Do vitamin D supplements do anything?

Vitamin D supplements may not be an effective defense against acute respiratory infections, a recent study has found.

It’s still possible the vitamin supplementation could reduce the severity and duration of such infections, “but [we] did not address this question,” said Adrian Martineau, a clinical professor of respiratory infection at Queen Mary University of London and the study’s senior author.

The paper appeared in the Lancet Diabetes & Endocrinology journal. It analyzed data pooled from nearly four dozen clinical trials involving more than 60,000 participants.

Despite this finding, vitamin D does provide other important health benefits. It helps prevent musculoskeletal diseases such as rickets and osteomalacia, for example, and is essential for absorbing calcium, reducing the risk of osteoporosis and fractures that can result from this bone-thinning disease.

“So if people are already taking a vitamin D supplement for musculoskeletal health, either prescribed or over-the-counter, they should not stop taking it,” Martineau said. “Our findings relate only to the question of protection against” acute respiratory infections.

Vitamin D is one of the supplements most commonly taken by U.S. adults, according to the Centers for Disease Control and Prevention. The recommended dietary allowance for vitamin D per day is 600 international units (IU) for most ages, 400 IU for infants and 800 IU for those older than 70.

A 2021 paper by the same team had found a slight but statistically significant protective effect against acute respiratory infections from taking vitamin D supplements.

But the most recent analysis added the results of three new large trials to the previous data and found a negligible — or nonstatistically significant — protective effect overall from the supplements.

“Our updated meta-analysis provides more evidence that we need only small-to-moderate amounts of vitamin D, and most people are already getting what they need without taking supplements,” said JoAnn E. Manson, one of the authors of the recent study.

Vitamin D levels in some of the study participants were low, even deficient, yet supplementation “didn’t lead to lower rates of infections in most of these randomized trials,” added Manson, chief of the division of preventive medicine at Brigham and Women’s Hospital and a professor of medicine at Harvard Medical School.

The vitamin D dosage amounts and frequency varied widely among the trials, Martineau said.

The researchers said they used standard statistical methods to determine the vitamin’s overall effects on acute respiratory infections and whether some groups might have benefited more than others. The impact of vitamin D supplementation on COVID-19 did not factor into their analysis because few studies were eligible for inclusion or the authors did not respond to a request to share data, Martineau said.

“Our major finding was that there was no statistically significant protective effect overall,” nor “did we show that any subgroup benefited more than any other,” Martineau said.

Extensive research shows that adequate vitamin D “is critically important for immune function and the ability to fend off viral and bacterial infectious diseases,” Manson said.

But the amount of vitamin D needed for that defense may be lower than previously understood and achievable through diet and sun exposure for most people, she said. For them, “adding vitamin D supplements wasn’t better than taking an inert placebo,” Manson said.

However, she added, several studies “have shown that those who have profoundly deficient blood levels of vitamin D do benefit from supplementation in reducing their risk of infection.”

In the randomized trials examined in the recent study, few participants had such levels, which may have contributed to the “lack of efficacy of vitamin D supplementation in these trials overall,” she said.

Donald Hensrud, a nutrition specialist at the Mayo Clinic, who was not involved in the study, pointed out the 2021 paper and the recent study both showed a relatively small effect of vitamin D on acute respiratory infections, 8% vs. 6%. “In other words, vitamin D probably does not help reduce acute respiratory infections according to the updated study, but even if it did, the effect is relatively small,” he said.

People should try to obtain at least the recommended daily allowance, Hensrud said. “This can sometimes be provided in the diet, especially if people consume dairy products, but if people want to be more certain they are getting enough, they could take a multivitamin or a small amount of vitamin D,” he said.

Manson stressed that individuals should consider supplementation under certain circumstances - for example, nursing home residents who may have little sun exposure; people with dietary restrictions, or malabsorption conditions such as Crohn’s or celiac disease; and those with osteoporosis or other bone health problems.

“If you’re part of a high-risk group for vitamin D deficiency, then talk to your doctor about taking a supplement and being tested for vitamin D blood levels,” she said. “Importantly, be physically active outdoors and eat healthfully to lower your risk of chronic disease. Dietary supplements will never be a substitute for a healthy diet and lifestyle.”