Background: Existing research suggests that irisin increases in response to exercise in humans. However, results are controversial and a majority of the studies lack a control group. The present study aimed to analyze the effects of both one-bout, and three-week intense resistance training on physical fitness (body composition and strength) and serum irisin levels when compared to a control group. Methods: A total of 26 healthy young adults (n = 14 males; 12 females) completed the pre-assessment phase, and were randomly assigned to either intervention or control group. Physical activity, diet, and physical fitness (strength, cardiorespiratory fitness, and body composition) were assessed. Blood samples were collected at baseline, during and post one-bout of exercise, and baseline on sessions 1, 3, 6 and 9 of 3-week high intensity resistance training (3 times per week). Results: None of the ANOVA effects on irisin concentration were significant after one-bout of exercise or 3 weeks of resistance training. The intervention group showed large significant changes from pre to post in relative body fat (%BF) (t (13) =-3.37, P = 0.003), and lean body mass (P = 0.016, d = 0.72). All muscle strength variables (1RM bench press (F (1,22) = 19.54, P < 0.001, η 2 G < 0.01); 1RM leg press (F (1,22) = 20.84, P < 0.001, η 2 G = 0.03); bench press-to-body weight ratio (F (1,22) = 18.93, P < 0.001, η 2 G = 0.01); leg press-to-body weight ratio (F (1,22) = 23.03, P < 0.001, η 2 G = 0.05)) showed significant group by time interaction effects. Conclusions: Serum irisin concentrations did not change during or post one-bout, nor during three-weeks of high-intensity resistance training compared to matched controls, while the program elicited significant changes in body composition and muscle strength in a group of healthy young adults. Only irisin levels in the control group were significantly increased. Additionally, no significant associations were found between irisin levels and physical activity, diet, or physical fitness. However, negative associations were found between baseline serum irisin concentrations and body composition (body weight and skeletal muscle mass) in males.
