FNDC5 and anorexia nervosa: In particular study groups—especially in those with overweight and obese participants [31], and also across the groups with a wide BMI spectrum, including both anorexia nervosa and extremely adipose patients—irisin levels directly correlated with body mass, BMI, fat, and fat-free mass [25,26], indicating possible modification of irisin secretion from muscle and fat tissue when body metabolism and tissue proportions are changed.