Excess vitamin D can cause calcium to build up in the blood, leading to fatigue, nausea, constipation, headaches and other symptoms.
The proportion of Americans who are taking vitamin D supplements at amounts that exceed the recommended limit has risen dramatically in recent years — jumping 590 percent between 1999 and 2014, according to a University of Minnesota study published this week in the Journal of the American Medical Association (JAMA).
The study also found that more than 3 percent of adults in the United States currently take vitamin D supplements at doses that have been associated with serious health problems.
Excess vitamin D can cause calcium to build up in the blood, leading to fatigue, nausea, constipation, headaches and other symptoms, and may cause, over time, the development of kidney stones. High levels of vitamin D have also been associated with a greater risk for certain cancers (particularly prostate and pancreatic cancers) and even early death.
The study’s findings underscore the ongoing confusion that many people have about vitamin D recommendations — confusion that has led millions of people to take the supplement unnecessarily and, in some cases, dangerously.
“One in five U.S. adults are consuming vitamin D supplements,” said Mary Rooney, the lead author of the study and a doctoral student at the U of M, in a phone interview with MinnPost. “That’s pretty notable.”
It’s particularly notable given that experts say only 13 percent of Americans aged 1 to 70 are at risk of being deficient in vitamin D — and that less than 6 percent are actually deficient.
A source of the confusion
As I’ve written here before, in 2011, the Institute of Medicine (IOM, now the National Academy of Medicine) determined that healthy people (those who do not have a specific medical condition that causes a vitamin D deficiency) require, on average, 400 international units (IUs) of vitamin D per day — until they reach the age of 71, when they require, on average, 600 IUs per day.
But to ensure that everybody gets enough of the nutrient, the IOM set the recommended dietary allowance (RDA) for vitamin D at amounts that reflect the upper end of the distribution of the biological requirement — in other words, at the highest amount that any healthy person would need: 600 IUs per day for people aged 1 through 70, and 800 for those 70-plus.
Yet, as already noted, most people need less than that amount — and very few need to take supplements to reach the recommended levels. A single cup of fortified milk contains, for example, 115 to 124 IUs of vitamin D, while three ounces of cooked salmon contains 447 IUs.
Furthermore, just 5 to 30 minutes of sunlight exposure on the face, arms or legs between 10 a.m. and 3 p.m. twice a week is usually enough to meet most people’s vitamin D needs — including those living in Minnesota and other northern latitudes.
For their study, Rooney and her U of M colleagues examined the supplement usage of a nationally representational sample of almost 40,000 adults aged 20 or older who participated in the National Health and Nutrition Examination Survey (NHANES) for the years 1999-2000 and 2013-2014.
They found that in 2013-2014, 18.2 percent of the survey’s participants said they were taking vitamin D supplements of 1,000 IU or more. That compared with only 0.3 percent of the participants in 1999-2000. These trends were found for most age groups, races/ethnicities and both genders.
In addition, 3.2 percent of the survey’s participants in 2013-2014 said they were taking daily vitamin D supplements of 4,000 IU or more — levels believed to pose considerable health risks. Fifteen years earlier, this figure was less than 0.1 percent.
This trend — the use of exceptionally high doses — was highest among women (4.2 percent), whites (3.9 percent) and those 70 years or older (6.6 percent).
Limitations and implications
The study has several limitations, particularly the fact that the participants self-reported their supplement use. Such reports are not always reliable.
Still, as Rooney and her colleagues point out in the study, the results have “important and complex public health and clinical implications.”
Many of those 1 in 5 Americans taking vitamin D supplements may, at the very least, be wasting their money. And some may be endangering their health.
Before taking vitamin D, individuals should talk with their healthcare provider and “weigh any potential benefits with any harms,” said Rooney.
Also, be wary of all the health claims swirling around vitamin D — and other supplements.
“If you see anything that’s being [called] a cure-all, have some skepticism,” she said.
FMI: An abstract of the study, which was published as a research letter, is available on the JAMA website, but — despite being partially funded by taxpayer money through the National Institutes of Health — the full study is behind a paywall.