The Myokine FGF-21 Responds in a Time-Dependent Manner to Three Different Types of Acute Exercise

<b>Background</b>: The myokine response to various types of exercise may differ and influence the adaptations to various physiological systems in response to training. This study aimed to compare systemic myokines' (apelin, interleukin-6 [IL-6], interleu-kin-15 [IL-15], fibroblast-growth factor-21 [FGF-21], and irisin) responses to acute moderate-intensity cardiovascular exercise (MICE), high-intensity interval exercise (HIIE), or resistance exercise (RE). <b>Methods</b>: Six healthy, recreationally active adults (n = 4 males, n = 2 females) completed this crossover pilot study. After baseline testing, in a balanced randomized order, participants completed all three exercise sessions with one week between each of the exercise sessions. Blood samples were obtained at rest, immediately post-exercise, and 1 and 3 h post-exercise. Myokine response was analyzed using a 3 (exercise condition: MICE, HIIE, RE) × 4 (time: baseline, post-exercise, 1 and 3 h post-exercise) repeated-measures ANOVA. <b>Results</b>: Our results showed no significant interaction of time × exercise type in any of the analyzed myokines (all <i>p</i> > 0.05). A significant main effect of time was found for FGF-21, where concentrations at baseline (188.96 ± 127.34 pg/mL; <i>p</i> = 0.038) and immediately post-exercise (206.27 ± 135.95 pg/mL; <i>p</i> = 0.006) were higher than 3 h post-exercise (111.08 ± 127.65 pg/mL). No other main effects for time or exercise type were identified (all <i>p</i> > 0.05). <b>Conclusions</b>: The three exercise types, when analyzed together in this study, demonstrated a reduction in FGF-21 3 h post-exercise, suggesting this myokine was removed from the systemic circulation following exercise. The negative results of this study are inconclusive given the lower statistical power observed in this research. These preliminary results indicate the need for a larger trial to evaluate the effects of different types of exercise on the specificity of myokine responses and how acute exercise responses may translate into long-term exercise training adaptations.