FGF21 and obesity due to melanocortin 4 receptor deficiency: The administration of recombinant FGF21 protein to mice fed an ob/ob, db/db, or high-fat diet (HFD), or obese Zucker diabetic fatty (ZDF) rats, has been shown to result in a significant reduction in obesity, lower blood glucose and triglyceride levels, and improved insulin sensitivity (87, 88).