Exercise enhances muscular endurance and strength, calories expenditure, and prevents the development of obesity and type 2 diabetes. These effects of exercise cannot be explained solely by the expenditure of calories in muscle. A recent study indicated that a novel hormone, irisin, plays an important role in the increase of total body energy expenditure by exercising muscle. Irisin is a cleaved and secreted fragment of a membrane protein FNDC5. And its circulating concentrations are increased after regular physical activity. Exercise induces the transcriptional regulator PGC-1α in the skeletal myocyte, which in turn drives the production of the FNDC5. Animal experiments suggested that irisin, cleaved form FNDC5, activates thermogenic programs in white adipose tissue (“browning”), including mitochondrial biogenesis and the expression of uncoupling protein 1 (UCP1), leading to mitochondrial heat production and energy expenditure. Irisin is highly conserved across species, and exercise also increases circulating irisin concentrations in humans. Therefore, the role of irisin in humans may be that it links physical activity to energy metabolic homeostasis, including weight control.
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