Implication of circulating irisin levels with brown adipose tissue and sarcopenia in humans

Context: Irisin is an exercise-induced novel myokine that drives brown-fat-like conversion of white adipose tissue and has been suggested to be a promising target for the treatment of obesity-related metabolic disorders. Objective: To assess the association of circulating irisin concentrations with brown adipose tissue (BAT) and/or sarcopenia in humans. SettingandDesign: Weexaminedirisin levels in40BAT-positiveand40BAT-negativewomendetected by 18F-fluorodeoxyglucose positron emission tomography (18FDG-PET). In a separate study, we also examined 401 subjects with or without sarcopenia defined by skeletal muscle mass index (SMMI) and appendicular skeletal muscle (ASM)/height2 using dual-energy x-ray absorptiometry. Results:Among6877 consecutive 18FDG-PET scans in 4736 subjects, 146 subjects (3.1%) had positive BAT scans. The BAT-detectable group and the matched BAT-undetectable group did not differ in circulating irisin levels measured using two different ELISA kits (P=.747 and P=.160, respectively). Serum irisin levels were not different between individuals with sarcopenia and those without sarcopenia using either kit (P = .305 and P = .569, respectively). Also, serum irisin levels were not different between groups defined by ASM/height2 using either kit (P = .352 and P = .134, respectively). Although visceral fat area and skeletal muscle mass showed significant difference according to tertiles of SMMI levels, irisin concentrations did not differ. Conclusions: Circulating irisin levels were not different in individuals with detectable BAT or those with sarcopenia compared with control subjects and were not correlated with SMMI.