IL34 and obesity due to melanocortin 4 receptor deficiency: Our main findings are (1) obesity, reflected by elevated fat mass, is strongly associated with VDD, and likely predispose to it, along with age and female sex; (2) VDD was significantly associated with IL-34 levels, suggesting a potential direct link between VD status and IL-34 expression; (3) IL-34 elevation is associated with both OP and KOA and most likely contributes to their etiology.