FGF21 and obesity due to melanocortin 4 receptor deficiency: Our previous studies showed that mice lacking the mitochondrial fusion protein optic atrophy 1 (OPA1 BKO) in brown adipose tissue (BAT) have high metabolic rates and are resistant to diet‐induced obesity (DIO) via effects partially mediated by independent actions of fibroblast growth factor 21 (FGF21) and growth differentiation factor 15 (GDF15) secretion from BAT.