The World Health Organisation (1994) recognises five categories of diabetes and in addition lists a sixth condition. These different types are listed below:

Type 1 diabetes mellitus occurs in about 20% of all diabetic patients.

This is characterised by insulin deficiency resulting from immune- mediated pancreatic beta-cell destruction. Type 1 diabetes results from the destruction of the pancreatic beta-cells, through an autoimmune process of unknown aetiology, leading to insulin deficiency and associated ketoacidosis. Pancreatic beta-cell destruction eventually results in absolute insulin deficiency. The pathological process occurs in genetically predisposed individuals over many years and appears to be triggered by environmental factors such as viral infections and nutritional factors such as ingestion of cow's milk in early childhood.

Type 2 diabetes mellitus occurs in about 75% of all diabetic patients.

This is generally characterised by peripheral insulin resistance and relative insulin deficiency which may range from predominant insulin secretory defect with insulin resistance. Some patients develop severe insulin deficiency. Obesity is common in patients with type 2 diabetes and itself causes insulin resistance. Peripheral plasma insulin levels are usually high with relative insulin deficiency being characterised by a delayed initial first phase insulin response with the second phase insulin response being weakened over several years. Resistance to the action of insulin takes the form of a decrease in the ability of skeletal muscle to store glucose (due to a reduction in activity of the enzyme glycogen synthase) and oxidise glucose (due to a reduction in pyruvate dehydrogenase activity). The risk of developing type 2 diabetes increases with age, obesity (particularly central obesity), family history of diabetes or cardiovascular disease(particularly hypertension or dyslipidaemia), and a lack of physical activity.

The other forms of diabetes account for 5% of all diabetic patients.

These are:

1. Malnutrition related diabetes mellitus(MRDM)

2. Diabetes associated with other conditions and syndromes:

• Pancreatic disease

• Disease of hormonal aetiology

• Drug-induced or chemical-induced diabetes

• Abnormalities of insulin or its receptors

• Certain genetic syndromes

• Miscellaneous

3. Gestational diabetes

4. Impaired Glucose Tolerance(IGT)