Diabetes mellitus is a chronic metabolic disorder caused by insufficient activity of the hormone insulin and a subsequent loss of control of blood glucose levels. There may be a lack of the hormone itself or resistance to its action. Insulin is produced by the beta cells of the pancreas in response to rising blood glucose levels and mainly regulates the metabolism of carbohydrates, but also of proteins and fats. There are two types of diabetes mellitus. Type 1 diabetes mellitus is caused by the destruction of insulin-producing cells, leading to an absolute lack of the hormone. Type 2 diabetes mellitus is characterised by insulin resistance and is often associated with obesity. In type 2 diabetes mellitus, the pancreas initially responds by increasing insulin production, but over time this excess production cannot be maintained, leading to a decrease in insulin production and a lack of insulin. Both types of diabetes mellitus are characterised by hyperglycaemia – an elevation of blood glucose levels above normal.