INSR and obesity due to melanocortin 4 receptor deficiency: Several processes contribute to diet-induced, obesity-associated ghrelin reduction, including increased engagement of insulin receptors expressed by ghrelin cells, reduced sensitivity of ghrelin cells to the ghrelin secretagogue norepinephrine, increased activation of ghrelin cell–expressed fatty acid receptors, and/or changed numbers of ghrelin cells (39, 45–48).