Diabetes mellitus is a metabolic disorder that is characterized primarily by chronic hyperglycemia (excessive sugar (glucose) in the blood) resulting from defects in insulin secretion, insulin action, or both. The symptoms of hyperglycemia include those caused by osmotic diuresis (increased urination because of the presence of certain substances, such as glucose, in the kidney tubules) and the resultant loss of calories (including polyuria (passage of copious amounts of urine), polydipsia (excessive thirst), weight loss, and polyphagia (excessive eating)); blurring of vision caused by fluctuations in the diameter of the ocular lens; and increased susceptibility to infections, particularly urinary tract infections and skin infections.

Severe life-threatening acute metabolic complications include diabetic ketoacidosis (excessive amounts of acidic ketones in the blood) and hyperosmolar nonketotic state (a dangerous condition caused by very high levels of blood sugar and dehydration). Chronic effects of poorly controlled diabetes include microvascular complications that affect the eyes (retinopathy, blindness), kidneys (nephropathy, renal failure), and nerves (neuropathy, peripheral and autonomic), as well as macrovascular complications that affect the cardiovascular system, increasing the risk of heart attacks and strokes fourfold compared to the nondiabetic population.