The incidence of type 1 diabetes (T1D) appears to be leveling off in Finland, new research shows.
An analysis of data from the Finnish National Institute for Health and Welfare on T1D diagnosed in children aged 0 to 14 years during 2006–2011 is published July 23 in a research letter in the Journal of the American Medical Association by Valma Harjutsalo, PhD, and colleagues.
Finland has the highest incidence of T1D in the world, and its T1D registry is one of the longest-running. "The encouraging observation in this study is that the incidence of T1D in Finnish children younger than 15 years has ceased to increase after a period of accelerated increase," they observe.
However, while similar findings have been reported in Sweden, rates of newly diagnosed T1D have seemingly continued to rise in other European countries and in the United States.
"It's interesting, and it should be reported…but with a small number of cases reported every year, it may be too early to draw a conclusion," Dana Dabelea, MD, PhD, professor of epidemiology and associate dean at the Colorado School of Public Health, University of Colorado, Denver, told Medscape Medical News.
The reasons behind the shifting worldwide trends in T1D incidence are unclear. Environmental influences, such as a lack of vitamin D or enteroviruses, have been proposed as possible causes but are controversial. Other unproven theories include a shifting of T1D onset to older ages and an exhaustion of genetic susceptibility.
Dr. Harjutsalo, a senior researcher at Finland's National Institute for Health and Welfare, Helsinki, told Medscape Medical News, "It is difficult at this stage to elucidate whether the observation of a flattened incidence rate of type 1 diabetes can be generalized and whether it can predict future changes in other populations. The answer will be provided in the coming years. Naturally, it depends on whether the reasons for change are local or global."
A True Plateau, or Just a Blip?
The Finnish research includes a total 14,069 children (7695 boys and 6374 girls) diagnosed with T1D between 1980 and 2011. Of these, 24% (3332) were newly diagnosed between 2006 and 2011.
Between 1988 and 2005, the incidence increased by 3.6% per year until reaching a peak incidence of 64.9 per 100,000 person-years in 2006. After that, it dropped yearly to a low of 58.3/100,000 in 2009 and 59.1/100,000 in 2010. In 2011, the rate was 64.3/100,000.
The important thing to note is that the incidence has not increased since 2006, Dr. Harjutsalo said. "If the incidence pattern had prevailed in a similar fashion as it had from the beginning of the 1990s until 2006, another 475 children would have been affected by diabetes between 2007 and 2011 in Finland, and the incidence would be near 80 per 100,000 in 2011, while it was [actually] 64.3."
She added that although "it is true and of course also worrying that there was a new peak in incidence in 2011," that increase was seen only in boys (from 63.8 per 100,000 in 2010 to 75.8 per 100,000 in 2011), while the incidence continued to decline among girls (from 54.2 to 52.2, respectively).
"At this stage it is impossible to envision which way the incidence is going and whether the peak in 2011 was only temporary. The future will tell. We do not have any preliminary data from 2012 at the moment," said Dr. Harjutsalo.
A similar plateau was not seen in the ongoing U.S. multicenter SEARCH for Diabetes in Youth study, which found an increase of about 2.7% per year for T1D in youths aged 0 to 19 years from 2002 to 2009.
Elizabeth Mayer-Davis, MD, who cochairs the SEARCH study with Dr. Dabelea, told Medscape Medical News, "It's really too early to draw the conclusion that the US and Finn data are different, given the timeline and analyses of both studies — including that the SEARCH data only go to 2009; we really need surveillance in both countries to continue into the future."
Theories on What Is Behind the Changes
The Finnish investigators suggest that vitamin-D consumption might play a role in both the previous rise in T1D diagnoses and the current plateau.
Dr. Harjutsalo told Medscape Medical News that a growing body of evidence is showing that vitamin D plays an immunomodulatory role through the vitamin-D receptor present on insulin-secreting pancreatic beta cells and also in the immune cells.
In Finland, the amount of vitamin D recommended for infants had been reduced 10-fold since the 1950s, during which time the incidence of T1D quintupled. The subsequent fortification of dairy products with vitamin D after 2003 might have contributed to the leveling off now being seen, she said.
But Dr. Mayer-Davis and Dr. Dabelea aren't so sure about the vitamin-D theory.
Dr. Mayer-Davis said, "The literature on vitamin D and development of diabetes autoimmunity and incidence of type 1 diabetes is mixed at best, and there are many factors that one could identify that changed over time along with trends in diabetes incidence. This sort of ecologic association is very weak evidence."
Dr. Dabelea noted, "Vitamin D is a very interesting hypothesis. It's a testable hypothesis, but it's not a proof."
She added that another possibility is that populations at high risk may have exhausted their genetic susceptibility, so that all of the individuals who were genetically predisposed to develop T1D have already done so, leaving the background population at lower risk. A similar phenomenon may be occurring with the incidence of type 2 diabetes among the Pima Indians in the United States, which also appears to have plateaued in recent years.
Another theory, mentioned by the Finnish authors, is that the onset of T1D may have simply shifted to an older age group. Indeed, the current data show slight deceases in T1D incidence from 2006 to 2011 among children aged 0 to 4, 5 to 9, and 10 to 14 years, but an increase from 58.1 to 68.6 per 100,000 among 10- to 14-year-olds.
Dr. Harjutsalo told Medscape Medical News, "Unfortunately there are no studies in Finland concerning [those] older than 14 years during the same time when the incidence among children stopped increasing."
In fact, very few T1D registries include individuals older than 19 years at onset, the maximum age in SEARCH. And those that do include adult-onset T1D aren't long-term, Dr. Dabelea said.
"This is an important area that has not been covered. You need a registry that goes up to age 35 or 40 years and for long-term. Nobody is really doing that. Unless you have a registry that covers the entire age range, you're not going to see or be able to understand what's going on," she noted.
Another theory is that obesity may be contributing to T1D by promoting insulin resistance, but the obesity rate hasn't decreased in Finland since 2005, the authors note.
In all, Dr. Harjutsalo told Medscape Medical News, "Continuous monitoring of the incidence of type 1 diabetes is essential not only in Finland but also worldwide. Unfortunately, there is no way to prevent type 1 diabetes at the moment."
The study was supported by grants from the Folkhälsan Research Foundation, the Wilhelm and Else Stockmann Foundation, the Academy of Finland, and the Liv och Hälsa Foundation. Dr. Harjutsalo has disclosed no relevant financial relationships; disclosures for the coauthors are listed in the paper. Dr. Mayer-Davis and Dr. Dabelea have disclosed no relevant financial relationships.
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