Introduction Low vitamin D status has been associated with higher risk of type 2 diabetes in cross-sectional studies. The evidence from prospective studies is limited. The aim was to examine the association between vitamin D status and risk of type 2 diabetes and markers of glucose homeostasis in a prospective cohort study.

Methods The study is part of the INTER99 study, based on a random sample of the general population of Copenhagen, Denmark. The current study included 6045 men and women aged 30–65 years at baseline (1999–2000). 4296 participated in the follow-up examination 5 years later (2004–2006). Data were collected with self-administered questionnaires, a physical examination, a 2 h oral glucose tolerance test, and various blood tests including measurement of serum 25-hydroxyvitamin D (25 (OH)D). Data were examined in multivariate logistic and linear regression models.

Results Low vitamin D status (25 (OH)D <25 nmol/l) was significantly associated with increased prevalence (OR 95% CI 1.62 (1.13 to 2.32)) and incidence (OR 95% CI 2.04 (1.38 to 4.17)) of diabetes compared to normal status (25 (OH)D ≥50 nmol/l). Moreover, low vitamin D status was significantly associated with markers of glucose homeostasis (glucose, insulin, c-peptide, Haemoglobin A1c, and insulin resistance (assessed by the HOMA model and the BIGTT test)) as well as unfavourable changes in these during follow-up.

Conclusion Low vitamin D status was associated with increased risk of type 2 diabetes and markers of glucose homeostasis in a Northern European general population sample.