Guide to Vitamins

Should I Take Vitamin D?

30 to 50% of Americans are low in vitamin D. Are supplements the answer? It’s complicated.

Illustrations by Kieran Blakey

This story is a part of The Elemental Guide to Vitamins. Here are the vitamins and supplements that are covered: multivitamins, vitamin D, vitamin C, calcium, B vitamins, omega-3, vitamin E, fiber, protein, and probiotics.

VVitamin D is an essential nutrient. That means the human body needs it but can’t make it on its own. While people can get vitamin C, calcium, and pretty much all of the other essential nutrients from their diet, few foods contain much vitamin D. Humans have historically gotten their vitamin D fix from sunlight; ultraviolet (UV) light exposure causes a chemical reaction in the skin that allows the body to produce vitamin D.

As more and more people have adopted indoor lifestyles and sun-protection habits that largely deprive their skin of UV light, rates of vitamin D deficiency have increased. By some estimates, between 30% and 50% of Americans have low blood levels of vitamin D. Most doctors define vitamin D deficiency as anything below 20 nanograms per milliliter of blood, although there’s some expert disagreement when it comes to optimum and insufficient vitamin D levels.

Worldwide, low vitamin D status is so prevalent that some doctors have referred to it as a pandemic. Studies have found evidence that low vitamin D levels are associated with a wide range of health issues, including bone weakness, obesity, cardiovascular disease, cancer, and autoimmune disorders. Research has also found links between low levels of vitamin D in developing fetuses and later risk for autism spectrum disorders.

Taking a vitamin D pill can seem like a great way to safely boost the body’s levels of the vitamin and also avoid or reduce the health risks associated with a deficiency. But over and over again, high-quality studies have mostly failed to find evidence that vitamin D supplements improve health outcomes.

“I do think there’s emerging evidence that vitamin D supplementation may reduce the risk of cancer-related death.”

“The randomized controlled trials on vitamin D supplementation have shown limited benefits,” says Dr. JoAnn Manson, a professor of medicine at Harvard Medical School. Manson was the lead researcher on the five-year, National Institutes of Health-funded VITAL trial, which is one of the largest and most comprehensive vitamin D studies to date. She and her colleagues found that people taking 2,000 IU of vitamin D3 a day were at slightly lower risk for cancer, but that drop in risk was not statistically significant. When it came to heart attacks, stroke, and cardiovascular disease-related deaths, vitamin D pills did not provide a benefit, her trial found.

“I do think there’s emerging evidence that vitamin D supplementation may reduce the risk of cancer-related death,” she says. But vitamin D supplements don’t seem to be the panacea many make them out to be.

So why is low vitamin D associated with so many health problems? Some researchers have argued that reduced vitamin D levels are a consequence, rather than a cause, of disease or disorder. For example, there’s evidence that fat cells absorb vitamin D, which helps explain why lower levels of the vitamin turn up in the blood of people with obesity — and also why taking vitamin D supplements does not seem to aid weight loss.

Even when it comes to a person’s risk for bone diseases like osteoporosis, some studies have found that vitamin D supplements do no good. They may even be harmful at high doses; a 2019 study from the University of Calgary found people who took either 4,000 international units (IU) or 10,000 IU of vitamin D daily for three years were at a greater risk for weak bones than those who took a placebo. “[Vitamin] D and calcium may possibly be useful for osteoporosis,” says David Jenkins, a professor of nutritional sciences at the University of Toronto who has conducted research on vitamin D. “But we are not as certain as we used to be.”

Also dubious is the now-common practice of testing a person’s blood for a vitamin D deficiency. The U.S. Preventive Services Task Force has found “insufficient” evidence that screening people for a deficiency can help doctors improve patient health outcomes, in part because the research on the benefits of vitamin D supplements is inconsistent or underwhelming.

The bottom line: It’s possible that future research efforts will bolster the now-shaky case that vitamin D pills can help people avoid cancer and, maybe, some other medical conditions. But as of today, America’s enthusiasm for vitamin D supplements is mostly not supported by the evidence.

This story is a part of The Elemental Guide to Vitamins.

I write about health and science. I live in Detroit with my wife and kids. I’m trying to learn German, but my progress so far is nicht gut.

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