Vitamin D is good for what ails you. Or at least that's what patients and doctors might conclude if they read only the headlines.
In the past few months, deficiency in the substance has been linked to chronic obstructive pulmonary disease, tuberculosis, spinal inflammatory diseases, age-related macular degeneration ... and the list goes on.
On the other hand, taking high doses of vitamin D didn't help patients with multiple sclerosis, MedPage Today reported, and it was of no benefit in reducing left ventricular mass in patients with chronic kidney disease.
Read this story on www.medpagetoday.com.
But overall, vitamin D gets pretty good press. The trouble is that hard evidence to back up the vitamin's benefits is lacking, according to Dr. Clifford Rosen of the Maine Medical Center Research Institute in Scarborough. "There's no data," Rosen told MedPage Today. "It's all weak association studies."
Benefit Beyond the Bones
At the American Heart Association meeting earlier this month, several studies suggested associations between low vitamin D and various aspects of heart disease.
But as one observer said at the time, there are no clinical trials yet that show improving vitamin D status does anything to reduce cardiovascular risk.
One study, the Vitamin D and Omega-3 (VITAL) trial, may shed some light on the issue. It is a randomized trial that is enrolling some 20,000 patients to see if daily vitamin D supplements prevent cancer and cardiovascular disease.
It will be one of the few randomized trials -- if not the only one -- to look at the issue directly, according to Dr. JoAnn Manson of Brigham and Women's Hospital in Boston, who is the principal investigator.
Although other randomized trials have produced evidence for a vitamin D benefit in several important clinical categories, Manson told MedPage Today it was mostly as an afterthought.
"Many of the randomized trials people have heard about were trials designed to look at the effect of vitamin D on fractures and falls," she said, with other effects as secondary outcomes.
It's in the nature of statistics, she pointed out, that if researchers look at enough outcomes, some will be significant just on the basis of chance.
The vast mass of the evidence for any kind of nonskeletal benefit is observational, and therefore suspect until confirmed by a properly designed, randomized trial, Manson said.
Among other things, a host of confounding factors -- obesity, poor nutrition, lack of exercise -- might play a role. No matter how carefully an observational study is done, she said, confounding is always possible. "Correlation does not prove causation," Manson reminded.
She noted that randomized trials have demolished observational evidence many times in the past, notably in the cases of such former fads as beta-carotene and selenium.
According to Rosen, there is reasonable evidence that improved vitamin D status leads to better bone health and some evidence that supplements reduce all-cause mortality in elderly women.
For almost everything else, he said, hard evidence is missing.