FGF21 and obesity due to melanocortin 4 receptor deficiency: FGF-21 was found to reduce blood glucose during nutrient sufficiency (for example, obesity or during feeding) (Xu et al., 2009a; Berglund et al., 2009; Xu et al., 2009b; Li et al., 2009; Ding et al., 2012; Holland et al., 2013; BonDurant et al., 2017), whereas it increased blood glucose during nutrient-deficient periods (for example, fasting) (Potthoff et al., 2009; Liang et al., 2014).