Reading any medical or nutritional textbook it becomes apparent that vitamin D and calcium metabolism are intertwined. Vitamin D, actually a steroid hormone, is required for the correct absorption of calcium, and in this way, levels of vitamin D regulate the absorption of the mineral. One way that vitamin D regulates calcium metabolism is through control of the absorption of calcium from the gut. The vitamin D metabolite, 1,25-dihydroxyvitamin D (calcitriol) binds to high affinity receptors in enterocytes and here it regulates the transcription of new protein to increase the availability of calcium transporters. An increase in circulating calcitriol therefore allows an increase in the absorption of calcium and this increases plasma levels of calcium. For this reason, calcium supplements are often found with vitamin D within the formula, the theory being that by optimising vitamin D levels, the absorption rate of calcium will increase from the supplements and in this way optimise the availability of the calcium to the tissues.
The vitamin D status of an individuals is therefore an important determinant of an individual, and low levels of vitamin D can detrimentally affect calcium intake. Associations between the vitamin D status of women and their calcium absorption rates, for example, have been demonstrated in the nutritional literature1. Absorption of calcium from milk increases as the blood levels of vitamin D (measured as 25-hydroxyvitamin D) increase. Low circulating vitamin D levels decrease calcium absorption, and this results in a paradoxical situation whereby cellular levels of calcium become elevated. This may result in the desensitisation of the insulin receptor to insulin, and this explains the association between vitamin D status and insulin resistance. Optimising calcium status, therefore requires optimisation of vitamin D. However, other minerals also need balancing with calcium intake as divalent cations such as magnesium, calcium and zinc compete with one another for absorption.
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