In contrast, people who ate dairy-rich diets had about a 30% lower risk for the metabolic syndrome, and about a 15% lower risk for type 2 diabetes than those who got little dairy in the diet, the investigators wrote in the June issue of the Journal of Clinical Endocrinology & Metabolism.

"Although the evidence to date suggests that vitamin D and calcium deficiency influences postprandial glycemia and insulin response while supplementation [with both nutrients] may be beneficial in optimizing these processes," said Anastassios G. Pittas, M.D., M.Sc., of Tufts-New England Medical Center here, and colleagues.

There is evidence to suggest that vitamin D and calcium may have direct or indirect effects on the function of insulin-secreting beta cells of the pancreas, or on insulin action in tissues, the authors said, noting that it's "not clear whether the effects are additive or synergistic."

To further explore the question, the investigators reviewed the medical literature and performed a meta-analysis to determine the association between vitamin D and calcium levels and diabetes risk, and to see whether supplementation with the nutrients could improve glucose metabolism.

They found that in observational studies the odds ratio for type 2 diabetes occurrence among non-blacks with the highest vs. lowest levels of 25-hydroxyvitamin D was 0.36 (95% confidence interval, 0.16-0.80). The odds ratio for occurrence of the metabolic syndrome for the highest versus lowest levels of dairy intake was 0.71 (95% CI, 0.57- 0.89).

The authors also detected inverse associations between vitamin D and/or calcium intake with incident type 2 diabetes and the metabolic syndrome. For example, the odds ratio for incident diabetes was 0.82 (95% CI, 0.72- 0.93) for the highest versus the lowest levels of combined vitamin D and calcium intake, and 0.86 (95% CI 0.79-0.93) for the highest levels of dairy intake compared with the lowest levels.

Evidence from trials with vitamin D and/or calcium supplementation suggests that combined supplementation may have a role in the prevention of type 2 diabetes, but only in populations at high risk, that is, those with glucose intolerance, the investigators said.

"The available evidence is limited," they said, "because most observational studies are cross-sectional and did not adjust for important confounders, whereas intervention studies were short in duration, included few subjects, used a variety of formulations of vitamin D and calcium, or did post hoc analyses."

Practice Pearl: Explain to patients that, in addition to being essential for bone health, dietary vitamin D and calcium, abundant in fortified milk and other dairy products, may help to reduce the risk for type 2 diabetes by improving how the body metabolizes glucose.  And also that the currently recommended intakes of vitamin D and calcium may not be sufficient.

Journal of Clinical Endocrinology & Metabolism 2007; 92: 2017 -2029. Pittas AG et al. "The Role of Vitamin D and Calcium in Type 2 Diabetes. A Systematic Review and Meta-Analysis."

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