Too much of this vitamin can be detrimental to your health

By Charlie Williams
Published September 17, 2020

Key Takeaways

When vitamin D was discovered in 1920, it marked the culmination of a long search for a cure for rickets, a childhood disease that weakens bones. Quickly, scientists began wondering how else this vitamin could promote health.

They soon determined that vitamin D keeps bones healthy by increasing the intestinal absorption of calcium. Without it, not only can children get rickets, but adults find themselves at increased risk of osteoporosis, fractures, and falls. The risk of certain cancers may also increase when vitamin D levels are insufficient. Studies suggest prostate, colon, and breast cancer rise in latitudes far from the equator—sun exposure and vitamin D levels may be part of the explanation. On the other hand, when we get sufficient vitamin D, our bodies may be better positioned to ward off dementia, heart disease, mood disorders, and diabetes. Vitamin D may even play a role in warding off COVID-19, as suggested by a recent study from University of Chicago Medicine.

“I would not mind recommending--and I do it myself—taking vitamin D supplements,” said Dr. Anthony Fauci during a recent interview.

Whether vitamin D helps or harms us depends on dose (and in the case of this unique vitamin, the amount our bodies produce). Unfortunately, new research suggests that in the quest to ensure we get sufficient vitamin D, many Americans are taking far too much of the supplement, which can lead to health risks that are as dangerous as those associated with getting too little of the vitamin.

Too little or too much?

There are two ways our bodies take in vitamin D—through sun exposure and by consuming vitamin D-rich foods and supplements. But there are also challenges to each approach. Too much sun exposure may lead to sun poisoning, melanoma, and aesthetic concerns like wrinkles and sun spots. Likewise, dietary sources of vitamin D present their own problems. The vitamin is naturally present in a few foods (including some fish, milk, orange juice, eggs, and cheese), which aren’t compliant with many diets. Supplements are a good choice for some, but for many, they are unnecessary and increase the likelihood of undermining the benefits of vitamin D and can even increase health risks.

National survey data gathered between 1999 and 2014 found a 2.8% increase in the number of people taking potentially unsafe amounts of vitamin D. The daily recommended dose for vitamin D is 600-800 IU. The same survey identified the upper limit of vitamin D consumption to be 4,000 international units (IU) daily. Anything more was deemed unsafe. During the survey period, there was an 18% increase in the number of people supplementing with 1,000 IU or more of vitamin D daily, which is beyond the recommended daily dose for most people.

Daily intake beyond 4,000 IU increases risk for hypercalcemia, a condition characterized by too much calcium in the blood, which can weaken bones, create kidney stones, and interfere with heart and brain function. Research suggests excess vitamin D can also cause soft tissue or vascular calcification—mineral deposits that can increase the risk of stroke and blood clots. We know that too little vitamin D might lead to certain cancers, but the same appears to be true for too much. Some epidemiological investigations have reported adverse associations of high vitamin D levels with prostate cancer, pancreatic cancer, and all-cause mortality.

Another study published in the New England Journal of Medicine with more than 25,000 participants found that those taking a vitamin D supplement didn’t have lower rates of heart attack, stroke, or cancer. There was one potential advantage: Among trial participants who eventually developed cancer, those who took vitamin D supplements for at least two years had a 25% lower chance of dying from their cancer than those who received a placebo.

“Research on vitamin D and calcium supplementation has been mixed and, especially when it comes to randomized clinical trials, has been generally disappointing to date,” said Dr. JoAnn E. Manson, professor of women’s health at Harvard Medical School, in an interview with Harvard Health Publishing.

Vitamin D and moderation 

Getting enough vitamin D used to be as simple as spending time outdoors. Your body and that big, shiny star in the daytime sky would take care of the rest. But, times have changed. Modern living—including desk jobs, diet, the pandemic, and ensuing quarantine—makes getting enough vitamin D more difficult. 

Our understanding of how vitamin D affects health has advanced in recent years, but much remains to be discovered. The studies we use to draw conclusions on the effects of vitamin D supplementation are controversial because their methodology has been inconsistent—some include only women, others both men and women, and others only elderly people. What’s more, certain studies measure people who use vitamin D alone, while others use combinations of vitamin D and calcium. Most of the participants use different doses, too.

Until we have more evidence, scientists recommend that healthy people limit supplement intake to between 600 and 800 IU daily, or, if possible, forego supplements entirely and get their vitamin D from foods like salmon, tuna, sardines, milk, and eggs.

Share with emailShare to FacebookShare to LinkedInShare to Twitter