‘Appetite, fat, and obesity’ considers the role of insulin in the body’s fat storage process. In the condition diabetes mellitus, the sugar level in the blood rises uncontrollably so the kidneys cannot stop it leaking into the urine. Type 1 diabetes is an autoimmune disease in which a person’s immune system attacks the insulin-secreting β-cells of the pancreas. The only effective treatment is to inject the missing insulin. Type 2 diabetes is considered more of a lifestyle disease. But what exactly is the connection between obesity and poorly regulated blood glucose? The body has more hormones capable of responding to energy shortage than to energy abundance, which may be why keeping control of body weight can be difficult. The hormones affecting appetite are also discussed.