Low BMD in AN is considered multifactorial and related to lower BMI, lower lean mass [5, 25], hypogonadism [5, 26], Growth Hormone resistance and low IGF-1 [27], high cortisol levels, high PYY levels [28], and low levels of leptin, oxytocin [29], insulin and amylin [30]. The gene discussed is PYY; the disease is hypogonadism.