Where will the health benefits of vitamin D end? Lately this supplement has been enjoying glowing coverage in the medical literature and popular press alike. Based on a recent trip to the vitamin section of my grocery, D appears to be muscling in on shelf space traditionally occupied by vitamin C, glucosamine and the grandaddy of all supplements, king calcium.
Trouble is, high-quality data are lacking as to vitamin D’s true value. In most D studies, researchers try to link intake of the vitamin with various health outcomes. Connecting the dots between two variables — say, vitamin D and the risk for colon cancer — is irresistibly tempting. But as any researcher will tell you, association does not ensure causation.
For example, estrogen was mistaken as a beneficial supplement for postmenopausal women back in the 1980s. Women who reported taking estrogen were found to exhibit lower incidences of heart disease, stroke, even dementia.
Sadly, these findings weren’t rigorously tested until 1991. The Women’s Health Initiative, a series of studies involving more than 160,000 patients, gave one group of volunteers a sugar pill and the other an estrogen pill, then followed participants for more than five years. To everyone’s dismay, all the assumptions about estrogen’s benefits were found to be incorrect.
How did researchers get it so wrong? Turns out estrogen consumption was really only a “marker” for a healthier lifestyle. Despite attempts to control for other confounding variables (exercise or smoking rates, for example) a few were still missed. These unrecognized behaviors then drowned out the negative effects of estrogen, making the hormone look beneficial.
Could D end up overreaching as well? Some associations have already been disproven by randomized studies. The same Women’s Health Initiative tested the hypothesis that vitamin D and calcium supplementation could prevent colon cancer. Based on the study duration of seven years, vitamin D had no beneficial effect on colon cancer rates.
This doesn’t mean we should ignore all of D’s claims. Without vitamin D, for example, absorption of calcium is very difficult. And we know that calcium and vitamin D provide modest protection from hip fracture in women older than age 60.
Evaluating other commonly cited effects of vitamin D — reductions in heart attacks, improvements in mood and memory, and protection from prostate and pancreatic cancer — remains challenging. Until high-quality, randomized, prospective studies are completed, we are left with tantalizing clues that may prove nothing more than epidemiologic dead-ends.
So how do I advise my patients? It’s recommended to take a vitamin D supplement of 800-1000 units daily if you are a woman over the age of 60, based on results from the Women’s Health Initiative. Additionally, vitamin D does not appear to cause harm in other populations (such as men or women younger than 60), so there appears to be little downside for these groups.
If you do decide to supplement, though, do it in a safe fashion: get your D in a pill or from foods, not from the sun. There’s no safe dose of sunshine — but we’ll save that topic for another time.