"We examined intakes of total calcium and vitamin D, and plasma concentrations of 25 hydroxyvitamin D [25(OH)D] in relation to fasting plasma concentrations of C-peptide in two cross-sectional studies among healthy men from the Health Professionals Follow-up Study and among healthy women from the Nurses' Health Study I. Intake of total calcium was modestly inversely associated with C peptide concentration in women (P-trend = 0.05); however, the inverse association was not significant in men(P = 0.7). Concentrations of C-peptide were 20% lower among men who had plasma concentrations of 25(OH)D in the highest quartile compared with those in the lowest quartile (P-trend = 0.08); there was no association in women (P = 0.3). The inverse association between calcium intake and the plasma C-peptide concentration was stronger in hypertensive individuals of both sexes. The difference in the C-peptide concentrations between extreme quartiles of calcium intake was 17% in men and 20% in women. Plasma concentrations of C-peptide for the combination of the highest tertiles of calcium intake and plasma 25(CH)D compared with the opposite extreme were 35% lower (P = 0.03) in men and 12% lower (P = 0.01) in women," wrote T.Y. Wu and colleagues, Harvard University, Medical Department.

The researchers concluded, "The results suggest that calcium intake or systemic vitamin D status, after adjustment for intake of dairy products, is associated with decreased insulin secretion. J. Nutr. 139: 547-554, 2009."