['The newly discovered myokine irisin has been proposed to affect obesity and metabolism by promoting browning of white adipose tissue. However, clinical and functional studies on the association of irisin with obesity, muscle mass, and metabolic status remain controversial. Here we assessed the effect of 4 distinct exercise regimens on serum irisin levels in children and young adults and systematically evaluated the influence of diurnal rhythm, anthropometric and metabolic parameters, and exercise on irisin.', 'Serum irisin levels did not show diurnal variations, nor were they affected by meal intake or defined glucose load during oral glucose tolerance testing. Irisin levels decreased with age. In adults, irisin levels were higher in men than in women, and obese subjects had significantly higher levels than lean control subjects. Irisin levels were closely correlated with muscle-associated bioimpedance parameters such as fat-free mass and body cell mass. Of the 4 exercise regimens that differed in duration and intensity, we identified a clear and immediate increase in serum irisin levels after acute strenuous exercise (cycling ergometry) and a 30-minute bout of intensive exercise in children and young adults, whereas longer (6 weeks) or chronic (1 year) increases in physical activity did not affect irisin levels.'] Are serum irisin levels regulated by acute strenuous exercise?