GIP and obesity due to melanocortin 4 receptor deficiency: However, it is well established that circulating levels of GIP are elevated in human obesity and obesity-driven forms of T2DM (Ebert & Creutzfeldt 1980, Salera et al. 1982), with studies using obese rodents reporting expansion of intestinal K-cell mass and elevated circulating GIP in genetically inherited obesity (Flatt et al. 1983) and following prolonged exposure to a high-fat, high-calorie diet (Bailey et al. 1986).