Epidemiology of type 1 diabetes
Type 1 diabetes may present at any age, but most typically presents in early life with a peak around the time of puberty. Its incidence varies 50–100-fold around the world, with the highest rates in northern Europe and in individuals of European extraction. Both sexes are equally affected in childhood, but men are more commonly affected in early adult life. The distinction between type 1 and type 2 diabetes becomes blurred in later life, and the true lifetime incidence of the condition is therefore unknown. A variant form known as latent autoimmune diabetes in adults (LADA) has been described. The incidence of childhood type 1 diabetes is rising rapidly in all populations, especially in the under 5-year-old age group, with a doubling time of less than 20 years in Europe. The increasing incidence of type 1 diabetes suggests a major environmental contribution, but the role of specific factors such as viruses remains controversial.
Type 1 diabetes has historically been most prevalent in populations of European origin, but is becoming more frequent in other ethnic groups. Within Europe the highest rates of childhood diabetes are found in Scandinavia and north-west Europe, with an incidence range from 57.4 cases/100,000 per year in Finland to 3.9/100,000 in Macedonia for children aged 0–14 years. Genetically related populations may differ in incidence: for example, type 1 diabetes is more common in Norwegians than in Icelanders of largely Norwegian descent, while Finnish children have a threefold risk compared with Estonians.
New cases of type 1 diabetes according to the IDF The incidence of type 1 diabetes remains relatively low in populations of non-European descent around the world, but many of these now report a rising incidence of the disease. Kuwait, for example, now has an incidence of 22.3/100,000. India and China have relatively low incidence rates for diabetes (4.2 and 0.6/100,000, respectively), but account for a high proportion of the world's children with type 1 diabetes because of their large populations. Statistics for the worldwide incidence of type 1 diabetes in children are available via the IDF Atlas.
The incidence of type 1 diabetes reaches a peak at puberty, and declines rapidly thereafter. Classification of diabetes becomes more problematic in older age groups, and there are therefore no accurate estimates of incidence over the age of 40 years. A subgroup of individuals with develop diabetes in later life with clinical features of type 2 diabetes but test positive for GAD autoantibodies. As a group, these individuals are leaner and more likely to progress to insulin therapy, but there is controversy as to whether they represent a distinct subtype of diabetes or merely the tail-end of the distribution of type 1 diabetes within the population.
An international survey of sex ratios in children presenting under the age of 15 years noted a minor male excess in Europe and populations of European origin, while a female excess was noted in populations of African or Asian origin. High incidence populations are characterised by male excess, and low incidence populations by female excess. In contrast, clear male preponderance has emerged from most studies of patients with type 1 diabetes diagnosed at 15–40 years. Adult type 1 diabetes therefore appears to differ from other common autoimmune diseases, which typically show a strong female excess, as does diabetes in the non-obese diabetic (NOD) mouse.
Evidence of a rising incidence
Childhood-onset diabetes was a rare condition at the start of the 20th century. While many children may have died from undiagnosed diabetes, contemporary reports are consistent in suggesting that western populations had a low and relatively constant rate of the disease over the first half of the century at levels equivalent to those seen today in parts of southern Asia. From the middle of the 20th century, or soon after, a number of populations showed an upturn in incidence that has continued in more or less linear fashion to the present day. The current overall rate of increase in Europe is about 3–4% per annum, and the most rapid increase is seen in the 0–5-year-old age group; the incidence in this age group is expected to double by 2020. There are, however, important regional differences, with signs of a rapid increase in parts of eastern Europe and of a levelling off in high-incidence countries such as Sweden.
What explains the increase?
It has been suggested that the rising incidence of childhood diabetes might represent a left shift in age of onset rather than an absolute increase in the lifetime risk of the disease within a population, and this effect would be consistent with increased penetrance of susceptibility genes in the face of a more permissive environment. Consistent with this, there is some evidence that the increase in the 0–14-year-old age group has been partially compensated by a decrease in young adults, so that the cumulative incidence by age 30 years remains unchanged,  and there is also evidence that the proportion of children with the highest risk HLA haplotypes has declined over time. This, and the speed of the increase within genetically stable populations, strongly suggests environmental influences, but is not convincingly explained by the specific factors currently under consideration.
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