Dairy-Deficient Diet May Determine Diabetes Risk

BOSTON, July 11 -- A lack of vitamin D and calcium in the diet may increase the risk for the metabolic syndrome and type 2 diabetes in patients with impaired glucose tolerance, nutrition researchers found.

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."

The investigators recommended that future prospective cohort studies should focus on quantitative rather than self-reported measures of calcium and vitamin D, and suggested randomized controlled studies of calcium and vitamin D intake and their effects on metabolic endpoints such as glucose tolerance, insulin secretion, and insulin sensitivity.