NEW YORK (Reuters Health) Apr 04 - Women with high intakes of vitamin D and calcium appear to have a lower risk of type 2 diabetes, according to Boston-based researchers.

Dr. Anastassios G. Pittas, of Tufts-New England Medical Center and colleagues evaluated data for 83,779 women enrolled in the Nurses' Health Study. The women had no history of diabetes, cardiovascular disease or cancer at baseline. Vitamin D and calcium intake from foods and from supplements were evaluated every 2 to 4 years. The results of the study are published in the March issue of Diabetes Care.

A total of 4843 incident cases of diabetes were documented over 20 years of follow-up. The mean daily cumulative intakes of vitamin D and calcium in these women were 309 IU and 867 mg, respectively.

"Based on the latest guidelines set by the Institute of Medicine, only 3% of women in our cohort had adequate vitamin D intake, and only 24% had adequate calcium intake," Dr. Pittas's group reports.

Total vitamin D intake was not significantly associated with type 2 diabetes after adjusting for multiple potential confounders, but there was a 13% lower risk of diabetes among women in the highest versus the lowest category of vitamin D intake from supplements (p = 0.04).

Women with the highest versus total calcium had a 21% lower risk of diabetes than those with the lowest intakes (p < 0.01). The risk was 18% lower among women in the highest versus the lowest category of calcium intake from supplements (p < 0.001).

The lowest risk of diabetes was observed among women with the highest combined intakes of calcium and vitamin D compared with those with the lowest.

The mechanism by which vitamin D affects diabetes risk is unclear. Insulin resistance and impaired pancreatic beta-cell function have been linked with vitamin D insufficiency. "These observations together with the finding of vitamin D receptors in beta-cells and the finding of impaired insulin secretory capacity in mice lacking a functional vitamin D receptor indicate an important role for vitamin D in regulating beta-cell function," the researchers report.

The mechanism by which calcium may affect the risk of diabetes is also unclear. "Abnormal regulation of intracellular calcium affecting both insulin sensitivity and insulin release has been suggested as a potential mechanism to explain the putative association between calcium insufficiency and risk of diabetes."