FNDC5 and osteoporosis: The results showed that the body weight (Fig. 6A), serum Ca2+ (Fig. 6B), serum 25 (OH) D (Fig. 6C), serum osteocalcin (Fig. 6D) in the osteoporosis (OP) group were all lower than that in Healthy group and Irisin treatment (Irisin) group, but the level of DPN in urine in OP group was significantly higher in OP group than that in Healthy group and Irisin group (P < 0.001, Fig. 6E).