Diabetes is the most common disease of the endocrine system. There are two main types, Type 1 and Type 2. The overall prevalence among the whole population is between one and two percent.

Type 1 diabetics have destruction of the beta-cells of the pancreas, usually as a result of problems within the immune system. They are usually insulin-deficient, and can develop ketosis.

Type 2 diabetics are the most common in the population. They show insulin-resistance, and eventually also develop a problem with the secretion of insulin.

There is a third class of diabetics which includes those with genetic defects in insulin secretion and action; diseases of the exocrine part of the pancreas; other hormone problems, which induce high blood-sugar; drug-induced types of diabetes; infectious causes of diabetes; and genetic syndromes, which are associated with an increased incidence of diabetes.

Gestational diabetes, is diabetes which arises in pregnancy.

The overall prevalence among the whole population is between one and two percent. However, in certain populations the incidence of diabetes, particularly type 2 diabetes, rises to alarming levels, affecting more than 30 percent of people. In South Africa, Type 2 diabetes is very common in older people.

Although they are regarded as separate diseases, with different causes and treatment, both lead to profound metabolic abnormalities in the body. These abnormalities can lead to long-term complications involving the eyes, kidneys, nerves and blood vessels.

There are several distinct diabetic syndromes which will be discussed separately.

Classification

Diabetes is now classified according to what causes it.

Type 1 diabetes

Immune-mediated
Idiopathic, or of unknown cause

Type 2 diabetes

This may range from predominantly insulin resistance with relative insulin deficiency, to a situation in which the main problem is lack of insulin secretion, with insulin resistance.

Other specific types

Genetic defects of beta-cell function ? this is sometimes called maturity onset diabetes of the young (MODY). It is now know that this encompasses several different chromosome abnormalities.
Genetic defects in insulin secretion ? these are a series of relatively rare genetic diseases.
Diseases of the exocrine (non-hormonal) pancreas ? these include pancreatitis, loss of the pancreas, cancer of the pancreas, and cystic fibrosis.
Other hormone problems ? such as acromegaly, Cushing's syndrome, and hyperthyroidism.
Drug- or chemical-induced diabetes ? which can include diabetes induced by thyroid hormones, thiazide diuretics, dilantin, and steroids.
Infections ? which includes congenital rubella and cytomegalovirus.
Uncommon forms of immune-mediated diabetes ? includes very rare diseases.
Other genetic syndrome sometimes associated with diabetes ? including Down's syndrome, Klinefelter's syndrome, Turner's syndrome, Huntingdon's chorea, porphyria and others.
Gestational diabetes ? which develops in pregnancy, and will be dealt with separately.

Symptoms of diabetes

These are similar for both Type 1 and Type 2.

The most common symptoms are tiredness, excessive thirst, frequent urination, including getting up during the night regularly, and weight loss. These, however, are late symptoms.

There may be more subtle signs which develop before the onset of the above symptoms, particularly in adults who are developing Type 2 diabetes.

Adults with Type 2 diabetes may have recurrent fungal infections of the skin under the breasts, in the groin and under the foreskin in men. They may also develop oral thrush.

Erectile dysfunction in men may be the first sign of diabetes.

High blood pressure may be a sign of diabetes in an older person, since there is a strong link between the two.

What causes Type 1 diabetes?

The pancreas in a normal person produces insulin from what are called the beta cells. These are special cells which lie within the body of the pancreas and respond to certain stimuli, particularly rising levels of glucose and fat in the blood, to produce insulin.

By the time Type 1 diabetes has developed, most of these beta cells have been destroyed. It is thought that this destructive process is autoimmune, that is, the body's own immune system is responsible.

For Type 1 diabetes to arise, the person must have a genetic tendancy towards the illness. Although there are definite clusters of Type 1 diabetes in families, the way in which it is inherited is not clear. There is a definite association between Type 1 diabetes and the HLA-D complex on the sixth chromosome.

In this genetically susceptible individual, there then has to be an environmental event which acts as a trigger. This is thought to be a virus capable of infecting a beta-cell.

There is a definite seasonal variation in the onset of the disease. There is also an abnormally high incidence of people developing Type 1 diabetes after an episode of mumps, hepatitis, infectious mononucleosis (glandular fever), congenital rubella and coxsackievirus infections.

However, this does not mean that onset is always caused by a virus. The evidence is confusing, and other factors certainly come into play.

There is usually an inflammation of the beta-cells, called insulinitis, in those who are developing Type 1 diabetes, but this may not be part of the cause.

The immune system then starts to attack the beta-cells and destroy them, the pancreas can no longer produce enough insulin, and Type 1 diabetes begins.

What causes Type 2 diabetes?

Although there appears to be a link between obesity, high lipid levels and Type 2 diabetes, the exact cause is not known.

The disease does run in families, but the way in which it is inherited is poorly understood. However the relationship is strong. If one of an identical twin develops diabetes, then the other has an almost 100 percent chance of doing so as well. Nearly four-tenths of siblings of those with Type 2 diabetes, and one-third of the children of those with Type 2 diabetes will develop the disease.

People with Type 2 diabetes have two main physiological problems. They do not have normal insulin production, and their bodies are resistant to the insulin they do produce. Ultimately, they also develop a problem with insulin secretion.

The majority of people with Type 2 diabetes are overweight, many to the point of being obese. There is a definite relationship between Type 2 diabetes and the distribution of body fat. The so-called male type fat distribution with a large belly and most fat around the abdomen, called truncal obesity, carries the most risk, not only of Type 2 diabetes, but also of heart disease. This fat distribution is generally associated with high blood lipid levels as well, leading to what is called the plurimetabolic syndrome. This is the combination of truncal obesity, high lipid levels and abnormal glucose metabolism. Many people with this combination of factors then go on to develop full-blown Type 2 diabetes.

However, there are people who develop Type 2 diabetes who are normal weight, have a normal lipid profile and take plenty of exercise. And there are also massively obese people, who take no exercise, who do not develop Type 2 diabetes.

Whatever the true cause of Type 2 diabetes, it is increasing in incidence worldwide. It is common among both developed and developing populations, the latter particularly where urbanisation is occuring.

Diagnosing diabetes

Diabetes is diagnosed quite simply by measuring the level of glucose in the blood. The normal levels are between 3.3 and 5.9 mmol/litre.

The World Health Organisation defines diabetes as being when either:

Fasting blood glucose is greater than or equal to 8 mmol/litre on two separate occassions
Random blood glucose is greater than or equal to 11 mmol/litre

If the blood glucose level is between 8 and 11 mmol/litre, then the person is said to have impaired glucose tolerance. Only a small proportion of people who have impaired glucose tolerance will go on to develop diabetes. It is not possible to predict who will and who will not.