Circulating Vitamin D Concentrations in Two Neighboring Populations With Markedly Different Incidence of Type 1 Diabetes

Recent studies have suggested that vitamin D deficiency may increase the risk of type 1 diabetes (1). In Finland, the incidence of type 1 diabetes is the highest in the world, while in the neighboring Karelian Republic of Russia, the incidence is approximately one-sixth that in Finland, despite no difference in HLA-conferred susceptibility (2). Thus, the reason(s) must be linked to environmental factors.

We assessed vitamin D status in the Russian Karelian and Finnish populations to determine whether vitamin D could play a role in the huge difference observed in diabetes incidence. The geographical location in terms of daily sunlight exposure is approximately the same in both populations (∼62–66°N). Circulating concentrations of 25-hydroxy (25-OH) vitamin D were analyzed using a commercial enzyme immunoassay kit (Immunodiagnostic Systems Limited, Boldon, U.K.) in cohorts representing the background population (schoolchildren and pregnant women). The schoolchildren series included 100 subjects from Finland and 100 subjects from Russian Karelia matched for age, sex, and month of sampling (mean age ± SD, 10.9 ± 1.7 years, 52% male subjects). Serum samples were collected during the years 1994–2000, and 86% were drawn during March–April. The series of pregnant women included 103 female subjects from Russian Karelia and 172 women from Finland matched for age and date of sampling (mean age 26.7 ± 5.3 years). Samples were collected at the end of the first trimester of pregnancy as a part of the routine prenatal follow-up during the year 2000, and 81% of them were drawn during October–December. The study was approved by the local ethical committees and the Finnish Maternity Cohort steering group. The study was carried out in accordance with the Declaration of Helsinki.

The median serum concentrations of 25-OH vitamin D were approximately the same in both countries among schoolchildren (35.0 in Karelia vs. 39.3 nmol/l in Finland, P = NS Wilcoxon test) and pregnant women (28.4 vs. 28.9 nmol/l, P = NS by conditional logistic regression test). Sex had no effect on vitamin D status among the schoolchildren. According to the previously suggested cutoff limit for vitamin D deficiency (serum 25-OH vitamin D <25 nmol/l) (3,4), the proportion of vitamin D–deficient subjects was not higher in Finland compared with Karelia (16 vs. 27% in schoolchildren, P = 0.091; 38 vs. 41% in pregnant women, P = NS, respectively).

The results suggest that circulating vitamin D concentrations do not differ markedly between Finland and Russian Karelia. Accordingly, vitamin D status may not contribute to the marked difference in the incidence of type 1 diabetes between the countries, and there must be other factors protecting the Karelian children from developing type 1 diabetes.

Nevertheless, vitamin D substitution remains an important issue in these countries, even beyond infancy, and regular substitution has long been recommended in both countries (5,6,7), especially during pregnancy, infancy, and the dark months of the year.