Every time we eat sugary or starchy food, the amount of glucose available to the body rockets. Yet the levels of glucose in the bloodstream are maintained within narrow limits by two key hormones – insulin and glucagon – working to prevent hyperglycaemia (abnormally high glucose levels) or hypoglycaemia (low glucose).
Both insulin and glucagon are released by the pancreas, a long, tapered gland that lies behind the stomach. Most of the pancreas produces digestive enzymes, which travel via ducts into the small intestine. But embedded in this tissue are nests of hormone-producing cells – the islets of Langerhans – which secrete insulin and glucagon into a network of surrounding blood vessels.
High blood sugar levels stimulate the release of insulin (produced by the beta cells in the islets), which increases the uptake of glucose by cells. Inside the cells, the glucose may be used as energy, converted to glycogen for storage (mainly in the liver and muscles), or used in the production of fats.
Glucagon is produced by the alpha cells in the islets, and is released by the pancreas when blood glucose is low. It stimulates the breakdown of stored glycogen to glucose, which is then released into the bloodstream.
What is diabetes?
Diabetes mellitus arises when insufficient insulin is produced, or when the available insulin does not function correctly. Without insulin, the amount of glucose in the bloodstream is abnormally high, causing unquenchable thirst and frequent urination. The body's inability to store or use glucose causes hunger and weight loss.
There are two main types of diabetes. Insulin-dependent diabetes – type 1 diabetes – occurs when there is a severe lack of insulin due to the destruction of most or all of the beta cells in the islets of Langerhans. This type of diabetes develops rapidly, usually appearing before the age of 35, and most often between the ages of 10 and 16. Regular insulin injections are required to survive.
Non-insulin-dependent diabetes – type 2 diabetes – occurs when the body does not produce enough insulin, and the insulin that is produced becomes less effective. This type of diabetes usually appears in people over the age of 40, and tends to have a more gradual onset. In most cases, glucose levels in the blood can be controlled by diet, or diet and tablets, although sometimes insulin injections may be needed. About 90 per cent of diabetics are non-insulin dependent.
What causes diabetes?
In type 1 diabetes, the insulin-producing beta cells are destroyed by an autoimmune process, whereby the body's immune system – its defence mechanism against disease – for some reason recognises the cells as being 'foreign' rather than 'self', and therefore attacks them.
In susceptible individuals, this autoimmune process is thought to be influenced by environmental factors – which are as yet unknown. Such susceptibility is genetically determined – two genes have been identified that appear to put an individual at risk, but there are certain to be more genes involved.
Type 2 diabetes is thought to be due both to defects in the islet beta cells, so that less glucose is produced, and to an impairment of insulin's ability to stimulate the uptake of glucose in muscles and other tissues. The cause of this insulin resistance has not yet been fully established, but may involve defects in the action of insulin after it has bound to the insulin receptor on the surface of cells. There is a genetic influence, as type 2 diabetes tends to run in families even more strongly than type 1 diabetes, and several genes are likely to be involved. But increasing age, obesity and a sedentary lifestyle also increase the risk of type 2 diabetes.
How is diabetes treated?
Treatments aim to keep the level of blood glucose as normal as possible. For type 1 diabetes, this involves regular injections of insulin, a regulated diet and the careful monitoring of blood glucose levels. As people with type 2 diabetes still produce some insulin, diet alone is often effective, although oral antidiabetic drugs and insulin injections may also be required. Regular exercise is usually recommended as it helps the body to use blood glucose.
What are the long-term complications?
All diabetics are at risk from complications, although the risk is reduced if blood glucose is well controlled. Complications include damage to the retina, kidney and peripheral nerves, while ulcers on the feet can develop into gangrene if untreated. High blood pressure and narrowing of the major arteries also increase the risk of heart disease and strokes.
Who discovered insulin?
The symptoms of diabetes were accurately described by the ancient Egyptians, Hindus, Chinese and Greeks. A firm connection between the pancreas and diabetes was made in 1889, when the German researcher Dr Minowski removed the pancreas from dogs and found that they became diabetic.
In 1921, two Canadians, Frederick Banting and Charles Best, tested pancreatic extracts on de-pancreatised dogs and discovered the active ingredient – insulin.