A new study from the School of Public Health shows that the number of U.S. adults taking daily vitamin D supplements above the recommended levels has increased dramatically since 1999, and three percent of the population exceeds the daily recommended amount, which poses a potential risk of adverse effects.

“While there has been a lot of buzz surrounding potential health benefits of vitamin D in the media, research findings have been inconsistent and currently the evidence is inconclusive,” says lead author and PhD student Mary Rooney. “More may not always be better. The potential for harm at higher doses — including too much calcium in the blood, fractures, and falls — has been noted in some randomized controlled trials.”

The study was published in the Journal of the American Medical Association and co-authored by Rooney’s adviser, Associate Professor Pamela Lutsey, Professor Lisa Harnack, and PhD student Rachel Ogilvie.

Vitamin D is considered beneficial for bone health. The recommended dietary allowance for vitamin D is 600 IU a day for adults 70 years or younger and 800 IU a day for those older than 70 years.

However, exceeding those limits can be dangerous. A 2011 report concluded that vitamin D intake of more than 4,000 IU a day could be harmful and result in abnormally high levels of calcium in the blood, and soft tissue or vascular calcification.

In looking at just how much vitamin D Americans are taking, Rooney and the research team found that the prevalence of daily supplemental use of 1,000 IU or more in 2013-2014 was 18.2 percent; in 1999-2000, it was 0.3 percent. In 2013-2014, the prevalence of daily supplemental intake of 4,000 IU or more was 3.2 percent; this figure was less than 0.1 percent prior to 2005-2006.

Trends of increasing supplemental vitamin D use were found for most age groups, race/ethnicities, and both sexes as well. In 2013- 2014, intake of 4,000 IU or more daily was highest among women (4.2 percent), non-Hispanic white individuals (3.9 percent), and those 70 years or older (6.6 percent).

“Vitamin D supplements can easily be purchased over-the-counter with or without input from a health care professional,” says Rooney, who recommends consumers use vitamin D supplements with care. “Consumers should be aware of both potential benefits and harm before supplementing vitamin D, particularly at high doses.”