Diabetes mellitus results mainly from a deficiency or diminished effectiveness of insulin that is normally produced by the beta cells of the pancreas. It is characterised by high blood sugar, altered sugar and glucose metabolism and this affects blood vessels and causes several organ damage. Causes of diabetes can be classified according to the types of diabetes.
Type 1 diabetes mellitus
This results from the body's failure to produce sufficient insulin. Here the pancreatic beta cells are irreversibly damaged and so they cannot produce adequate insulin. This is believed to be due to an over active immune system that instead of fighting foreign microbes turns on the body’s own cells and begins to destroy the pancreatic cells.
Since type 1 diabetes has been found in both identical twins in studies, four genes are thought to be important. One (6q) determines the sensitivity of the islet cells of pancreas to damage. This damage could be due to viruses or cross-reactivity from cow's milk-induced antibodies.
In addition, associations with HLA DR3 and DR4 and islet cell antibodies around the time of diagnosis have been noted. Risks of developing type 1 diabetes are similar in all ethnic groups. This could be due to diet during childhood or due to genes.
Type 2 diabetes mellitus
Type 2 diabetes mellitus results from a resistance to the insulin. There may be a normal or increased level of insulin initially. The pancreatic beta cells try to secrete more insulin initially to meet the raised demands of the body. When it fails, type 2 diabetes develops.
Risks for type 2 diabetes mellitus include excess body weight and physical inactivity. All racial groups are affected but increased prevalence in people of South Asian, African, African-Caribbean, Polynesian, Middle-Eastern and American-Indian ancestry is noted.
Other risk factors for type 2 diabetes include history of gestational diabetes, impaired glucose tolerance, impaired fasting glucose, drug use like thiazide diuretic along with a beta-blocker, low-fibre, high-glycaemic index diet, metabolic syndrome, Polycystic ovarian syndrome, family history and those who have a history of a low birth weight.
Gestational or pregnancy associated diabetes
Pregnant women who have never had diabetes before may develop increased demands for insulin during pregnancy. This may not be met by a raised insulin secretion and gestational diabetes results. This affects 4 to 5% of all pregnant women. It may precede development of type 2 (or rarely type 1) diabetes.
Maturity onset diabetes of the young
This is a combination of several forms of diabetes all resulting from a single genetic defect affecting the beta-cell function resulting in impaired insulin secretion. There may be slight high blood sugar at a young age. This genetic defect is usually inherited in an autosomal-dominant manner.
Secondary diabetes occurs due to a disease affecting the pancreas or other endocrine organs. This accounts for 1 to 2% of all diabetics. Some of the causes of secondary diabetes include:-
- Diseases of the pancreas that may affect the beta cells – this includes cystic fibrosis, chronic pancreatitis, after surgical removal of the pancreas or due to pancreas cancer.
- Diseases of the hormonal system of endocrine system – Cushing’s syndrome (affecting adrenal glands), acromegaly (affecting the pituitary gland), thryrotoxicosis (excessive activity of the thyroid gland), peochromocytoma (affecting adrenal glands), glucagonoma (affecting glucagon producing cells of the pancreas).
- Due to intake of certain drugs over long term - this includes water pills or diuretics like thiazides, corticosteroids, atypical antipsychotics, protease inhibitors used in HIV infection.
- Patients with Congenital lipodystrophy, Acanthosis nigricans etc.
- Those with genetic conditions like Wolfram syndrome also known as DIDMOAD standing for diabetes insipidus, diabetes mellitus, optic atrophy and deafness. Other genetic conditions predisposing to diabetes include Friedreich's ataxia, dystrophia myotonica, haemochromatosis, glycogen storage diseases etc.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)