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A meta-analysis of 25 randomized controlled trials involving over 11,000 participants confirmed that vitamin D supplementation can stave off acute respiratory infections.

“Most people understand that vitamin D is critical for bone and muscle health,” said Carlos Camargo, MD, DrPH, of the Department of Emergency Medicine at Massachusetts General Hospital (MGH) and the study’s senior author. “Our analysis has also found that it helps the body fight acute respiratory infection, which is responsible for millions of deaths globally each year.”

This was a highly challenging work since many of these studies had different designs or participant qualifications. Some concluded that low Vitamin D levels were linked to a greater risk of developing an acute respiratory infection. Other clinical trials, on the other hand, which investigated the protective abilities of Vitamin D supplements reached opposing conclusions. Some found Vitamin D supplementation staves off infections while other found no conclusive evidence that this is the case.

The team led by Adrian Martineau from the Queen Mary University of London aggregated all of the data from these 25 trials by conducting an individual participant data meta-analysis. Typically, a meta-analysis averages data from all participants in each study. This was not the case. The researchers, instead, separated out the data from each individual participant to obtain a higher resolution analysis of the data from all these mammoth studies.

The conclusion is that daily or weekly supplementation had the greatest benefit for those individuals that had a Vitamin D deficiency, to begin with. Those with the lowest levels of Vitamin D (blood levels below 10 mg/dl) cut their risk of respiratory disease by half. All participants experience some beneficial effect from regular Vitamin D supplementation. Occasional high doses of vitamin D had no effect.

“Acute respiratory infections are responsible for millions of emergency department visits in the United States,” says Camargo, who is a professor of Emergency Medicine at Harvard Medical School. “These results could have a major impact on our health system and also support efforts to fortify foods with vitamin D, especially in populations with high levels of vitamin D deficiency.” The study was funded by a grant from the National Institute of Health Research (U.K.).

According to recommendations from the Institute of Medicine, most adults need about 600 IU (international units) of vitamin D per day while the elderly (over 70 years) are advised to intake 800 IUs per day.

We get our Vitamin from what we eat but also sourced from our own bodies as these produce Vitamin D when in contact with sunlight. A good Vitamin D-rich diet might include milk and other dairy products, orange juice, cereal, as well as sardines and other fish products which contain a high level of Vitamin D.

Since this is a meta-study, you should take the conclusion with caution. After all, some of the studies found that supplements don’t work. The standard multivitamin has about 400 IUs but if you’re already intaking 600 IUs, it’s not clear you need to take a supplement despite the current meta-analysis suggests ‘all participants can experience beneficial effects from supplementation.’

It’s best you visit your doctor who can check your Vitamin D levels. She will tell you the right course of action. Chances are you’re not deficient.

The findings appeared in The New England Journal of Medicine.