FNDC5 and Obesity: Depending on sex, lower concentrations of circulating irisin are present in men with obesity and without chronic degenerative diseases than in women [39, 42, 43], and this puts forward a possible irisin secretory mechanism related to women's own body fat distribution [43] and to possible implications of anabolic hormones such as estradiol, which favors the increase in muscle mass and has been positively associated with irisin in middle-aged women regardless of BMI [14].