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, VO(2) 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 VO(2) 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 VO(2) 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 VO(2) 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.).