FNDC5 and Osteopenia: At baseline, the serum irisin levels were significantly lowered in patients with osteoporosis and osteopenia in contrast to the controls (159.68 ± 41.08 vs. 184.37 ± 51.20 vs. 422.13 ± 95.22, p < 0.001), and additional within-group comparisons revealed no differences among the osteopenia and osteoporosis groups (159.68 ± 41.08 vs. 184.37 ± 51.20, p = 0.267) (Figure 2).