Writing in the Journal of Nutrition, Tianying Wu, Walter C. Willett, and Edward Giovannucci from Harvard School of Public Health report that women with high intakes of calcium had 20 per cent lower levels of C-peptide, a marker of insulin levels, while men with high vitamin D levels had similarly lower levels of the marker.
“The results suggest that calcium intake or systemic vitamin D status, after adjustment for intake of dairy products, is associated with decreased insulin secretion,” they wrote.
The study is in-line with previous reports, including a meta-analysis and review published in 2007 in the Journal of Clinical Endocrinology & Metabolism (Vol. 92, pp. 2017-2029), which found that the data from observational studies showed a "relatively consistent association" between low intakes of calcium, vitamin D, or dairy intake and type-2 diabetes, with highest levels associated with a 64 per cent lower prevalence of the disease, and a 29 per cent lower prevalence of metabolic syndrome among non-blacks.
When intake of calcium and vitamin D was combined, the inverse associations were still observed, with the highest versus lowest combined intake being associated with an 18 per cent lower incidence of diabetes.
D, Ca, or dairy?
The new study used data from healthy men participating in the Health Professionals Follow-up Study, and healthy women participating in the Nurses' Health Study I. Intakes of total calcium and vitamin D, and blood levels of 25 hydroxyvitamin D (25(OH)D), the storage form of vitamin D in the body, and fasting levels of C-peptide were measured.
C-peptide is often used as a measure of insulin levels - insulin is produced by splitting so-called pro-insulin, which forms one insulin molecule and one C-peptide.
The Boston-based researchers report that C-peptide levels were 20 per cent lower in men with the highest blood levels of 25(OH)D, but no such association was observed in women.
On the other hand, the highest levels of calcium were associated with a 20 per cent reduction in C-peptide levels in women, and a 17 per cent reduction in men, compared to those with the lowest levels.
When Wu, Willett, and Giovannucci combined calcium intake and blood levels of 25(OH)D, they found that the highest levels were associated with 35 and 12 per cent lower levels of C-peptide.
However, no benefits were observed when the researchers considered dairy intake, a result that is at odds with the earlier meta-analysis and review.
Diabetes – a growing problem
An estimated 19 million people are affected by diabetes in the EU 25, equal to four per cent of the total population. This figure is projected to increase to 26 million by 2030.
In the US, there are over 20 million people with diabetes, equal to seven per cent of the population. The total costs are thought to be as much as $132 billion, with $92 billion being direct costs from medication, according to 2002 American Diabetes Association figures.
Source: Journal of Nutrition 2009, Volume 139, Pages 547-554, doi:10.3945/jn.108.089920“Plasma C-Peptide Is Inversely Associated with Calcium Intake in Women and with Plasma 25-Hydroxy Vitamin D in Men” Authors: T. Wu, W.C. Willett, E. Giovannucci