The irisin level dispersion was significantly different among the groups of girls (p < 0.001), with the greatest dispersion observed in girls with obesity (95% CI of median [2.29–6.30]), compared with girls with the MS (95% CI of median [5.26–6.65]) and normal weight girls (95% CI of median [7.39–8.80]) (Fig. 2b). Here, FNDC5 is linked to obesity due to melanocortin 4 receptor deficiency.