A newfound exercise-induced hormone helps the body increase its ability to burn energy and confers the beneficial characteristics of “brown” fat cells on unhealthy “white” fat cells, according to a study (Nature, DOI: 10.1038/nature10777). Bruce M. Spiegelman of Harvard Medical School and colleagues say the hormone, which they named irisin after the Greek messenger goddess Iris, is likely responsible for at least some of the beneficial metabolic effects of exercise, which include weight loss and lower risk of type 2 diabetes. These effects raise the possibility that irisin could find therapeutic use. The researchers uncovered the hormone when studying the protein PGC1-α, which is expressed in muscle and is involved in the biochemistry that boosts resistance to obesity and diabetes. They found that PGC1-α increases the expression of another protein, FNDC5, which is later cleaved to form irisin. The team subsequently studied the effects of irisin in mice and in fat cells in culture. Spiegelman and coworkers found that the hormone’s metabolic benefits can be induced in mice by increasing irisin levels in the blood, even without changes in exercise or food intake.