The vitamin D story exudes teaching points: It offers a master class in critical appraisal, connecting the concepts of biologic plausibility, flawed surrogate markers, confounded observational studies, and slews of randomized controlled trials (RCTs) showing no benefits on health outcomes.
Yet despite the utter lack of benefit seen in trials, the hype continues. And the pandemic has only enhanced this hype as an onslaught of papers have reported the association of low vitamin D levels and COVID-19 disease.
My questions are simple: Why doesn’t the evidence persuade people? How many nonsignificant trials do we need before researchers stop studying vitamin D, doctors stop (routinely) measuring levels, and patients stop wasting money on the unhelpful supplement? What are the implications for this lack of persuasion?
Before exploring these questions, I want to set out that symptomatic vitamin deficiencies of any sort ought to be corrected.
Biologic plausibility and the pull of observational studies
It has long been known that vitamin D is crucial for bone health and that it can be produced in the skin with sun exposure. In the last decade, however, experts note that nearly every tissue and cell in our body has a vitamin D receptor. It then follows that if this many cells in the body can activate vitamin D, it must be vital for cardiovascular health, immune function, cancer prevention: basically, everything health related.
Oodles of observational studies have found that low serum levels of vitamin D correlate with higher mortality from all causes, cancer, cardiovascular disease, and now even COVID-19. Yet no matter the amount of statistical adjustment in these studies, we cannot know whether these associations are due to true causality.
The major issue is confounding: That is, people with low vitamin D levels have other conditions or diseases that lead to higher rates of ill health. Consider a patient with obesity, arthritis, and cognitive decline; this person is unlikely to do much exercise in the sun and may have low vitamin D levels. The low vitamin D level is simply a marker of overall poor health.
The randomized controlled trials tell a clear story
There are hundreds of vitamin D RCTs. The results simplify into one sentence: Vitamin D supplements do not improve health outcomes.
Here is a short summary of some recent studies.
VITAL, a massive (N > 25,000) RCT with 5 years of follow-up, compared vitamin D supplements to placebo and found no differences in the primary endpoints of cancer or cardiac events. Rates of death from any cause were nearly identical. Crucially, in subgroup analyses, the effects did not vary according to vitamin D levels at baseline.
The D-Health investigators randomly assigned more than 21,000 adults to vitamin D or placebo and after 5.7 years of follow-up reported no differences in the primary endpoint of overall mortality. There also were no differences in cardiovascular disease mortality.
Then you have the Mendelian randomized studies, which some have called nature’s RCT. These studies take advantage of the fact that some people are born with gene variations that predispose to low vitamin D levels. More than 60 Mendelian randomization studies have evaluated the consequences of lifelong genetically lowered vitamin D levels on various outcomes; most of these have found null effects.
Then there are the meta-analyses and systematic reviews. I loved the conclusion of this review of systematic reviews from the BMJ (emphasis mine):
“Despite a few hundred systematic reviews and meta-analyses, highly convincing evidence of a clear role of vitamin D does not exist for any outcome, but associations with a selection of outcomes are probable.”