From the WebMD Archives

April 26, 2019 -- Vitamin D has been proclaimed a potential protector against everything from cancer, heart disease, and diabetes to mood disorders like depression and dangerous falls. But recent research has challenged several of those promising roles and revealed how little is really known about the essential nutrient.

“It’s been a hot topic for about 10 years,” says Kelly Pritchett, PhD, a registered dietitian who studies vitamin D at Central Washington University in Ellensberg, WA. “But there are a lot of different trains of thought about what’s optimal.”

In 2014, the U.S. Preventive Services Task Force, a federally funded panel of health experts, said there was no common agreement on how to define vitamin D deficiency or what level was ideal. That remains the case today.


Still, vitamin D supplements have become more popular. Two decades ago, fewer than 1% of U.S. adults took them, according to a 2017 study. But less than 15 years later, nearly 1 in 5 adults took one every day. Why?

“The bottom line is, there’s a lot of wishful thinking,” says vitamin D researcher Clifford Rosen, MD, the director of clinical and translational research and a senior scientist at the Maine Medical Center Research Institute in Scarborough. “Whatever sounds good is often what people like to use.”

Latest Research: Bone Heath, Cancer, Heart Disease

While low vitamin D levels have been linked to many health problems -- such as a higher risk of some types of cancer, diabetes, heart disease, and depression -- research is inconclusive. Researchers do know that vitamin D helps our bodies absorb calcium, which helps keep bones healthy. So it’s perhaps natural to assume that vitamin D supplements may help strengthen our bones and protect against fractures and falls. But a large review of the research, published in October, concluded that vitamin D supplements, in low or high doses, play no such role.

“Everybody once thought that it would, but we now have become much more skeptical,” says Rosen. “Supplementation doesn’t increase bone density, so how would it reduce the risk of fractures? The answer is we don’t know. But if your vitamin D levels are sufficient, there’s no rationale for adding more.”

In November, another significant study, funded by the National Institutes of Health, appeared. In this one, researchers recruited nearly 26,000 healthy U.S. adults over 50 to take either a vitamin D supplement or a placebo. After following them for an average of almost 5 1/2 years, the study authors concluded that vitamin D supplements did not lower the risk of cancer, stroke, or heart attack.

And in February, researchers published an analysis of past studies on the link between vitamin D supplements and cancer risk and survival. They found no link between taking a supplement and a lower risk of cancer, but they did connect taking vitamin D supplements to a 13% lower risk of dying from cancer. The study did not look at whether vitamin D supplements were directly responsible for that drop in the cancer death rate by, perhaps, revving up the body’s immune system to fight the disease.

“Could vitamin D improve the immune system enough that it could improve cancer mortality? The answer is maybe,” says Rosen. “It’s not that I don’t believe it. I think it’s attractive but it needs to be tested in a randomized trial.”

Irina Todorov, MD, a family medicine doctor at the Cleveland Clinic, agrees: “Though there are many, many studies about cancer and heart disease and so on, we need more research to identify whether specific numbers, specific levels may be beneficial.”

Supplements, Your Diet, and the Sun

Does this new research mean that your vitamin D supplements serve no purpose? Maybe.

“If you have an adequate level, I don’t see that you’ll get benefits from taking a supplement,” says Pritchett.

But getting enough vitamin D through your diet is difficult. “Very few foods are naturally high in vitamin D, unless you’re talking about fatty fish,” she says.

Some foods, like milk, are fortified with vitamin D, but even if you drink 3 cups of milk a day, says Pritchett, you’ll still fall far short of the 600 IUs that current recommendations advise for people between the ages of 1 and 70.

The best way to be sure to get enough of the nutrient is to go outside and get some sun. When the sun’s ultraviolet rays hit your skin, that light triggers the production of vitamin D in your body. To benefit, you don’t need to -- and, because of skin cancer risk, shouldn’t -- sunbathe. Instead, Todorov recommends only 10 to 15 minutes, two to three times a week between 10 a.m. and 3 p.m.

“Expose yourself, then put on your sunscreen,” she says.

Some medical groups disagree with this advice. The American Academy of Dermatology, for example, recommends against getting vitamin D via sunlight because sun exposure can make skin cancer more likely. The preventive services group also says sun exposure is not generally recommended as a treatment.

But she points out that not everyone can do this. Health reasons or your job may keep you indoors. And, as we age, our skin produces less vitamin D. Also, if you live in an area with high pollution or in a part of the world that gets limited sun, your time outdoors may not give you much vitamin D. In those circumstances, a supplement may make sense.

Certain health conditions also can make you less able to meet your vitamin D needs and may make supplements a useful addition to your diet and outdoor activities. These include Crohn’s disease and celiac disease, as well as liver and kidney diseases. If you’ve had gastric bypass surgery, your body may not absorb nutrients as well as it might have before. Obesity also has been tied to low levels of vitamin D, but, says Rosen, the reason why remains unclear.

If you have any of those health conditions or other things that raise your risk, says Todorov, your doctor may run tests to see if you have enough vitamin D. You may also be tested if you have the bone-softening disease osteomalacia or, in children, a similar condition called rickets. Both often result from a lack of vitamin D, which may be treated with vitamin D supplements. In general, though, such blood tests are not recommended. The Preventive Services Task Force says there’s no evidence that such screenings provide any benefits.

Don’t want to give up your vitamin D supplements while you wait for the experts to answer the many questions about their value? You likely have nothing to worry about as long as you go easy on them. Getting too much vitamin D can bring nausea, vomiting, fatigue, and other unpleasant symptoms caused when vitamin D leads to the production of too much calcium in your body.

“I don’t think there’s any harm in doses up to 4,000 IUs a day,” says Rosen, “but you have to be realistic that it’s unlikely to have an impact on most chronic diseases.”

Show Sources

Kelly Pritchett, PhD, registered dietitian, Central Washington University, Ellensberg, WA.

Clifford Rosen, MD, director of clinical and translational research and senior scientist, Maine Medical Center Research Institute, Scarborough.

Irina Todorov, MD, Center for Integrative Medicine, Cleveland Clinic.

The Lancet: Diabetes & Endocrinology: “Effects of vitamin D supplementation on musculoskeletal health: a systematic review, meta-analysis, and trial sequential analysis.”

Cleveland Clinic: “Vitamin D & Vitamin D Deficiency.”

Annals of Oncology: “Vitamin D supplementation and total cancer incidence and mortality: a meta-analysis of randomized controlled trials.”

New England Journal of Medicine: “Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease.”

NIH Office of Dietary Supplements: “Unit Conversions.”

Journal of the American Medical Association: “Trends in Use of High-Dose Vitamin D Supplements Exceeding 1000 or 4000 International Units Daily, 1999-2014.”

U.S. Preventive Services Task Force: “Final Recommendation Statement: Vitamin D Deficiency: Screening.”

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