FGF21 and obesity due to melanocortin 4 receptor deficiency: Given the recent evidence indicating that FGF21 plays a role in the regulation of sugar intake, which is likely mediated through a central nervous system mechanism (3,11,12), and prior evidence suggesting that FGF21 is affected by obesity and diet (13,14), we aimed to test the hypotheses that greater circulating FGF21 levels would be negatively associated with brain responses to oral sucrose in regions involved in appetite regulation, and that BMI and dietary added sugar intake would be independently associated with sucrose-induced increases in FGF21 in humans.