FGF21 and obesity due to melanocortin 4 receptor deficiency: Many previous reports have confirmed that FGF21 is highly expressed in patients with chronic metabolic diseases such as obesity, diabetes, coronary heart disease, etc. The explanation of this paradox is that in these diseases, the human body has resistance to FGF21, and the sensitivity is reduced, which leads to the high compensatory increase of FGF21 in the circulation (34).