The realisation that vitamin D is a steroid hormone and not a true vitamin facilitates a greater understanding about its biological roles. Traditionally vitamin D was seen as important in calcium regulation only, and in this regard a deficiency was associated with an increased risk of osteomalacia and rickets in adults and children, respectively. However, more recently this narrow physiological role has been expanded rapidly to incorporate a functional role in energy utilisation and immune system modulation. In the case of the latter, a wide ranging role for vitamin D has been unveiled that includes direct regulation of innate and acquired immunity through interaction with the vitamin d receptor in the nucleus of immune cells. A role for vitamin D in protection from the development of cancer is also strongly implied from the nutritional literature with many epidemiological studies supporting an association between vitamin d and cancer incidence. Mechanistically cell culture experiments also support the theory that vitamin D may be protective of cancer.
A recent study published in the American Journal of Clinical Nutrition1 investigated the association between vitamin D and lung cancer using data taken from the Women’s Health Initiative Study. The subject sample included almost 130,000 women, and within this group there were 1771 cases of lung cancer that developed during the course of the study. Of the women in the study, 12 % were given a 400 IU supplement of vitamin D per day (also containing 1 gram of calcium). When the data was analysed, the result showed that there was a significant protective effect of a vitamin D intake of more than 400 IU against lung cancer in non-smoking women. In addition, those women receiving the vitamin 400 IU per day D supplement had a significant reduction in lung cancer as long as their vitamin A intake was over 1000 μg per day retinol equivalents. Therefore these results suggest that vitamin D may be protective of lung cancer in women. This supports other evidence in the literature showing associations between vitamin D and various types of cancer.
It is true that some studies have failed to find associations between vitamin D and cancer. In fact in this study, no association was found for the population as a whole, only subgroup analysis showed positive results with respect vitamin D and lung cancer. A number of explanations for this are possible and confounding variables are always a consideration. However, it might simply be that the general intake of vitamin D amongst the population is too low to show a strong protective effect and that a threshold intake is required for the protective effects against cancer to become evident. In addition, the duration of sun exposure is often not considered in dietary studies, and this can have a considerable impact on the outcome of any analysis. For example, exposure of even small parts of the body to the sun during the warmest months can increase vitamin D levels considerably. Generally, it is considered that 2000 IU of vitamin D3 is required per day in order to maintain optimal plasma levels of the accepted biomarker 25-hydroxyvitamin D. Studies using 400 IU as a daily intake of the vitamin may therefore still allow the subjects to remain in a vitamin D insufficient state.
Dr Robert Barrington’s Nutritional Recommendation: If you have not considered a vitamin D supplement during the winter months you should. Those living at high latitudes are not able to maintain an adequate vitamin D plasma level from autumn to spring in the absence of a dietary supplement. Food sources of vitamin D are limited, and as such most people are not able to obtain enough vitamin D in the diet alone. Around 2000 IU of vitamin D is required to maintain plasma levels of 25-hydroxyvitamin D above the recommended 40 nmol/L (100 mg/mL).
RdB
