For example, plasma FGF21 levels are elevated in both rodents and humans in response to high carbohydrate levels [4], [5], [6], protein restriction [7], [8], fasting [9], [10], [11], [12], [13], [14], and obesity and insulin resistance [14], [15], [16], [17], [18], [19], [20], [21], [22]. Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.