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More People Are Taking Vitamin D Than Ever. Here’s Why That Might Be Risky

More people are taking vitamin D supplements today than they were 20 years ago, but that’s not necessarily a good thing. According to a new paper published today in JAMA, more than 18% of the population gets excess vitamin D—and more than 3% takes doses so high they could be dangerous.

Vitamin D is known as the sunshine vitamin; in order for humans to produce it, their skin must be exposed to sunlight. People can also get vitamin D from fortified foods (like milk, orange juice, and cereal in the United States), some fatty fish, and from dietary supplements.

Scientists know that vitamin D is important for bone and brain health, and in recent years, the nutrient has been touted as a potential “miracle” treatment for everything from headaches to heart disease to the flu. Vitamin D deficiencies have been linked to numerous health problems, and low levels are common in the U.S., especially during the winter.

For these reasons, supplement companies have touted the potential benefits of a daily vitamin D pill, and it’s not unusual for doctors to recommend them for certain patients.

But studies on vitamin D haven’t been overwhelmingly positive. A 2011 Institute of Medicine report concluded that doses of more than 4,000 IU a day could cause unusually high levels of calcium in the blood, which could lead to hardening of blood vessels or soft tissues. Other studies have found little to no benefit, in terms of cancer or heart-disease risk, of vitamin D at higher than “normal” levels.

With all that in mind, researchers at the University of Minnesota School of Public Health wanted to see if there’s been an increase in people taking high doses of these supplements. “There’s been a lot of buzz around vitamin D and its potential benefits, and we were curious if that meant people were taking more of it,” says lead author and PhD student Mary Rooney.

To find out, she and her colleagues analyzed data from nearly 40,000 adults who responded to health questionnaires between 1999 and 2014. The participants were asked about their daily supplement intake for the past 30 days, and were asked to bring along their pill bottles to ensure accuracy in their responses.

The recommended dietary allowance for vitamin D is 600 IU a day for people 70 and younger, and 800 IU a day for older adults—an amount that most people can get naturally from food and/or a few minutes of sun exposure. And in the first year of the study, 1999–2000, only 0.3% of the participants reported taking vitamin D in doses significantly higher than that, at 1,000 IU a day or more.

By 2013–2014, however, that number had jumped to 18.2%. “I thought we would see an increase, but it was to a greater extent than expected,” says Rooney.

Similarly, less than 0.1% of people took doses of more than 4,000 IU a day in the years prior to 2005–2006. But by the end of the study, 3.2% were taking these potentially dangerous doses.

Vitamin D supplementation increased over the study period across most ages groups and ethnicities, and for both genders. Extremely high doses of vitamin D were most common among a few specific groups: In the last year of the study, 4.2% of women, 3.9% of non-Hispanic white people, and 6.6% of adults 70 and older reported taking at least 4,000 IU a day.

But along with concerns about dangerously high calcium levels, high-dose vitamin D supplementation has also been associated with increased risks of fractures, falls and kidney stones. Some observational studies have even linked high vitamin D levels to higher rates of prostate cancer, pancreatic cancer, and early death.

Vitamin D screenings aren’t currently recommended for healthy adults who don’t have symptoms of a deficiency, so ask your doctor if you have concerns you might not be getting enough.

While multivitamins typically contain about 400 IU of Vitamin D, other over-the-counter supplements can contain 1,000, 2,000, or even 5,000 IU per pill. Rooney cautions against taking Vitamin D—or any supplement—without talking to your doctor first.

“More may not always be better,” says Rooney. “A lot of the discussion on vitamin D has been around the potential benefits, but people should also be aware that there may be potential harms.”