FNDC5 and vitamin D deficiency: Between-group comparisons, correlation analyses with false discovery rate (FDR) correction, and multivariable regression analyses were performed.<h4>Results</h4>Children with vitamin D deficiency exhibited significantly lower median circulating levels of irisin (13.76 vs. 29.52 ng/mL), myostatin (363 vs. 1108 ng/L), and myonectin (2.24 vs. 5.28 ng/mL) compared with controls (all p < 0.05).