Vitamin D is essential for normal bone growth and development in children, and in adults it's needed for maintenance of bone strength all because it allows the absorption of calcium from the diet. But now it is also being touted by some as a sort of miracle vitamin, one that can help prevent, or treat, Alzheimer's disease, heart disease, multiple sclerosis and possibly other ailments.
For example, a recent study in JAMA Neurology correlated decline in cognitive function in older adults with deficits in blood levels of vitamin D. This, although a well-designed study, was still only correlative one that can't be used to determine causality. There may well have been other characteristics of the study participants that accounted for the observed impairment. Still, it was covered by several media outlets such as Medscape, Time, and NPR.
While these outlets were responsible in their coverage, others are not. As always, the American Council on Science and Health is here to make sense of it all.
The bad news is that the data supporting such "extra-curricular" vitamin D functions are not strong with respect to showing a causal relationhip, since they're observational types of studies, as is the JAMA study cited above. And we know from earlier studies, of beta-carotene for example, that even strong observational studies often don't pan out when tested in well-controlled interventional trials. There is good news, though: some stronger studies are in the offing and we should know more in just a couple of years.
For example, as reported in The Washington Post, the so-called VITAL study is testing vitamin D and Omega-3 fatty acids from fish oil to see if either, or both, decreases the risk of stroke, cancer, heart disease or other health issues. Participants who receive the active pills will get 2,000 IU vitamin D and 1 gram of fish oil daily. This is a five-year, randomized, double blind study that involves nearly 26,000 men and women. It will continue until October 2017.
Another controlled trial, the D2d study, is enrolling about 2,800 people at risk for developing Type 2 diabetes. This study will determine if 4,000 IU of vitamin D will prevent the disease. It will run for four years.
A third trial examines the potential of 5,000 IU of vitamin D to slow the progression of multiple sclerosis in 172 patients who already have the disease.
In the meantime, what should people do?
The current recommendations for vitamin D intake are 600 IU for those up to age 70, and 800 for the 71-and-up crowd. For those who want to hedge their bets, it's important to note that the recent Institute of Medicine report on vitamin D sets a safe upper limit of 4,000 IU per day.
We certainly understand that people are interested in using vitamin D to help prevent disease, but we must emphasize that the data to support that use, except as it involves bone health, simply aren't there yet.
However, it's also true that some reasonable health behaviors, such as the use of sunscreens, can limit the amount of vitamin D the body makes on its own. So there is some rationale for people to be concerned about having enough vitamin D. Also, as people age their bodies become less efficient at using sunlight to produce vitamin D. Still, it is important to observe the upper limits of vitamin D consumption to avoid long term overdoses which can cause deposition of excess calcium in the body.