The traditional view of vitamin D was that is was required for the prevention of the bone disorders osteomalacia and rickets, in adults and children, respectively. However, recent research has expanded this narrow range of functions for vitamin D. The fact that that vitamin D is a steroid hormone that interacts with the nuclear vitamin D receptor to regulate cell function has been known for many decades. However, the wide range of physiological functions that are now attributed to vitamin D have only recently been discovered. In particular vitamin D appears necessary for the production of proteins that have particular anti-infective properties. Insufficient intakes of vitamin D below the 40 ng/mL (100 nmol/L) threshold, in the absence of sunlight, are now thought to leave an individual susceptible to infection. Vitamin D may therefore be effective at decreasing infective diseases, which explains the higher incidence of colds and influenza in the winter months, a time when subcutaneous vitamin D production ceases.
A number of studies have investigated the cellular effects of vitamin D in order to understand its immune modulatory effects. In addition, many clinical trials have assessed the ability of vitamin D to prevent infections in human subjects. For example, one study1 investigated the effects of 30,000 and 60,000 IU of oral vitamin D per month on the risk of infection in 644 elderly residents aged 64 to 80 years. Each subject received a monthly dose of vitamin D3 (cholecalciferol) and was then monitored for antibiotics use as a marker for infection. The results showed that those subjects who were assigned 60,000 IU per month had a 28 % lower risk of having to be prescribed antibiotics compared to the placebo group. However, this effect did not reach statistical significance in the study population as a whole. Subgroup analysis of different age ranges revealed that in those subjects over 70 years old, the 60,000 IU dose did cause a significant reduction in the use of antibiotics compared to the placebo.
The 60,000 IU of vitamin D per month in this study would equate to a daily intake of 2000 IU per day. This is now roughly the recommended intake for an adult in the absence of strong sunlight. The 60,000 IU was administered to the elderly subjects in a single dose, presumably because this would increase compliance considerably. The usual regimen is to take a daily dose of the vitamin, but being fat soluble, there is a propensity to store the vitamin in the adipose tissue. This store that can then release vitamin D slowly to the plasma over a long period of time. The elderly are at particular risk of vitamin D deficiencies because they tend not to expose themselves to strong sunlight. It is therefore interesting that the vitamin D was protective of infection in those subject over, but not under, 70 years of age. The authors noted however, that the lack of significance in the 60,000 IU treatment group was a result of high variability in the plasma 25-hydroxyvitamin D levels at baseline, and therefore despite a lack of statistical significance these result infer anti-infective effects for vitamin D.
Dr Robert Barrington’s Nutritional Recommendation: Current recommendations are for adults to consume around 2000 IU of cholecalciferol (vitamin D3) as a supplement to the diet if exposure to strong sunlight is not possible. Therefore most adults should consume 2000 IU of vitamin D from around mid autumn to late spring, a period where subcutaneous sunlight production is low or absent. Those who avoid sunlight in the summer, or who use sun cream to prevent tanning, should also consume vitamin D during this period. Vitamin D plasma levels begin to decline in the autumn and continue to do so up until early summer. The immunomodulatory properties are a function of its metabolism in the liver to 25-hydroxyvitamin D and then further in the kidney to 1,25-dihydroxyvitamin D. These hydroxylation steps appear pivotal in providing the required cellular signal molecules for correct immune function.
RdB
