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Nathan C. Winn, Ying Liu, Timothy D. Heden, Lauryn M. Nyhoff, Leryn J. Boyle, and Jill A. Kanaley FACSM University of Missouri, Columbia, MO

Irisin is a recently discovered myokine that has been proposed to be secreted into circulation by skeletal muscle in response to exercise. In rodents, irisin activates thermogenic programs in white adipose tissue and improves glucose homeostasis. It is unknown how acute and chronic aerobic exercise alters circulating levels of irisin in obese humans. PURPOSE: We sought to determine whether circulating irisin is exercise-responsive in obese humans. METHODS: A) Eleven obese females (37.3 + 2.1 kg/m2) completed, in random order, a single bout of continuous moderate exercise (CME; 55 min, 55% VO2peak) and high-intensity interval exercise (HIIE; 4 x 4 min intervals at 85%VO2peak separated by 4 x 3 min active recovery periods at 50% VO2peak) at least 7 days apart. Plasma irisin concentrations were measured at 3 time intervals (rest t=0 min, during exercise t=30 min, and post-exercise t=100 min). B) Pre- and post- exercise training irisin concentrations were measured in 13 and 6 obese adults (34.6 + 3.02 kg/m2 and 34.55 + 3.03kg/m2) completing 15 d and 12 wk of aerobic exercise training. RESULTS: Body weight and resting blood chemistry did not differ between CME and HIIE training days (p >0 .05). A two-way repeated measures ANOVA did not reveal a significant interaction or main effect of exercise intensity (p > 0.05) on circulating irisin levels. Irisin concentrations were not different across time within CME (t= 0, 30, 100 min; 2.14 + 0.08, 2.02 + 0.19, 2.21 + 0.13 μg/ml, p >0.05) or HIIE (t= 0, 30, 100 min; 2.04 + 0.16, 2.21 + 0.11, 2.08 + 0.14 μg/ml, p >0.05), respectively. Chronic exercise training had no effect on plasma irisin (p > 0.05). In addition, no significant associations were made between plasma irisin and metabolic parameters. CONCLUSION: To our knowledge these are the first data that have directly measured plasma irisin during and after acute exercise of differing intensities in obese humans. Our novel findings suggest that circulating irisin is not responsive to acute or chronic aerobic exercise in obese adults. Future studies are needed to elucidate the mechanisms underlying the physiological effects of irisin levels in humans

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