In particular, plasma levels of FGF21 have been shown to increase in parallel with the severity of hepatic steatosis in humans [74,75,76], and a positive correlation has been observed with the severity and progression of MASLD, as well as with obesity, body mass index (BMI), triglycerides, and insulin resistance [53]. This evidence concerns the gene FGF21 and obesity due to melanocortin 4 receptor deficiency.