Vitamin D can (pick at least one):
- prevent/cure cancer
- prevent premature death
- treat multiple sclerosis
- prevent/treat rheumatoid arthritis
If this were a true/false exam, you'd do well to respond "false" by pretty much all of these propositions — and others besides. That's the bottom line of a recent review by Dr. G. Michael Allan from the Department of Family Medicine at the University of Alberta, Canada, and his colleagues. Their report, "Vitamin D: A Narrative Review Examining the Evidence for Ten Beliefs," was recently published in the Journal of General Internal Medicine.
The impetus for this review, the authors said, was the accumulation of a large number (over 800) of observational studies linking lower vitamin D status to a number of ailments. Obviously, such studies have led to the hypothesis that vitamin D, in addition to its usual function of mediating the absorption of calcium in the gastrointestinal tract, can treat or prevent these conditions. In addition, there have also been some randomized controlled trial of varying strengths, and in some cases meta-analyses. They evaluated the strength of the evidence from all types of studies for 10 widely-held beliefs about vitamin D's health-promoting value.
The following eight beliefs were those originating from such observational studies; the authors evaluated those studies and any evidence from randomized trials, and provided a "bottom line" for the validity of the belief as follows:
- Vitamin D reduces falls: The evidence is not strong — partly because the studies examined the number of "falls" rather than "fallers" so an individual may have been counted more than once. So vitamin D supplementation may reduce the number of falls among the elderly, and possibly the number of fallers.
- Vitamin D reduces fractures: The best available evidence indicates that vitamin D, when given with calcium, can reduce the risk of non-vertebral or hip fractures by about 10 to 15 percent.
- Vitamin D reduces respiratory tract infections: In western populations, vitamin D doesn't reduce the risk of RTIs. It may benificial in profoundly deficient children in developing countries.
- Vitamin D improves depression and mental well-being: At least in the general population, vitamin D supplements don't improve mental well-being scores, even if the baseline levels of the vitamin are low. The data on depressed patients are poor quality, the authors state, and aren't recommended.
- Vitamin D can prevent or treat rheumatoid arthritis: There is no reliable, consistent evidence that vitamin D is useful for this disease.
- Vitamin D can treat multiple sclerosis (MS): Any evidence that currently exists is of poor quality, and doesn't support the idea that vitamin D supplements can benefit MS sufferers.
- Vitamin D reduces mortality: Although some studies suggest that there could be anywhere from a 2 percent increase to a 13 percent decrease in mortality risk, the authors note the results aren't consistently statistically significant, and if the reductions are real, they "are likely quite small."
- Vitamin D reduces cancer incidence and mortality: "Vitamin D supplementation does not reduce the incidence of cancer. The impact of vitamin D supplementation on cancer mortality is less clear and has been assessed only by low- quality evidence."
- Vitamin D dose — more is better: "High-dose vitamin D has been shown to increase the risk of falls and fractures. Single high-dose (≥300,000 IU) supplementation should not be recommended."
- Vitamin D levels should be tested routinely: The authors found little substantive evidence for routine vitamin D testing, and that vitamin D supplementation in the general adult population, is safe. They noted that the Institute of Medicine sets a level of 50 nanomoles/liter as the level of sufficiency.
In their summary, the authors acknowledged that, "Severe vitamin D deficiency causes important health problems (e.g., rickets)." But they also emphasized that while there have been associations between lower vitamin D levels and a number of medical conditions, on the whole these associations have not been shown to be causal links.
At present, they stated that the evidence supports Vitamin D to prevent fractures (especially accompanied by calcium), and maybe to prevent falls in older patients. But, they say: "No other effects are proven. For many other conditions, the evidence for vitamin D supplementation is plagued by the use of small, poor-quality trials."