Why Is Vitamin D Hype So Impervious to Evidence?
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.
@pipasha 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 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 their overall poor health.