Hippocrates observed that "walking is man's best medicine" and thus underscored the benefits of physical activity to health. More than two millennia later, the benefits of physical activity in lowering the risk of death from any cause and improving longevity have been well documented. Scientists are also beginning to understand the benefits of exercise on a molecular level and to consider skeletal muscle as an endocrine organ, capable of communicating with other tissues through myokines, which are released into the circulation during physical activity. In a recent study, Bostrom and colleagues identified a new myokine, irisin, which is released into the circulation during exercise and triggers the transformation of white fat cells into brown-in-white, or brite, cells - white fat cells with a phenotype similar to that of brown fat cells.
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