Preliminary findings on salivary and plasma irisin concentrations following high-volume resistance Exercise

INTRODUCTION: Irisin is a myokine mainly released in response to exercise, playing a role in energy expenditure, white adipose tissue browning, and metabolic regulation (1). While it is widely recognized that aerobic exercise consistently increases plasma irisin levels (2), the response to resistance exercise remains inconclusive. Some studies report increased irisin levels following resistance exercise, while others show no change or a decrease (3, 4). Additionally, irisin analysis is usually assessed in blood samples, which are invasive and may not always be practiced in some settings. Saliva sampling offers a non-invasive alternative, but limited research has evaluated its reliability for irisin detection to date (5). This study aimed to investigate acute irisin responses in both plasma and saliva following a high-volume resistance training session. METHODS: Seven healthy, resistance-trained men (23.5±2.5 years; training experience: 5±3 years) were enrolled. The study involved three preliminary test sessions (10RM, TUT, 1RM) and one experimental training session (TS). The TS consisted of 30 sets performed to failure, with a time under tension (TUT) of 5-1-2-1, emphasizing eccentric movements. Blood and saliva samples were collected at baseline (T0), 15 minutes (T1), 24 hours (T2), and 48 hours post-exercise (T3). Plasma and salivary irisin levels were measured by ELISA, while plasma creatine kinase (CK) and visual analogue scale (VAS) levels were analyzed as indirect markers of muscle damage. Nonparametric statistical tests (Friedman and Pearson correlation) were applied, with significance set at p<0.05. RESULTS: Plasma irisin levels significantly increased between T0 and T1 (10.44 ± 0.9 to 11.38 ± 1.4 ng/mL, p<0.05). Similarly, salivary irisin levels rose significantly from 0.051 ± 0.006 to 0.053 ± 0.008 ng/mL (p<0.05). CK values peaked at T2, confirming markedly exercise-induced muscle damage (p<0.001). Also, VAS values peaked at T2 to then decrease at T3, suggesting that muscle soreness was higher 24 h after exercise. A significant correlation was observed between percentage changes in plasma and salivary irisin (ρ= 0.86, p<0.05), suggesting a potential link between the two biological fluids in irisin release kinetics. CONCLUSION: This pilot study is the first to suggest a concurrent increase of plasma and salivary irisin following highvolume resistance exercise in trained men. The correlation between plasma and salivary irisin suggests that saliva sampling may serve as a viable and non-invasive method for assessing irisin responses to resistance exercise. Future research should investigate the increase of salivary irisin concentration in larger cohorts and different training modalities.