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.

SETTING AND DESIGN: We examined irisin levels in 40 BAT-positive and 40 BAT-negative women detected by (18)F-fluorodeoxyglucose positron emission tomography ((18)FDG-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)/height(2) using dual-energy x-ray absorptiometry.

RESULTS: Among 6877 consecutive (18)FDG-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/height(2) 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.