FNDC5 and type 2 diabetes mellitus: Among patients with T2DM, the circulating irisin level of those with OP was lower than that of normal BMD (9.98 ± 2.09 vs. 11.39 ± 1.57 ng/ml, P < 0.01); irisin had a negative correlation with β-C-terminal telopeptides of type I collagen (β-CTX) (r = −0.496, P < 0.001) and came back unrelated to Lumbar BMD; Lumbar BMD was negatively relevant to OC (r = −0.274, P < 0.05) and β-CTX (r = −0.410, P < 0.01).