks—on a number of major health fronts. So new research on brown fat brings good news. It seems we can bring brown fat back into play—especially when the play is exercise.
A home-made, fat-fighting health hero?Let me also introduce you to a hormone called irisin. It's a relatively recent discovery that showed up in the muscle tissue of lab mice when they exercised. A breakthrough study by the University of Florida in 2012 followed up on that discovery, and showed that when lab mice were injected with a prepared irisin formulation:
- Irisin turned some of their white fat into the more active, calorie-burning brown fat
- Irisin also seemed to keep the mice from becoming obese, even when they were fed a high-fat, high-calorie diet that should have puffed them up like little muffins
For man or just mouse?Of course, it first had to be determined whether irisin is even present in humans. Maybe the mice are keeping it for themselves? Then, if it's available and active in humans, would it affect human fat cells the same way as it did mouse fat cells? Answers?
- One study found traces of irisin in sedentary people—but lots of irisin in people who exercised often.
- Another study found that people with lower body mass indexes (BMIs), tended to have more brown fat. Low BMI is often associated with exercise, among other factors.
One more step toward realizing a promiseAnother study looked at both white and brown human fat from tissue extracted during surgery. Some of the white fat cells were mature. Others were younger—essentially stem cells that could grow into mature fat cells or other types of tissue. The researchers exposed both white and brown fat cells to varying strengths of irisin. They then looked for signs that the white fat was becoming brown fat. And there the signs were—especially in the white fat cells that were exposed to moderate or high doses of irisin. Those cells began to act brown, becoming significantly more metabolically active—ready, willing, and able to burn calories, instead of retaining calories. In other words, the study gave us significant evidence that irisin prompted “browning” of white fat. Which simultaneously led to increased metabolic energy and decreased formation of new white fat. There was more:
- Many of the young stem cells in the fat that was exposed to irisin became the kind of cell that matures into bone, rather than maturing into white fat cells.
- In the end, the irisin-treated tissue contained about 40 percent fewer mature fat cells than tissue unexposed to the hormone—which showed no change.
Recommendation: exercise—it produces irisinWe're still in the early days of learning about irisin. We've studied only human tissue samples, not live humans. So there's still much we don’t know:
- What types of exercise produce the most irisin?
- How much irisin is enough?
- If exercise produces irisin, and irisin reduces white fat, why do so many studies show that exercise alone rarely results in significant weight loss?
- Reynolds, Gretchen. "Exercise May Turn White Fat Into Brown." New York Times. Oct 12, 2016.
- Harding, Anne."What Is Brown Fat? 5 Fascinating Facts" Live Science. Jan 30, 2015
- Boström P, et al. “A PGC1-α-dependent myokine that drives brown-fat-like development of white fat and thermogenesis”.2012 Jan 11;481(7382):463-8.
- Zhang Y “Irisin exerts dual effects on browning and adipogenesis of human white adipocytes.” Am J Physiol Endocrinol Metab. 2016 Aug 1;311(2):E530-41. Epub 2016 Jul 19.
- Jedrychowski MP, et al “Detection and Quantitation of Circulating Human Irisin by Tandem Mass Spectrometry.” Cell Metab. 2015 Oct 6;22(4):734-40. Epub 2015 Aug 13.