Plasma irisin in runners and nonrunners: no favorable metabolic associations in humans

Irisin is a hormone which mimics the favorable metabolic effects associated with regular exercise, by converting subcutaneous white fat into brownish fat, in rodents. Thirty-three human subjects (16 runners, 17 nonrunners) were measured for: resting energy expenditure (REE), body composition, VO2 Peak test, [irisin]p, and plasma metabolic profile. Nine female nonrunners then participated in a 10-week supervised 5 km training program and tested after the race. Two runners underwent (18)F-FDG-PET scans to quantify brown fat. No gender or age (28 ± 10 years) differences noted between matched cohorts. Runners averaged 58 ± 26 miles/week for 13 ± 6 years and had lower bodyweight (63 vs. 88 kg; P < 0.001), BMI (21 vs. 30 kg/m(2); P < 0.0001), triglycerides (58 vs. 123 mg/dL; P < 0.01), total (white) fat (14 vs. 32%; P < 0.0001), and had higher VO2 Peak (63 vs. 34 mL/kg-min; P < 0.0001) and HDL (65 vs. 48 mg/dL; P < 0.01) compared with nonrunners. [Irisin]p was lower in runners versus nonrunners both before (179 vs. 197 ng/mL; NS) and after (207 vs. 226 ng/mL; NS) the VO2 Peak test. Significant (P < 0.05) positive correlations were noted between [irisin]p versus BMI (r(2) = 0.15), triglycerides (r(2) = 0.40), and total body fat(g) (r(2) = 0.24) with a significant negative correlation between [irisin]p versus respiratory quotient (r(2) = 0.33). Total lean mass significantly correlated with REE (r(2) = 0.58) while total fat mass inversely correlated with VO2 Peak (r(2) = 0.64). Nonrunners had lower [irisin]p after completion of the training program (194 vs.181 ng/mL; pre- to post-training; P > 0.05). Neither runner selected for (18)F-FDG-PET scans had brown fat. Runners demonstrated significantly healthier metabolic and body composition profiles compared with nonrunners. None of these favorable exercise effects were positively associated with [irisin]p..