Unlike many other autoimmune diseases, where females are more at risk of disease, boys and girls under age 15 are diagnosed with type 1 diabetes at relatively equal rates. Some populations with a high incidence tend to have more males than females with type 1, while some with low incidence show more females than males, although this varies among studies. In people of European descent diagnosed at ages 15-40, however, there is a clear male predominance: more men than women are diagnosed with type 1 diabetes at these older ages (Soltesz et al. 2007).
Worldwide, incidence trends generally do not differ between genders-- the incidence of type 1 diabetes is rising in children of both genders (Diamond Project Group 2006).
On average, in children under age 15, type 1 diabetes incidence increases as a child gets older. In other words, a person 10-14 years old has a higher risk of developing type 1 diabetes, someone 5-9 years old has a middle risk, and someone 0-4 years old has a lower risk. Someone 10-14 has about twice the risk of developing type 1 diabetes as someone under 5. This trend generally does not vary by gender (Diamond Project Group 2006).
Overall, and especially in Europe, however, the rates of increase, however, have been highest in children under age 5, with a 4% annual increase in this age group (Diamond Project Group 2006).
A European study from 1989-1994 found that the average annual rate of increase was 6.3% in 0-4 year olds, 3.1% in 5-9 year olds, and 2.4% in 10-14 year olds (EURODIAB ACE Study Group 2000). In the U.S., a Colorado study also found that the increase was highest in the youngest age group, 3.5% annually in 0-4 year olds in this study, which covered a 26 year period (Vehik et al. 2007).
In most areas, the highest rates of increase, then, are seen in the youngest children, which in itself is a matter of real concern.
Most studies of type 1 diabetes only consider children under the age of 15. A recent study that looked at all new cases of diabetes diagnosed during a 3 year period in an area of Sweden, and actually tested people of all ages for antibodies, found that type 1 incidence peaked during ages 0-9 and then again at ages 50-80, showing that disease onset is not limited to children. In fact, nearly 60% of new type 1 cases were diagnosed in people over age 40 (Thunander 2008).
What about changes over time? As discussed above, incidence of type 1 diabetes among children is increasing over much of the world. Yet one study from Belgium makes an interesting point. That study was one of the few that included people over 14 years old, and the researchers found that even though there had been an increase in type 1 diabetes incidence for 0-14 year olds, there was no overall increase in incidence over the 12 year period, because fewer people over 14 were being diagnosed. In 0-4 year olds, the annual increase in incidence was 5%. This finding implies that at least in Belgium, the increasing incidence in type 1 in children may be due to an acceleration of the disease process, but not an overall increase in incidence (Weets et al. 2002). A follow-up study found that the rising incidence in Belgian children was largely due to an increase in incidence in boys under 10, but not girls (unlike most other studies that have not varied by gender, as mentioned above) (Weets et al. 2007).
Another study shows a similar finding: that during the period of 1983-1998 in Sweden, the incidence of type 1 diabetes did not show an overall increase in the 0-34 year age group, but instead, the average age at diagnosis decreased. A shift to younger age at diagnosis seems to explain the increasing incidence of type 1 diabetes in Sweden during this time period (Pundziute-Lyckå et al. 2002). More recent data from Sweden, covering the period from 1983-2007, shows that incidence was higher in children under age 15, which peaked and then declined, while there were decreases in the older age groups (25-34 years), suggesting that there was a shift to a younger age of onset, instead of a uniform rate of increase among all age groups (although the overall incidence is still increasing) (Dahlquist et al. 2011). However, a new look at the Swedish data shows that these previous studies may have underestimated the number of people with type 1 diabetes in the 0-34 age range-- the actual number is 2-3 times higher than previously thought (Rawshani et al. 2014).
Long-term data from Norway from the 1930s to the 1970s show not only that the age of diagnosis decreased, but also that overall incidence increased, in people up to age 30. In the 1930s, diabetes was more common in people aged 15-29 than in people age 0-14, and by the 1970s it was more common in the younger age group instead (Gale 2002b; Gale 2005b).
Until there are more studies that include older age groups, it will be difficult to say how much of the increasing incidence in children is actually due to a decreasing age of diagnosis, and whether or not incidence is also increasing in adults. In the meantime, we can say that type 1 diabetes incidence is increasing in children of both genders, especially the youngest children, in countries around the world.
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