Exercise Intensity Effects on Irisin in Master Athletes and Links With Maximal Oxygen Uptake and Body Fat.

Circulating irisin, a myokine linked to exercise-induced metabolic adaptation, may respond to exercise intensity and serve as a marker of training status. This study assessed intensity-dependent acute irisin responses in master athletes and recreationally trained men, baseline irisin associations with maximal oxygen uptake and body fat, and fibronectin type III domain-containing protein 5-related pathways using bioinformatics. Master endurance athletes (n=11; 49.9±5.8 y) and recreationally trained men (n=10; 49.9±7.2 y) completed two running sessions (moderate and severe), and plasma irisin was measured at rest and 15- and 120-minutes post-exercise. Exercise intensity significantly modulated acute irisin responses (time×intensity, p=0.006), with greater and earlier increases after severe exercise. Master endurance athletes exhibited consistently higher irisin than recreationally trained men (p<0.001), with intensity effects in both groups; baseline irisin was inversely associated with body fat (p=0.018) and positively with maximal oxygen uptake (p<0.001). Complementary RNA-seq analyses showed an age-related decline in skeletal muscle fibronectin type III domain-containing protein 5 expression, preserved by long-term endurance training via upregulated mitochondrial and energy metabolism pathways. Together, these findings indicate that exercise intensity and lifelong training modulate irisin regulation, supporting its use as a biomarker of aerobic fitness, metabolic adaptation, and healthy aging.