Studies showing that homozygous GIP receptor knockout (KO) mice (GIPRKO) were resistant to obesity when fed a HF diet [22] and the presence of K-cell hyperplasia and elevated GIP and insulin levels in HF fed rodents, led to the suggestion that GIP may contribute to the development of obesity, with associated insulin resistance and glucose intolerance [23]–[24]. This evidence concerns the gene INS and obesity due to melanocortin 4 receptor deficiency.