*Article* **Associations of Circulating Irisin and Fibroblast Growth Factor-21 Levels with Measures of Energy Homeostasis in Highly Trained Adolescent Rhythmic Gymnasts**

**Jaak Jürimäe 1,\*, Liina Remmel 1, Anna-Liisa Tamm 2, Priit Purge 1, Katre Maasalu <sup>3</sup> and Vallo Tillmann <sup>3</sup>**


**Abstract:** The aim of this investigation was to determine the associations of serum irisin and fibroblast growth factor-21 (FGF-21) with the measures of energy homeostasis, training stress and other energy homeostasis hormones in highly trained adolescent rhythmic gymnasts (RG). Thirty-three RG and 20 untrained controls (UC) aged 14–18 years participated in this study. Body composition, resting energy expenditure (REE), peak oxygen consumption, and different energy homeostasis hormones in serum, including irisin, FGF-21, leptin, and resistin, were measured. Irisin and FGF-21 were not significantly different (*p* > 0.05) between RG and UC groups. In RG, serum irisin was positively associated with REE (*r* = 0.40; *p* = 0.021) and leptin (*r* = 0.60; *p* = 0.013), while serum FGF-21 was related to body fat mass (*r* = 0.46; *p* = 0.007) and leptin (*r* = 0.45; *p* = 0.009). Irisin was related to FGF-21, independent of age, body fat, and lean masses (*r* = 0.36; *p* = 0.049) in RG. In conclusion, serum irisin concentration was associated with energy expenditure and serum FGF-21 level with energy availability measures in lean adolescent athletes, while no relationships of irisin and FGF-21 with energy status measures were observed in lean nonathletic adolescents.

**Keywords:** rhythmic gymnasts; irisin; fibroblast growth factor-21; energy homeostasis; training stress
