Vitamin D is an immensely popular supplement. Known as the “sunshine pill” for its supposed ability to mimic the effects of sunshine on health, vitamin D supplements are a billion-dollar industry.
People take it when they feel tired or sad, and are worried the cause is vitamin D deficiency, maybe as a result of not getting enough sun (think winter blues). They also take it because vitamin D is associated with reduced risk for hundreds of common diseases, conditions, and viruses.
Swallowing down a vitamin pill is an easy way to stay healthy and prevent scary health conditions. But as vitamin D’s popularity continues to soar, an increasing number of researchers warn that the science just doesn’t support the hype.
That’s not to say the vitamin isn’t helpful at all — but taking it as a supplement doesn’t always help you in the way that you might want it to. At the same time, a growing number of people are taking too much vitamin D — eager for benefits that likely won’t happen.
What do we know about vitamin D?
Vitamin D plays an important role in the body: It helps the body absorb calcium. Getting enough calcium is essential for your bone health — not getting enough can put you at risk of problems like osteoporosis or rickets. Vitamin D can come from several sources, including supplements, diet, or exposure to ultraviolet-B radiation from sunlight. But the reasons why people take vitamin D often have nothing to do with its relationship to calcium.
Vitamin D came to prominence in the public’s mind on the back of several small studies that showed it could benefit health beyond your bones. Taking the supplement could reduce the risk of a number of illnesses and conditions, including type 2 diabetes, chronic kidney disease, and high blood pressure, they claimed.
For years, it seemed the supplement was “riding high,” Dr. Myles Wolf, professor at Duke University School of Medicine, argued in a recent JAMA editorial. But randomized clinical trials — designed to validate these small studies’ claims — changed all that. The trials just didn’t bear out the studies’ findings.
Based on the results of those clinical trials, we shouldn’t expect that vitamin D supplementation will have any benefits beyond supporting skeletal health because “the body of clinical trial data do not support these broader claims,” Wolf tells Inverse.
Why did the larger trials fail? Wolf says that it comes down to the difference between association and causation. Too low vitamin D levels may be associated with a problem, but they may not be the direct cause. There are myriad reasons why people with health conditions may also have vitamin D deficiency, including changes to diet and decreased physical activity — both repercussions of being sick.
Dr. David Jenkins, professor at the University of Toronto, tells Inverse that, as time goes on and more significant clinical studies on the supplement are conducted, “the studies that were interesting to begin with fall away.”
In a 2018 review, Jenkins and his colleagues evaluated 179 published studies that looked at the health benefits of multivitamins, calcium, vitamin C, and vitamin D.
They revealed that these common supplements had no advantage or added risk in the prevention of cardiovascular disease, stroke, heart attack, or premature death.
“I have to say that I was quite surprised that we didn’t see more positive effects,” Jenkins says. “The studies that came out, in general, didn’t show that vitamin D did any harm — so the chance of it having a placebo effect is very good.”
How does Vitamin D boost health?
Currently, there are no studies that definitively prove vitamin D does the average adult any good, according to Jenkins.
Where it may be of some help is when it is given in certain doses to individuals with multiple sclerosis, he says. As for vitamin D’s other supposed health benefits, it’s “very difficult for one to build a strong case.”
But the vitamin’s established ability to help maintain bone health does justify “ensuring normal vitamin D stores, even if some of the other hypothesized benefits are not demonstrated in clinical trials,” Wolf cautions.
"Having a decent vitamin D level in the blood is correlated with good health."
Unfortunately, even that claim is subject to debate. In a 2018 review, scientists looked specifically at the effect of vitamin D supplementation on musculoskeletal health across 81 studies. They concluded that vitamin D has no effect on preventing bone fractures or on increasing bone mineral density.
Some doctors criticized the findings, afraid they would be misunderstood and used to take away much-needed vitamin D from children, who have to get enough to ensure their bones grow properly. Other researchers also argued that more research is needed to say whether or not it helps before coming to any conclusions.
At the time, the review’s first author Mark Bolland, associate professor at the University of Auckland, told Inverse he found the criticism frustrating.
“Why don’t people believe the evidence that’s out there?” he said. “I think there are people who passionately believe that vitamin D will be beneficial even though it’s not.”
Is too much vitamin D bad for you?
Taking vitamin D likely isn’t going to do you any harm — even if it’s not leading to the preventative results you want, according to Jenkins’ research. People who take 600 to 1,000 international units (IU) of vitamin D have “sufficient” blood levels of the vitamin, it suggests. This is particularly reasonable, says Wolf, “especially in colder climates during the winter.”
But here the water gets muddy once more: It’s not clear if taking vitamin D pills are entirely responsible for raising blood levels of the nutrient.
“Having a decent vitamin D level in the blood is correlated with good health,” Jenkins says. “But you have to question: Can these levels be explained biochemically, by the supplement, or is it a much more complex lifestyle reason that is leading to higher vitamin D levels?”
To complicate matters further, there’s conflicting data over what counts as a “normal” or safe level of vitamin D in the blood.
In the VITamin and OmegA-3 Trial (VITAL), 2,000 IUs of Vitamin D a day was pegged as the safety limit. In that study, the researchers settled on that number in part because previous studies focused on much smaller amounts. VITAL’s findings became a hot topic of conversation: It found that vitamin D was associated with a 25 percent reduction in cancer deaths, although the authors cautioned that they found “signals” of its benefits, not causation. The data have since been used to underpin a number of other studies looking at similar topics.
Even if we don’t know the exact safe level, some people are taking way too much vitamin D. A 2018 review found that, since 2010, there’s been an uptick in reports of vitamin D intoxication. Symptoms range from the mundane — feeling thirsty — to the life-threatening — having a seizure. The researchers suggest that people should be safe from vitamin D intoxication if they take less than 10,000 IU a day.
The average vitamin D pill is 400 IU, so you’d have to take a lot of them to become intoxicated by that measure. But single pill amounts can vary by brand, and the 10,000 IU recommendation is just one of many. A separate group of scientists pegged the level much lower: In a 2017 paper, they argued that the tolerable upper limit is just 4,000 IU a day. The National Institutes of Health sets its daily recommended daily amount for adults under 70-years-old at 600 IU.
Despite its sunny reputation, the science of vitamin D is murky at best. Overhyped claims that it can do you a lot of good have historically overshadowed the real picture. Ultimately, larger clinical trials are needed to establish what if anything the supplement does to the body. In the meantime, taking a vitamin D pill every day isn’t going to do you any harm — but it also might not do you any good, either.