Exercise in the heat induces similar elevations in serum irisin in young and older men despite lower resting irisin concentrations in older adults.

Irisin is thought to play a cytoprotective role during acute stressors, such as exercise, by reducing oxidative stress and inflammation. Relative to young adults, older individuals exhibit an impaired capacity to dissipate heat during exercise, which can exacerbate elevations in oxidative stress and the acute inflammatory response especially in the heat. In turn, this could induce a greater increase in circulating irisin. Thus, we evaluated age-related differences in irisin expression during prolonged exercise in a non-heat stress and high-heat stress environment. Specifically, we assessed serum irisin in 12 young (22 ± 3 years) and 12 older (59 ± 4 years) men before and after 3-h moderate-intensity exercise (metabolic rate: 200 W/m) and 60-min post-exercise recovery in temperate (wet-bulb globe temperature (WBGT) 16 °C) and hot (WBGT 32 °C) environments. Core temperature (T) was measured continuously. Post-exercise T was similarly higher in the hot compared to the temperate condition for both groups (p < 0.001), although T remained elevated at end-recovery in the heat in older but not young adults (p = 0.006). Absolute serum irisin concentrations were significantly higher (p ≤ 0.002) under all conditions in the young relative to older adults. Post-exercise and end-recovery irisin was elevated above baseline in both groups in the hot (+39.3 pg/mL SEM 8 and + 48.9 pg/mL SEM 10, respectively; both p ≤ 0.043) but not the temperate condition. When comparing between conditions, the change in irisin concentrations at post-exercise did not differ, although serum irisin was elevated in the hot (+48.9 pg/mL SEM 10) relative to the temperate (+0.88 pg/mL SEM 0.2) condition in both groups at end-recovery (p = 0.004). Our findings indicate that irisin concentrations were elevated after exercise compared to rest in hot, but not temperate conditions across groups. However, older adults may still have greater cellular vulnerability to heat stress given their blunted circulating irisin levels.