Association of Circulating Irisin with Adiposity, Sarcopenia-Related Indices, and Phenotypes in Premenopausal and Postmenopausal Women

Background: Irisin is an exercise-induced myokine also produced by adipose tissue and thus considered an adipo-myokine. Its relationship with adiposity and sarcopenia-related indices remains unclear. We examined these associations and compared circulating irisin concentrations across operationally defined phenotypes in premenopausal and postmenopausal women. Methods: In this cross-sectional study, 79 women underwent body-composition assessment by BIA and HGS measurement by dynamometry, of whom 78 underwent serum irisin measurement by ELISA. Participants were classified into four operational phenotypes (Control, Obesity, Sarcopenia, and Sarcopenic Obesity) using BMI ≥ 30 kg/m2 for obesity and HGS < 16 kg for low muscle strength. Irisin was compared across phenotypes and correlated with adiposity and sarcopenia-related indices. Results: Circulating irisin was significantly lower in women with BMI-defined obesity than in those without obesity (173.70 [89.03–357.20] vs. 347.30 [188.90–716.00] pg/mL; p = 0.0024) but did not differ when obesity was defined by %BF (≥35%; p = 0.1034). Overall, irisin did not differ by menopausal status. Among postmenopausal women, irisin concentrations were lower in those with obesity than in those without obesity (179.70 [85.30–321.90] vs. 403.60 [219.20–801.70] pg/mL; p = 0.0043), whereas no significant difference was observed among premenopausal women (p = 0.2666). Across the operational phenotypes, irisin was lowest in the sarcopenic obesity group (128.90 [68.57–321.90] pg/mL) and significantly lower than in controls (322.40 [164.10–498.40] pg/mL; p = 0.0076). Irisin was not significantly associated with HGS, ASM, or MQI, whereas ASMI showed an inverse association (r = −0.2773, p = 0.0153). Significant inverse associations were observed with FMI (r = −0.3835, p = 0.0006), BMI (r = −0.3609, p = 0.0013), and %BF (r = −0.3396, p = 0.0025). In exploratory univariable regression analyses, BMI, FMI, and %BF accounted for 15.9%, 15.2%, and 8.4% of the variance in circulating irisin, respectively. Conclusions: Circulating irisin showed consistent inverse associations with adiposity indices but a less consistent pattern across muscle-related indices, suggesting that adiposity may be particularly relevant to circulating irisin in women.