February 22, 2017 1256 GMT
A new suggests the sunshine vitamin can help reduce the risk of respiratory infections, including colds and flu — especially among people who don't get enough of the vitamin from diet or exposure to sunlight.
Researchers pooled data from 25 studies that included more than 10,000 participants. The studies looked at whether vitamin D supplements cut the number of infections, npr.org reported.
"We found that overall there was a modest protective effect," said Dr. Adrian Martineau, a clinical professor of respiratory infection and immunity at Queen Mary University of London who led the research team. Overall, he said, vitamin D supplements seemed to reduce the risk of infection about 10 percent.
People who had been vitamin D deficient when they enrolled in the studies saw more benefit. Their risk of infection was cut in half, according to the findings.
"What we found is that those with the lowest vitamin D levels experienced the greatest benefit from supplementation," Martineau said.
Acute respiratory infections are responsible for millions of emergency department visits in the United States each year, said Carlos Camargo, a professor of epidemiology at Harvard Medical School and a senior author of the study. He said their findings support the notion that more foods should be fortified with vitamin D.
The US has long fortified foods with vitamin D, including milk and other dairy products. Many food companies supplement orange juice products and cereals, too. And certain foods like sardines and other oily fish naturally contain significant levels of vitamin D. For instance, a serving of salmon can have almost a whole day's worth of vitamin D. And the body produces its own vitamin D when exposed to sunlight.
So not everyone is convinced that this study should lead us all to the supplement aisle.
According to recommendations from the Institute of Medicine, most adults need about 600 IU (international units) of vitamin D per day. Adults 70 years old and older are advised to increase their intake to 800 IUs per day.
Many Americans don't routinely get this amount from their diets, so a multivitamin can help make up the shortfall. "The standard multivitamin has about 400 IUs," said Steven Abrams, a professor of pediatrics at the Dell Medical School at the University of Texas at Austin. He was a member of the Institute of Medicine panel that wrote the vitamin D recommendations for children and adults.
Abrams said the guidelines, issued in 2010, were based on evidence that vitamin D helps protect bone health. "Now the controversy is whether there's enough evidence about other diseases to make [new] recommendations."
Over the last 10 years, a number of studies have suggested that the sunshine vitamin can help prevent disease. That has led people to think that higher doses of supplements are better. But Abrams said he's not convinced there's a benefit of taking a supplement for people who are not deficient. "It needs further studies to confirm."
Abrams said the importance of the new study is that it's a summary of 25 controlled trials. "And it shows that people with very low vitamin D [levels] do better when they're given supplements." He said this is not too surprising. "If you're deficient, getting an adequate amount will make a difference."
In other words, if you're getting the recommended 600 IUs of vitamin D from your diet, a supplement may not lead to any further benefit. But the growing interest in vitamin D has lots of people curious about their levels.
Certain groups of people are at higher risk of vitamin D deficiency, including people with digestive disorders such as celiac disease and people who cover up most of their skin or get very little exposure to the sun. And pregnant and nursing women, as well as women with osteopenia or osteoporosis, often need more vitamin D to maintain bone health.