Is Vitamin D Supplementation an Evidence-Based Practice?

This post originally appeared here.

Nearly one-third of Americans take a vitamin D supplement in an industry whose market value is around $1.3B. There has been a considerable increase in vitamin D supplementation in the past two decades. But a landmark study in 2019 and then an ancillary study in 2022 have called this into question and created a lot of controversy in the field. 

The study, called the Vitamin D and Omega-3 Trial, or VITAL, randomized 25,871 women 55 years or older and men 50 years or older to one of four conditions: vitamin D plus omega-3 fatty acids, vitamin D plus placebo, omega-3 fatty acids plus placebo, or double placebo. The researchers collected annual questionnaires on health outcomes and ultimately concluded that there was no impact of vitamin D supplementation on risk for cancer or cardiovascular disease. An ancillary study in 2022 that randomized participants to vitamin D and omega 3 conditions found that there was no benefit of vitamin D supplementation for prevention of fractures. 

In the aftermath of these studies, the authors published a commentary in the New England Journal of Medicine definitively stating that there is no benefit whatsoever of vitamin D supplementation in the general population, that physicians should stop screening for deficiencies and recommending supplementation, and that people should stop taking supplements “to prevent major diseases or extend life.” 

But many in the field are saying: “not so fast.” In the past, some have cast vitamin D supplementation as a “panacea,” capable of preventing or ameliorating a host of conditions, so these new studies represent quite a departure from that  wisdom about the supplement. To be fair, the two studies had some limitations, including the fact that there were large drop-out rates, so it wasn’t clear if the final sample was representative of the larger study population. But the most significant criticism of these studies is the fact that they only looked at people who had normal levels of vitamin D going into the study. So it’s exceedingly hard to say whether vitamin D supplementation helps people who actually need it, e.g. those who are deficient. That in itself is a tricky question, because the field has debated for many years exactly how to define lower than normal levels of vitamin D.

Many endocrinologists have also argued that the VITAL studies are too narrow. They argue that vitamin D supplementation probably does help prevent fractures in populations with osteoporosis, so we have to be careful not to throw the whole concept out because of studies in one population. 

Testing for vitamin D levels can be expensive so very real questions remain about when to do this. It is probably not recommended to test universally but only in cases where there’s a suspicion of deficiency. Physicians and patients should use shared decision-making in this context to decide if supplementation makes sense. Most recently, the Endocrine Society issued guidelines recommending against routine testing of vitamin D levels in healthy individuals. 

This example should provide a lesson in how physicians and medical bodies deal with potentially changing recommendations. The vitamin D story is replete with nuance: who counts as a “healthy” individual, for instance and what in fact are the normal upper and lower limits of blood vitamin D levels? It is imperative not to leap to conclusions when a handful of new studies seem to contradict ongoing medical practice, but at the same time clinicians must be flexible in the face of new data and not cling to traditions when contrary evidence accumulates. The main problem here is that debate among medical professionals can confuse the public and, as we know, confusion often leads to distrust. Hence, it is critical in situations like this that everyone emphasizes the need to individualize health decisions based on individual needs, values, goals, and preferences. Some people still need to have their vitamin D levels checked. Most people probably do not. The subtlety here is a matter to be taken up between people and their healthcare providers.