What we know — and don't know — about the benefits of vitamin D

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I have a lot of questions about vitamin D.

Our bodies synthesize vitamin D from sun exposure and absorb it from many foods. It’s important for bone health, and you may have heard about observational studies that suggested people with higher blood levels of vitamin D had a number of better health outcomes, including improved cardiovascular and brain health.

But I keep seeing compelling studies that show supplements of vitamin D do not seem to convey all of the health benefits expected, and articles poking holes in the claim that many people are deficient in D.

With the end of winter around the corner, I figured this might be a moment when a person living in a chilly, northern clime and sitting indoors most of the workweek — like me — might have vitamin D on the mind. So I called the leader of the largest, randomized vitamin D trial ever conducted to see if she could help me sort fact from fiction.

This researcher’s name is Dr. JoAnn Manson, and she’s chief of the Division of Preventive Medicine at Brigham and Women’s Hospital and a professor of medicine at Harvard Medical School. She’s also the principal investigator of the Vitamin D and Omega-3 Trial (VITAL), which followed nearly 26,000 participants across the U.S. for a little over five years.

Manson told me vitamin D is actually a hormone that stimulates receptors in nearly every cell in the body. This may help explain why there was so much optimism about its potential to improve health.

However, she said the results of randomized clinical trials — including VITAL — have not borne out the many hopes for vitamin D, at least taken by itself. Here are some highlights from our conversation:

‘Limited benefits’ from vitamin D supplements 

Participants in the VITAL trial received a daily vitamin D3 softgel with a dose of 2,000 IUs, a daily omega-3 capsule of one gram, both supplements or a placebo.

Manson and her colleagues “saw no effect of vitamin D supplementation on cardiovascular disease [or] on total cancer incidence,” she told me. They also saw no effect for reducing depression, cognitive decline, atrial fibrillation or macular degeneration. The supplements didn’t even significantly reduce the risk of bone fractures, the researchers found.

“The interpretation of this should not be that vitamin D doesn’t matter, but rather that we need only small to moderate amounts of vitamin D,” Manson said. “Most people do get enough from their diet and from incidental sun exposure to derive these benefits from vitamin D. Further supplementing vitamin D has really limited benefits.”

Cancer, autoimmune diseases and COVID

Some of the most promising evidence from the VITAL trial involved advanced and metastatic cancer. Although overall cancer diagnoses weren’t reduced, the researchers noticed fewer cancer deaths among participants receiving vitamin D supplements.

“[It’s] suggesting vitamin D may have a role in affecting tumor biology, [or] may make the tumors themselves less likely to become invasive and metastatic, and less likely to be fatal cancers,” Manson said.

Another promising area was autoimmune diseases such as rheumatoid arthritis and psoriasis.

“We saw [a] 22% reduction in autoimmune diseases,” Manson told me. “We do know that vitamin D can improve immune function, can tamp down inflammation, which is why we’re studying it now for reducing severity of COVID and preventing long COVID. We hope to have those results published by later this spring.”

There were hints in the data that Black adults might benefit more than other groups from vitamin D supplementation to reduce memory loss and total invasive cancer, but Manson cautioned more research is needed here.

Another intriguing finding suggested people with a body mass index under 25 might benefit more from supplementation than those with obesity or who are overweight. But again, more exploration is needed, Manson said.

The myth of vitamin D deficiency

Most people in the VITAL study were not vitamin D deficient – a condition that absolutely should be treated, Manson emphasized, because it has known risks for bone health. Those most at risk of deficiency include nursing home residents, people taking medication for osteoporosis, and people with certain conditions like Crohn’s disease, celiac and a few others. But for almost everyone else, testing and supplementation isn’t necessary.

If you worry about vitamin D levels, Manson recommends eating more vitamin D-rich foods, like fortified dairy products, wild mushrooms and fish such as salmon, sardines and tuna.

You can also go outside in the middle of the day for 15 minutes a couple of times a week, she said, although people should avoid prolonged sun exposure without sunscreen because that can increase the risk of skin cancer and skin aging.

If you still feel concerned, Manson said daily vitamin D supplements of 1,000 to 2,000 IUs a day are a “very reasonable” option, especially during winter months. “We know that dosage is very safe long-term,” Manson told me.

“However, it’s not essential to do that,” she added.

Manson recommends resisting any advice to megadose. The long-term safety of doses like 5,000 or 10,000 IUs a day has not been well tested, she said.

Go outside, and consider a multivitamin

One of my biggest takeaways from our conversation is to exercise outdoors when possible, because physical activity is great for health, and it can bring benefits in addition to vitamin D, such as improving mood and feeling a connection with nature. I’m also strongly considering adding a multivitamin to my daily routine.

When it comes to cognition, Manson said the evidence is “much stronger” for multivitamins than for vitamin D as an isolated supplement. One large-scale, placebo controlled trial known as the COSMOS Trial recently found promising evidence that a daily multivitamin can slow memory loss and cognitive decline in older adults.

Of course, it would be nice if one pill could magically improve many aspects of health. But as Manson notes, vitamin D and other dietary supplements “will never be a substitute for a healthy diet and a healthy lifestyle.”

“Being physically active outdoors is really as close to a magic bullet for good health as we’re going to come,” she said.

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