To take or not to take, that is the question many people have been asking themselves about vitamin D supplements. As is so often the case in science, there is no concrete answer. This in spite of close to 2000 studies published in the scientific literature. What that means is that if an effect exists, it is likely to be small, because if it were significant it would have revealed itself.
There is no question that a deficiency of vitamin D is responsible for rickets and that vitamin D supplements can help. But beyond that, the situation is quite murky. Given that rickets is a bone weakness problem, it is reasonable to explore whether vitamin D supplements can protect against fractures, particularly among the elderly. There are consistent observational studies showing an association between low vitamin D levels in the blood and greater risk of fractures. However, studies on supplementing the diet with vitamin D have not shown spectacular results. When the studies dealing with fractures are pooled, the evidence that emerges is that taking roughly 1000 IU of vitamin D and 500 mg of calcium can have an effect on fracture reduction, but not a very significant one. The data indicate that roughly fifty people would have to take vitamin D and calcium every day for ten years to prevent one fracture. There is no increased advantage to taking more than 1000 IU a day.
Vitamin D supplements have also been claimed to be of help in multiple sclerosis, depression, rheumatoid arthritis and respiratory tract infections. In the case of MS, it is well known that the incidence increases with latitude, suggesting that decreased exposure to ultraviolet light leading to a reduced formation of vitamin D in the body may play a role. But supplementation with vitamin D has not been shown to have a clinical effect. Neither has benefit been shown for depression or respiratory tract infections.
There has also been much interest in exploring the potential of vitamin D supplements in preventing cancer given that observational studies have consistently shown that people with low blood levels of the vitamin have a greater risk of cancer, especially of the breast. But the question here is whether low levels predispose to cancer, or whether cancer causes vitamin D levels to drop. A few studies, mostly in women, have shown that supplements reduce the incidence of cancer but in general the total number of cancer cases in these trials is too small for sweeping generalizations. Nevertheless, the trend in the cancer studies is towards showing at least a minor protective effect with vitamin D. So, the bottom line is that vitamin D is no panacea, but may play a small role in preventing fractures and possibly some cancers. Given that there is no recorded downside to dosing with 1000 IU a day, and that the supplement is cheap, it seems that taking a 1000 IU vitamin D supplement is not unreasonable, especially for women. The potential benefit is very small, but the risk is essentially zero.