Vitamin D Supplements May Be a Double-Edged Sword

Dr F. Perry Wilson comments on a new analysis showing different effects of vitamin D and calcium supplementation on cancer and cardiovascular mortality.
www.medscape.com/viewarticle/...
-- TRANSCRIPT --
Welcome to Impact Factor, your weekly dose of commentary on a new medical study. I'm Dr F. Perry Wilson of the Yale School of Medicine.
Imagine, if you will, the great Cathedral of Our Lady of Correlation. You walk through the majestic oak doors depicting the link between ice cream sales and shark attacks, past the rose window depicting the cardiovascular benefits of red wine, and down the aisles frescoed in dramatic images showing how Facebook usage is associated with less life satisfaction. And then you reach the altar, the holy of holies where, emblazoned in shimmering pyrite, you see the patron saint of this church: vitamin D.
Yes, if you've watched this space, then you know that I have little truck with the wildly popular supplement. In all of clinical research, I believe that there is no molecule with stronger data for correlation and weaker data for causation.
Low serum vitamin D levels have been linked to higher risks for heart disease, cancer, falls, COVID, dementia, C diff, and others. And yet, when we do randomized trials of vitamin D supplementation - the thing that can prove that the low level was causally linked to the outcome of interest - we get negative results.
Trials aren't perfect, of course, and we'll talk in a moment about a big one that had some issues. But we are at a point where we need to either be vitamin D apologists, saying, "Forget what those lying RCTs tell you and buy this supplement" - an $800 million-a-year industry, by the way - or conclude that vitamin D levels are a convenient marker of various lifestyle factors that are associated with better outcomes: markers of exercise, getting outside, eating a varied diet.
Or perhaps vitamin D supplements have real effects. It's just that the beneficial effects are matched by the harmful ones. Stay tuned.
The Women's Health Initiative remains among the largest randomized trials of vitamin D and calcium supplementation ever conducted - and a major contributor to the negative outcomes of vitamin D trials.
But if you dig into the inclusion and exclusion criteria for this trial, you'll find that individuals were allowed to continue taking vitamins and supplements while they were in the trial, regardless of their randomization status. In fact, the majority took supplements at baseline, and more took supplements over time.
That means, of course, that people in the placebo group, who were getting sugar pills instead of vitamin D and calcium, may have been taking vitamin D and calcium on the side. That would certainly bias the results of the trial toward the null, which is what the primary analyses showed. To wit, the original analysis of the Women's Health Initiative trial showed no effect of randomization to vitamin D supplementation on improving cancer or cardiovascular outcomes.
But the Women's Health Initiative trial started 30 years ago. Today, with the benefit of decades of follow-up, we can re-investigate - and perhaps re-litigate - those findings, courtesy of this study, "Long-Term Effect of Randomization to Calcium and Vitamin D Supplementation on Health in Older Women" appearing in the Annals of Internal Medicine.
Dr Cynthia Thomson, of the Mel and Enid Zuckerman College of Public Health at the University of Arizona, and colleagues led this updated analysis focused on two findings that had been hinted at, but not statistically confirmed, in other vitamin D studies: a potential for the supplement to reduce the risk for cancer, and a potential for it to increase the risk for heart disease.
The randomized trial itself only lasted 7 years. What we are seeing in this analysis of 36,282 women is outcomes that happened at any time from randomization to the end of 2023 - around 20 years after the randomization to supplementation stopped. But, the researchers would argue, that's probably okay. Cancer and heart disease take time to develop; we see lung cancer long after people stop smoking. So a history of consistent vitamin D supplementation may indeed be protective - or harmful.
Transcript in its entirety can be found by clicking here:
www.medscape.com/viewarticle/...