Childhood-onset type 1 diabetes in Sweden: why has it plateaued?
A new paper in Diabetologia shows that the incidence of childhood-onset type 1 diabetes in Sweden has levelled off over the past two decades, although it is still high. Increased immigration to Sweden by families at lower risk does not provide a complete explanation for this trend. Dr Susan Aldridge reports.
Mapping population-based trends of diseases such as type 1 diabetes is important for understanding the underlying causes and for healthcare planning. Between the early 1980s and the first years of the 21st century, the incidence of childhood-onset type 1 diabetes doubled in Sweden. There were similar increases in many other countries around the world. The usual explanation for these increases is a change in environmental risk factors, such as lifestyle or exposure to infectious diseases, affecting the autoimmune pathogenesis of the condition.
In the 30-year follow-up of the nationwide Swedish Childhood Diabetes Registry (SCDR), there is a plateau in the incidence trend in the years 2002-2007. Some other population-based registers have reported a similar finding. Finland recently found a tendency towards a decrease, but the majority of study centres globally are still reporting an increase in type 1 diabetes incidence.
After Finland, Sweden has the second highest incidence of childhood-onset type 1 diabetes in the world, suggesting a strong genetic risk. Research from the USA has shown that the incidence and prevalence of type 1 and type 2 diabetes in young people differ between ethnic groups, with the highest prevalence of type 1 being in non-Hispanic white people.
Therefore, one factor that may affect diabetes incidence in a country could be migration, which could both introduce new lifestyle habits and change the genetic risk of the overall population. The number of people living in Sweden but born (or having parents born) outside Sweden has gone up from 11.3% in 2000 to 19.6% in 2019. Could this explain the plateau in incidence of type 1 in Sweden described above?
Ingrid Waernbaum at Uppsala University and colleagues at Umeå University, both in Sweden, have done a follow-up of the SCDR data, analysing trends by age, sex and parental country of birth to see whether the 2000 to 2007 plateau was stable during the following 12 years, in order to assess the impact of migration.
Investigating trends in type 1
The study population consisted of 23,143 type 1 diabetes cases recorded in the SCDR from January 1978 to December 2019. To investigate the impact of immigration on time trends, children born in Sweden with two parents born in Sweden were compared with the rest of the study population, consisting of those who had at least one parent who was not born in Sweden. The former (18,606 children) were termed ‘Swedish’, the latter (4527 children) ‘Other countries’.
In addition, a sub-group of Asian children (586 children, part of the ‘Other countries’ population) was analysed. This Asian group actually represented the most numerous immigrant group (3%) coming to Sweden from elsewhere.
The researchers used the data to describe the incidence trends by age and sex from 1978 to 2019 for the total, Swedish, Other countries and Asian populations. The incidence trends in the total population and in the Swedish population showed a steady increase until 2000 and then began to level off – this flattening off continued up to 2019. Trend curves were broadly similar for total, Swedish, Other and Asian populations, but overall higher for Swedish and lower for Other and Asian, compared with the total population.
Understanding the trends
This new follow-up study to 2019 reveals a prolongation of the levelling off in type 1 diabetes incidence in Sweden that was noted in the early 2000s. Incidence, however, is still high. Similar trends have been noted in Norway, Western Australia and Finland, although with shorter observation times.
The hypothesis being tested in this study was that increased immigration to Sweden over the past decades has caused the plateauing of the incidence rate of type 1 diabetes because of an increased fraction of children with fewer susceptible genetic traits. Put simply, Swedish children are more likely to develop type 1 diabetes than children from elsewhere, so if the balance shifts towards the latter, this could explain the development of the plateau. However, statistical analysis led the researchers to conclude that the hypothesis does not, in fact, fully explain the incidence trends.
Instead, the Swedish population displayed a similar slowing down in incidence to the total population. The generally lower incidence for children with immigrant parents shows that genetics determines the incidence level, but the actual trend is mainly due to exposure to environmental risk factors. This is illustrated by the low, but still increasing, incidence among Asian children. They have a low genetic risk for type 1 diabetes (contrasting, of course, with their risk of type 2 diabetes). And, since risk genes may differ in Asian children and may respond differently to environmental triggers, it would be interesting to continue follow-up of this sub-population.
Of the non-genetic risk factors that could be involved in type 1 diabetes, dietary patterns and early growth rate have been suggested as triggers and/or accelerators of beta cell destruction. In Sweden, as elsewhere, the prevalence of childhood obesity increased in the late 1990s, but this reached a plateau in the first decade of this century.
Another possible risk factor for type 1 diabetes in the Swedish setting is maternal BMI, particularly during the first trimester. However, a recent study suggests that there have been lower rates of increase in BMI and obesity prevalence among Swedish women since the early 2000s. It can therefore be concluded that incidence trends in childhood-onset diabetes in Sweden may partly depend on changes in childhood growth and weight.
One strength of this new study is that it is nationwide, population-based and has a long period of follow-up. Linking individual cases to their parents and country of birth provides us with new insights into the trends in type 1 diabetes incidence over time.
The authors conclude that, after more than 25 years of rapid increase in the 1980s and 1990s, the incidence of childhood-onset type 1 diabetes in Sweden has levelled off over the past two decades. This pattern does not depend upon increased immigration over this period, but its association with obesity is clearly of interest.
The number of cases of childhood-onset type 1 diabetes remains high in Sweden. Given that type 1 diabetes is a lifelong condition that may impose a considerable burden on the child and their family, it is important to keep monitoring trends and investigating the underlying risk factors. And, of course, efforts to improve lifestyle habits among young children and their families should be intensified.
To read this paper, go to: Waernbaum I, Lind T, Möllsten A, Dahlquist G. The incidence of childhood-onset type 1 diabetes, time trends and association with the population composition in Sweden: a 40-year follow-up. Diabetologia 20 October 2022. https://link.springer.com/article/10.1007/s00125-022-05816-0
Any opinions expressed in this article are the responsibility of the EASD e-Learning Programme Director, Dr Eleanor D Kennedy.