FGF21 and obesity due to melanocortin 4 receptor deficiency: The insulin-sensitizing effects of FGF21 [23] correspond well to the findings of this study, i.e., (i) the positive correlation between FGF21 and the Matsuda index in the bariatric cohort, (ii) the counter-regulatory up-regulation in pregnant women with obesity, possibly due to the high risk for GDM and the ensuing surplus of insulin-resistance boosting factors [7], and (iii) the down-regulation in the normal-weight controls, potentially to further enforce the physiological late-gestational insulin resistance [24].