A disease in which glucose (sugar) is not adequately taken up from the bloodstream by the cells of the body and therefore cannot be metabolized to produce energy or stored in the liver. It is caused by an abnormality in the synthesis and secretion of the hormone insulin. Diabetes mellitus is characterized by high concentrations of glucose in the blood (hyperglycaemia) and in the urine, with symptoms of thirst, loss of weight, tiredness, and an excessive production of urine.

People affected with insulin-dependent (or type I) diabetes mellitus have little or no ability to produce the hormone and depend on daily injections of insulin. This type of diabetes can also be referred to as juvenile-onset diabetes because it usually starts during childhood or adolescence. The dosage of insulin must be carefully controlled and matched to the dietary intake of glucose. If cells receive insufficient glucose, fats may be used as an alternative source of energy, which can lead to alteration of the acidity of the blood, the accumulation of ketones (formed when fats are metabolized) in the bloodstream (ketoacidosis), and eventually to diabetic coma. Noninsulin-dependent (or type II or maturity-onset) diabetes mellitus develops in adults, usually over 40 years old, because of a decrease in the production of natural insulin, although the pancreas still functions to some extent; alternatively, the body becomes resistant to the effect of insulin. This condition occurs more often in people who are overweight; it is usually treated by diet, weight reduction, and oral hypoglycaemic drugs, but some patients may need insulin instead.

All people with diabetes, regardless of which type, must pay attention to their diet, in which the amount of carbohydrate should be carefully controlled to suit their body’s needs, and be aware of the importance of monitoring blood glucose concentrations. Very low blood glucose (hypoglycaemia) may be caused by excessive dosage of insulin or some oral antidiabetic drugs or by missing a meal. Good control of blood sugar is necessary to prevent such long-term consequences of diabetes as atherosclerosis (giving rise to heart disease), poor circulation in the extremities (causing foot ulcers), impairment
of eyesight, and kidney damage.