The traditional viewpoint is that vitamin D and calcium are required for correct bone formation. Until recently little attention was paid to either calcium or vitamin outside of this role. However, growing evidence in the nutritional literature has challenged this idea. Increasingly both calcium and vitamin D are being found to play a role in blood glucose metabolism, and this may explain the inverse associations between calcium and vitamin D with diabetes and obesity. Calcium and dairy products have also been shown to cause weight loss in animals and humans, which further supports an insulin sensitising effect for calcium. The possibility therefore arises that both calcium and vitamin D when deficient in the diet, cause reductions in the availability of these nutrients to tissues, and this causes a decline in the sensitivity of the insulin receptor for insulin, subsequently leading to elevations in blood sugar. Based on this hypothesis, calcium and vitamin D supplements should improve blood glucose regulation in deficient individuals.
And this is exactly what is found. Supplements of calcium and vitamin D in combination are effective at improving the blood sugar regulation in those with poor control, but not those with adequate control, the former likely including those subjects that are already deficient in these two essential nutrients. For example, in one study1, researchers assessed the effects of 3 years of supplementation of 500 mg of calcium citrate and 700 IU of vitamin D as the cholecalciferol D3 form, on bone related outcomes in a group of healthy adults without diabetes. However, when the results were analysed, the data showed that the supplementation had had a positive effect on the subjects blood sugar levels. The researcher spit the subjects into groups depending on whether they had baseline levels of blood sugar that were normal or irregular. Analysis of the treatment protocols showed that the calcium and vitamin D supplements were only effective at lowering the blood glucose levels in those with elevated blood levels of blood sugar.
As well as lowering blood sugar levels the calcium and vitamin D supplement also improved the insulin sensitivity in those with elevated levels of blood sugar at baseline. This increase in insulin sensitivity was likely the mechanism by which the supplements could lower fasting levels of blood sugar. The ability of the calcium and vitamin D supplements to lower blood sugar levels and improve insulin sensitivity, suggests that these people were deficient in these nutrients and that repleting tissue levels of these essential nutrients allowed metabolic pathways that were previously under active to become operable again. This then resulted in a more efficient blood sugar control. The ability of vitamin D and calcium to regulate blood sugar in this way explains the association between vitamin D and calcium tissue levels with the degree of adiposity that have been reported in epidemiological studies. The modest amount of calcium and vitamin D in this study were well below the amounts currently recommended by non-mainstream nutritionists.
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