Incidence Rate of Type 2 Diabetes is >50% Lower in GrassrootsHealth Cohort with Median Serum 25–hydroxyvitamin D of 41 ng/ml than in NHANES Cohort with Median of 22 ng/ml

Sharon L. McDonnell, Leo L. Baggerly, Christine B. French, Robert P. Heaney, Edward D. Gorham, Michael F. Holick, Robert Scragg, Cedric F. Garland

Higher serum 25–hydroxyvitamin D [25(OH)D] concentrations have been associated with lower risk of
type 2 diabetes. This study compared incidence rates of type 2 diabetes among participants aged ≥20 years in two U.S. cohorts with markedly different median 25(OH)D concentrations. The median 25(OH)D concentration in the GrassrootsHealth (GRH) cohort was 41 ng/ml (N = 4933) while in the 2005–6 National Health and Nutrition Examination Survey (NHANES) it was 22 ng/ml (N = 4078) (P < 0.0001). The adjusted annual incidence rate of type 2 diabetes was 3.7 per 1000 population (95% confidence interval = 1.9, 6.6) in the GRH cohort, compared to 9.3 per 1000 population (95% confidence interval = 6.7, 12.6) in NHANES. In the NHANES cohort, the lowest 25(OH)D tertiles (<17, 17–24 ng/ml) had higher odds of developing diabetes than the highest tertile (OR: 4.9, P = 0.02 and 4.8, P = 0.01 respectively), adjusting for covariates. Differences in demographics and methods may have limited comparability. Raising serum 25(OH)D may be a useful tool for reducing risk of diabetes in the population.

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