Traditionally the recommended intakes of vitamin D for adults were based on the amount of cod liver oil required to prevent rickets in children. Nutritionally, such an archaic method of calculating the vitamin D requirement of an adult is obviously flawed. Although most medical recommendations for vitamin D are still based on this flawed estimation, nutritional studies have reappraised the required intake and found that it is much higher than previously thought.
RDA Versus Current Recommendations
The RDA for vitamin D in the United Kingdom is 200 IU for an adult. However, research from the nutritional literature suggests that intakes should be above 2000 IU for those not exposed to strong sunlight. Therefore in the winter months in high latitude countries, failure to take additional vitamin D will almost certainly result in a vitamin D insufficiency. In those who have particularly low levels, this could result in a deficiency.
Vitamin D And Cancer
Most of the evidence for the efficacy of vitamin D in cancer protection has come from animal studies and epidemiological (observational) studies. Accumulating evidence from these sources suggests that low levels of plasma vitamin D increases the risk of cancer. In particular, vitamin D intake may protect from breast, prostate and ovarian cancer, which suggests that the vitamin (actually a hormone) may be protective of sex hormone related cancers.
How Does Vitamin D Prevent Cancer?
Vitamin D is able to inhibit cancer formation when it is activated into its metabolite forms (25-hydroxyvitamin D and 1, 25-dihydroxyvitamin D). These active forms of the vitamin are able to interact with cellular receptors and have physiological effects. One of these effects is to inhibit the production of new blood vessels, a process called angiogenesis. Because tumours require these blood vessels, vitamin D can inhibit tumour promotion and progression.
Are Supplements Necessary?
Vitamin D is present in a limited number of foods. Apart from fish and their oils, not many commonly eaten foods contain much vitamin D. Although it is fortified into milk in some countries, this is not currently the practice in the United Kingdom. Therefore it is not possible to attain the 2000 IU of vitamin D per day recommended for optimal health. For this reason supplements are necessary to boost dietary intakes, in the absence of sunlight.