The controversies about the benefits of vitamin D reflect how science evolves, said Dr. Barry Kramer, director of the cancer prevention division at the National Cancer Institute.
Early research on the benefits of vitamin D was mostly observational — large-scale, population-level studies — and did not look at endpoints that are important for long-term health, like whether a high vitamin D intake reduces one's risk for particular diseases or death.
Researchers found associations between higher levels of vitamin D intake and a range of health benefits. "But with the observational studies — especially when you're dealing with dietary supplements and diet — taking supplements is also associated with many other confounding factors that predict the outcome: being wealthier, being health-conscious, having health insurance and access to the health care system, low smoking prevalence, increased physical exercise," said Kramer.
In other words, the people who were taking these vitamins were doing many other things that might have caused them to have better health outcomes. Still, this early science encouraged people to hop on the vitamin D bandwagon.
Since then, randomized trials — that introduce vitamin D to one group and compare that group with a control group — have been disappointing, showing little or unclear benefit for vitamin D testing and supplementation in healthy people. That Institute of Medicine report noted that randomized trials had uncovered no health benefit for healthy people with vitamin D blood levels that were higher than 20 nanograms per milliliter.
There are also well-documented costs associated with overtesting and getting too much vitamin D: the cost to the health system for all those tests, and the potential harms from high vitamin D levels, such as kidney stones and high calcium — which can cause nausea, vomiting, and loss of appetite.
So until we have more and better studies on vitamin D, related testing and treatment are clouded with uncertainty and a lack of evidence for any benefit.
There's also the issue of defining vitamin D levels that are problematic. Experts agree that anything less than levels of 10 ng/mL of blood is worrisome or a deficiency, but when is someone insufficient? Is 20 ng/mL really enough? Should the minimum cutoff be 30 ng/mL?
According to the US Preventive Services Task Force — whose recommendations set the tone for medical practice in this country — this uncertainty led to a lot of inconsistency around how vitamin D insufficiency was defined in studies. Different professional bodies also back different minimum blood levels, usually ranging from 20 to 30 ng/mL.
Finally, there's some question of whether healthy (asymptomatic) adults who undergo routine screening for vitamin D actually see any health benefit as a result. The task force points out that there were no studies on the benefit of screening otherwise healthy adults, but it did find that putting them on treatment with supplements did not improve health outcomes for a range of issues, including cancer, Type 2 diabetes, and fractures.
"Although the evidence is adequate for a few limited outcomes, the overall evidence on the early treatment of asymptomatic, screen-detected vitamin D deficiency in adults to improve overall health outcomes is inadequate," the task force authors write in their latest guidance.
To clear up some of the uncertainty, the NIH has funded one of the largest randomized trials on vitamin D, with the results expected to be ready next year. Maybe then we'll have a better sense of what, if any, benefit this vitamin holds.
One of the authors on that study, Dr. JoAnn E. Manson, recently told the New York Times, "A lot of clinicians are acting like there is a pandemic," of vitamin D deficiency. "That gives them justification to screen everyone and get everyone well above what the Institute of Medicine recommends."