Irisin is a myokine that is thought to be secreted in response to
exercise that may help to prevent obesity and maintain normal glucose
metabolism. In this study we investigated the associations between
irisin and glucose homeostasis in middle-aged, overweight and obese men (n =  144)
with impaired glucose regulation, and the impact of exercise training
on these relationships. The participants underwent 12 weeks of
resistance or aerobic (Nordic walking) exercise training three times per
week, 60 minutes per session. Venous blood (n = 105) and skeletal muscle samples (n = 45)
were obtained at baseline and post-intervention. Compared to controls,
Nordic walking, but not resistance training, increased irisin levels in
plasma (9.6 ± 4.2%, P = 0.014; 8.7 ±  4.9%, P = 0.087;
respectively) compared to controls. When considering all subjects,
baseline irisin correlated positively with atherogenic index of plasma (r = 0.244, P = 0.013) and 2-hour insulin levels (r = 0.214, P = 0.028), and negatively with age (r = −0.262, P = 0.007), adiponectin (r = −0.240, P = 0.014) and McAuley index (r = −0.259, P = 0.008). Training-induced FNDC5 mRNA changes were negatively correlated with HbA1c (r = −0.527, P = 0.030)
in the resistance training group and with chemerin in the Nordic
walking group (r = −0.615, P = 0.033). In conclusion, 12-weeks of Nordic
walking was more effective than resistance training in elevating plasma
irisin, in middle-aged men with impaired glucose tolerance. Thus, the
change in irisin in response to exercise training varied by the type of
exercise but showed limited association with improvements in glucose
homeostasis.