Cell culture experiments show that the addition of vitamin D to breast cancer cell lines inhibits the proliferation of the cells. As well as this antiproliferative effects, vitamin D is also known to produce pro-differentiation effects and cell cycle stabilisation effects on cells in culture. This provides a clear mechanisms by which vitamin D might be protective of cancer in humans, as evidenced by epidemiological data. Vitamin D status can be described as sufficient, insufficient or deficient based on plasma levels of the vitamin D metabolite 25-hydroxyvitamin D. Generally, plasma levels of 25-hydroxyvitamin D above 40 mg/mL (100 nmol/L)* are thought to be sufficient and levels below 25 ng/mL (62.5 nmol/L) are thought to cause deficiency, the latter resulting in the classic vitamin D deficiency diseases of osteomalacia and rickets. However, plasma levels between 25 and 40 ng/mL are classed as being insufficient, and at this level although the classic deficiency diseases do not develop, the risk of cancer increases significantly.
An inverse association between 25-hydroxyvitamin D plasma levels and certain types of cancer exists. However, there is still insufficient evidence to claim that vitamin D is a definitely a causative factor in protection from cancer, although the evidence does hint towards this. A number of well designed studies have shown the inverse association of vitamin D status with cancer risk. For example, in one study1 involving women from Saudi Arabia, researchers evaluated the association between circulating levels of 25-hydroxyvitamin D and breast cancer risk. The results of the study showed that those subjects who developed breast cancer had a significantly lower vitamin D status (mean plasma 25-hydroxyvitamin D levels of 23.5 nmol/L) compared to controls (mean plasma 25-hydroxyvitamin D levels of 38.5 nmol/L). Those with the highest serum concentrations of 25-hydroxyvitamin D (50 nmol/L) had a significantly lower risk of developing breast cancer compared to those with the lowest (<25 nmol/L).
*NB: To convert vitamin D from ng/mL to mmol/L multiply by 2.496.
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