Insulin stops the use of fat as an energy source by inhibiting the release of glucagon. With the exception of the metabolic disorder diabetes mellitus and metabolic syndrome, insulin is provided within the body in a constant proportion to remove excess glucose from the blood, which otherwise would be toxic. When blood glucose levels fall below a certain level, the body begins to use stored sugar as an energy source through glycogenolysis, which breaks down the glycogen stored in the liver and muscles into glucose, which can then be utilized as an energy source. As a central metabolic control mechanism, its status is also used as a control signal to other body systems (such as amino acid uptake by body cells). In addition, it has several other anabolic effects throughout the body.

When control of insulin levels fails, diabetes mellitus will result. As a consequence, insulin is used medically to treat some forms of diabetes mellitus. Patients with type 1 diabetes depend on external insulin (most commonly injected subcutaneously) for their survival because the hormone is no longer produced internally. Patients with type 2 diabetes are often insulin resistant and, because of such resistance, may suffer from a "relative" insulin deficiency. Some patients with type 2 diabetes may eventually require insulin if other medications fail to control blood glucose levels adequately. Over 40% of those with Type 2 diabetes require insulin as part of their diabetes management plan.

Insulin also influences other body functions, such as vascular compliance and cognition. Once insulin enters the human brain, it enhances learning and memory and benefits verbal memory in particular.^[2] Enhancing brain insulin signaling by means of intranasal insulin administration also enhances the acute thermoregulatory and glucoregulatory response to food intake, suggesting that central nervous insulin contributes to the control of whole-body energy homeostasis in humans.^[3]

Human insulin is a peptide hormone composed of 51 amino acids and has a molecular weight of 5808 Da. It is produced in the islets of Langerhans in the pancreas. The name comes from the Latin insula for "island". Insulin's structure varies slightly between species of animals. Insulin from animal sources differs somewhat in "strength" (in carbohydrate metabolism control effects) in humans because of those variations. Porcine insulin is especially close to the human version.

Insulin glargine, marketed by Sanofi-Aventis under the name Lantus, is a long-acting basal insulin analogue, given once daily to help control the blood sugar level of those with diabetes. It consists of microcrystals that slowly release insulin, giving a long duration of action of 18 to 26 hours, with a "peakless" profile (according to the Lantus package insert). Pharmacokinetically, it resembles basal insulin secretion of non-diabetic pancreatic beta cells. Sometimes, in type 2 diabetes and in combination with a short acting sulfonylurea (drugs which stimulate the pancreas to make more insulin), it can offer moderate control of serum glucose levels. In the absence of endogenous insulin—Type 1 diabetes, depleted type two (in some cases) or latent autoimmune diabetes of adults in late stage—Lantus needs the support of fast acting insulin taken with food to reduce the effect of prandially derived glucose.

NPH insulin (or neutral protamine Hagedorn) (also known as Humulin N, Novolin N, Novolin NPH, NPH Iletin II, and isophane insulin), is an intermediate-acting insulin given to help control the blood sugar level of those with diabetes. NPH was created in 1936 when Nordisk formulated "isophane" porcine insulin by adding neutral protamine to regular insulin.

This is a suspension of crystalline zinc insulin combined with the positively charged polypeptide, protamine. When injected subcutaneously, it has an intermediate duration of action, meaning longer than that of regular insulin, and shorter than ultralente, glargine or detemir.