Diabetes mellitus is a clinical syndrome characterised by an increase in plasma blood glucose (hyperglycaemia). Diabetes has many causes (see Box 21.5, p. 807) but is most commonly due to type 1 or type 2 diabetes. Type 1 diabetes is caused by autoimmune destruction of insulin-producing cells (β cells) in the pancreas, resulting in absolute insulin deficiency, whereas type 2 diabetes is characterised by resistance to the action of insulin and an inability to produce sufficient insulin to overcome this ‘insulin resistance’. Hyperglycaemia results in both acute and long-term problems. Acutely, high glucose and lack of insulin can result in marked symptoms, metabolic decompensation and hospitalisation. Chronic hyperglycaemia is responsible for diabetes-specific ‘microvascular’ complications affecting the eyes (retinopathy), kidneys (nephropathy) and feet (neuropathy). There is a continuous distribution of blood glucose in the population, with no clear division between people with normal and abnormal values. The diagnostic criteria for diabetes (a fasting plasma glucose ≥ 7.0 mmol/L (126 mg/dL) or glucose 2 hours after an oral glucose challenge ≥ 11.1 mmol/L (200 mg/dL); see p. 807) have been selected to identify those who have a degree of hyperglycaemia which, if untreated, carries a significant risk of microvascular disease, and in particular diabetic retinopathy. Less severe hyperglycaemia is called ‘impaired glucose tolerance’. This is not associated with a substantial risk of microvascular disease, but is connected with an increased risk of large vessel disease (e.g. atheroma leading to myocardial infarction) and with a greater risk of developing diabetes in future. The incidence of diabetes is rising. Globally, it is estimated that 366 million people had diabetes in 2011 (approximately 8.3% of the world population, or 3 new cases every 10 seconds), and this figure is expected to reach 552 million by 2030. This global pandemic principally involves type 2 diabetes, the prevalence of which varies considerably around the world (Fig. 21.1), being associated with differences in genetic as well as environmental factors such as greater longevity, obesity, unsatisfactory diet, sedentary lifestyle, increasing urbanisation and economic development. A pronounced rise in the prevalence of type 2 diabetes occurs in migrant populations to industrialised countries, as in Asian and AfroCaribbean immigrants to the UK or USA. Type 2 diabetes is now being observed in children and adolescents, particularly in some ethnic groups, such as Hispanics and Afro-Americans. The incidence of type 1 diabetes is also increasing, such that between 1960 and 1996, 3% more children were diagnosed worldwide each year. Type 1 diabetes is generally more common in countries closer to the polar regions. Finland, for instance, has the highest rate of type 1 diagnosis per year at around 40 per 100 000 of the population, whereas in China the incidence is only 0.1 per 100 000 of the population. Type 1 diabetes is most common in Caucasians and more people are diagnosed in the winter months. Diabetes is a major burden upon health-care facilities in all countries. Globally, diabetes caused 4.6 million deaths in 2011, and health-care expenditure attributed to diabetes was estimated to be at least US$465 billion, or 11% of total health-care expenditure.
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