The incidence of childhood-onset type 1 diabetes, time trends and association with the population composition in Sweden: a 40 year follow-up

The present follow-up of the incidence trend of childhood-onset type 1 diabetes in Sweden over the last 40 years shows a prolongation of the levelling off indicated in the early 2000s, although still at a high level of incidence [2]. Similar trends have been observed in some other countries, albeit with shorter observation times [11,12,13]. The hypothesis that increased immigration to Sweden over the past decades has caused a stabilisation of the incidence rate due to an increased fraction of children with fewer susceptible genetic traits does not seem to explain the development of the incidence trends. Instead, a Swedish subpopulation displayed a similar deceleration in incidence rate, although with some variability. The lower incidence level for children with immigrant parents compared with those with Swedish parents is evidence that genetics may determine the incidence level, whereas the trend is mainly due to environmental risk factors. This is illustrated by the low, but still increasing trend in incidence in children with at least one parent born in an Asian country, that is, with an extremely low trait for type 1 diabetes [21]. This increase in incidence over time after exposure to environmental triggers has also been shown, for example, in Japanese children living in Japan, where a plateau in incidence was indicated after 1992 [22]. As the risk genes may also differ in Asian populations and may respond differently to environmental factors, it would be interesting to follow this low-risk population living in Sweden further.

Of the non-genetic risk factors suggested, lifestyle habits, for example dietary patterns and early growth rate, have been proposed to be triggers and/or accelerators of beta cell destruction in combination with a complex genetic background [7,8,9,10]. In Sweden, as in many other countries, the prevalence of childhood obesity increased in the late 1990s [23]. Swedish data based on routine weight–length measurements of schoolchildren, however, indicate that the obesity epidemic reached a plateau during the first decade of the 2000s [24, 25]. Moreover, a recent worldwide pooling of data from population-based studies, analysing trends in BMI and weight over the period 1975–2016 [26], showed that the increase in BMI in children and adolescents has plateaued in many high-income countries during recent years.

Another risk factor for type 1 diabetes in the Swedish setting is maternal BMI, particularly obesity in the first trimester [27]. However, a recent study indicates that there have been lower rates of increase in BMI and obesity prevalence in Swedish women since the early 2000s [28].

These ecological associations that indicate that the incidence trends in childhood-onset diabetes may partly depend on changes in childhood growth and weight are supported by several population-based case–referent studies showing such associations at the individual level [29,30,31,32,33].

A strength of our study is that it is population-based, is nationwide and represents a long period of follow-up and a large number of cases with data collected from reliable sources, both for the overall study population and for the subgroups. Linking individual cases to their parents, and in turn their countries of birth, provides new knowledge of the trends involved. The subpopulation analyses of cases born in Sweden with Swedish parents compared with other groups, and the more specific comparison made with cases with an Asian origin, provide further support for the observed incidence trends owing to the similar patterns of diabetes incidence over time. However, limitations of this study include lack of detailed comparisons of subgroups of other origins, and also the absence of individually linked variables that would support our hypothesis on the importance of changes in growth.

We conclude that, after more than 25 years of rapid increase in the 1980s and 1990s, the incidence of childhood-onset type 1 diabetes has levelled off in Sweden over the last two decades, despite a noted variability in the older age group. This pattern is not dependent on the increased immigration level over the same period. The association with the time trend of obesity in Sweden is of clear interest.

The number of incident cases of this burdensome disease remains high in Sweden and the results indicate that it is important to continue to monitor future developments and intensify efforts to improve lifestyle habits among young children and their families.

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