Glucose is used by all body tissue for energy, with the brain as a major consumer of glucose (25% of the total amount of glucose available). The brain and nervous system require a constant source of glucose for energy because they are unable to store glucose or to use fatty acids for fuel. Ingested food provides the body with more glucose than is needed. Approximately 66% of the glucose from each meal is stored in the liver as glycogen and some is converted to fat and stored. Between meals the liver releases glycogen, which is broken down into glucose (glucogenesis) to keep the serum glucose level constant (80-90 mg/dL). The liver can also manufacture glucose from amino acids, glycerol, and lactic acid (gluconeogenesis).

Insulin, produced in stages by pancreatic beta cells in the islets of Langerhans, allows glucose to enter cells for use as an energy source, promotes growth and development, decreases the production of glucose by the liver, increases protein synthesis, halts ketone production, and increases the production of very-low-density lipoproteins by the liver. The lipoproteins then transport triglycerides to fat tissue where they are stored.

During stage one of insulin production, preproinsulin is converted to proinsulin, a molecule composed of 81 amino acids. During stage two the beta cells remove a C-peptide structure from proinsulin to create insulin, which has a 51 amino acid structure. Because C-peptide is removed from each proinsulin molecule to create a unit of insulin, measuring the amount of C-peptide in the blood allows us to measure the amount of insulin produced daily by the beta cells. Approximately 40-50 units of insulin are released by the pancreas on a daily basis. In addition to the daily production, several hundred units are stored and can be released when the serum glucose level rises. Long-term untreated or under treated DM results in severe complications such as renal failure, cardiovascular disease, and blindness.