Irisin is a recently described exercise-induced hormone secreted by skeletal muscle in mice and humans. Irisin activates beige fat cells (beige cells have a gene expression pattern distinct from either white or brown fat and are preferentially sensitive to the polypeptide hormone Irisin). Irisin is cleaved from the type I membrane protein FNDC5 and improves systemic metabolism by increasing energy expenditure. Circulating irisin levels were shown to be upregulated after acute exercise. Increase of irisin under conditions of obesity may indicate a physiological function to improve glucose tolerance.