Plasma and Salivary Irisin Response to Resistance Training: A Comparative Study

Irisin is a myokine released in response to physical exercise that enhances energy expenditure, the browning of white adipose tissue, bone homeostasis, muscle hypertrophy, and the regulation of glucose and lipid metabolism1. Existing literature clearly indicates that aerobic exercise leads to increased plasma irisin levels2; instead, the effects of resistance exercise remain unclear. Some studies suggest that resistance exercise increases plasma irisin levels, while others report no change or a decrease3-4. Furthermore, irisin is typically analysed in blood samples, which is invasive and poses potential risks. The substitution of saliva samples for blood would represent a less invasive method for irisin detection; however, only a limited number of studies have evaluated irisin levels in saliva5. This preliminary study aimed to investigate the acute release of plasma and saliva irisin in response to resistance exercise. A total of seven healthy, trained men (age: 23.5 ± 2.5 years; training experience: 5 ± 3 years) were recruited. The protocol included three test sessions (10RM, TUT, 1RM) and one experimental training session (TS). TS sets (n.30) were carried out to muscular failure, with a time under tension (TUT) of 5-1-2-1, emphasising the eccentric phase of the movements. Blood and saliva samples were collected at baseline (T0), 15 minutes (T1), 24 hours (T2), and 48 hours (T4) post-exercise. Plasma samples were used to evaluate irisin and CK levels, while salivary samples were only used for irisin detection, assessed using an ELISA Assay Kit (#EK-067-29). Both plasma and salivary irisin levels demonstrated a significant increase in response to TS between T0 vs T1 (plasma 10.44 ± 0.9 to 11,38 ± 1,4 ng/mg, p:0.02*; saliva 0.051 ± 0.006 to 0.053 ± 0.008 ng/ml, 0.021, p:0.02*). CK values revealed significant differences between T0 Vs T1 (130.2 ± 27.9 to 295.4 ± 111.9 U/L, p:<0.001***), T0 vs T2 (406.4 ± 160.8 U/L, p:<0.001***), T0 vs T3 (248.4 ± 93.2 U/L, p:<0.001***); T1 vs T3 p:0.01**; and T2 vs T3 p:0.006**. A significant correlation was found between percentage changes in plasma vs saliva irisin increase between T0 and T1 (plasma 9.6 ± 15.2%, saliva 3.9 ± 4.4%; rho: 0.86, p:0.02*). This preliminary study is the first to explore irisin production following resistance training sessions in both saliva and plasma. The results show a moderate but significant increase for both plasma and saliva irisin. Correlation analysis, although preliminary, suggests that saliva sampling might be a sensitive method for detecting changes in irisin levels in response to resistance training exercises. Further studies with larger sample sizes and additional sampling points are required to comprehensively understand irisin's role in resistance training.