Vitamin D—the“sunshine vitamin”—is best known for its role in bone and brain health. But a collection of evidence suggests that it can also protect against cold and flu, according to a new scientific review of 25 published studies. The new research supports the theory that it also boosts immunity and helps fight acute respiratory infections.
Several observational studies have linked low vitamin D levels to greater susceptibility to respiratory infections. However, clinical trials on vitamin D supplementation have had conflicting results, with only some finding significant benefits. For the new study, a meta-analysis published in The BMJ, researchers looked at individual data from nearly 11,000 people who had participated in randomized controlled trials in more than a dozen countries.
The researchers found that people who took daily or weekly vitamin D supplements were less likely to report acute respiratory infections, like influenza or the common cold, than those who did not. Those who had low vitamin D levels before they started supplementation got the biggest benefit: For people with the most significant vitamin D deficiencies (blood levels below 10 mg/dl), taking a supplement cut their risk of respiratory infection in half.
People with higher vitamin D levels also saw a small reduction in risk: about 10%, which is about equal to the protective effect of the injectable flu vaccine, the researchers say. No significant benefits were associated with high doses of vitamin D spaced out over larger periods of time.
Vitamin D is thought to protect against illness by boosting levels of natural, antibiotic-like peptides in the lungs. This may be one reason why colds and flus are most common in the winter, when sunlight exposure (and therefore the body’s natural vitamin D production) is at its lowest, the researchers say. It may also help explain why vitamin D appears to be protective against asthma attacks, which can be triggered by respiratory viruses.
The study authors, who hail from 21 institutions worldwide, say that the findings should encourage wider efforts to fortify foods with vitamin D, especially in countries where deficiency rates are high. (The United States fortifies milk, orange juice and cereal, but other countries, such as the United Kingdom, do not.) Because respiratory infections are responsible for millions of hospital visits and deaths every year, such an action could have a major impact on public health, they say.
What the findings mean for individuals is less clear. In the United States, vitamin D deficiency is less common than in other countries, and screenings are not currently recommended for people who don’t have symptoms of a deficiency.
Measuring vitamin D levels can be expensive, says lead author Adrian Martineau, professor of respiratory infection and immunity at Queen Mary University of London. “The first priority from a public health perspective is to make sure that there are as few people as possible having the lowest levels,” he says in an email. “This is best achieved via strategies like fortification of foods with vitamin D, and not by testing individuals and topping up those with lowest levels.”
But, he adds, the study does provide “the strongest evidence yet that there are benefits for the immune system," in addition to the already-known benefits for bone and muscle. For people in the United Kingdom, he recommends taking a regular supplement at least in the winter and spring.
Even though people in the United States are less likely to have a deficiency, supplementation is worth discussing with a doctor if you get a lot of acute respiratory infections—particularly if you have asthma or chronic bronchitis, where the consequences of these infections can be particularly serious, he says.