Fortifying food with vitamin D would save more than 3 million people a year from suffering with chronic colds and flu, a landmark study suggests today.
It claims that consuming the vitamin at least once a week slashes the risk of some people falling ill with a respiratory infection by up to 50 percent.
The study, led by British scientists, suggests that if everyone in the UK took vitamin D pills, or ate vitamin-fortified food, it could cut the number of people infected with flu or colds by at least five percentage points, meaning 3.25 million people every year would avoid the illnesses.
Chronic respiratory infections are the most common illnesses in the world, and are responsible for a quarter of GP appointments in Britain.
Every year, 70 percent of the UK population suffer with at least one bout of flu, coughs or colds, causing 300,000 hospital admissions and 35,000 deaths.
The researchers in the study, from Queen Mary University of London, admitted it was 'unrealistic' to expect everyone to take supplements. Instead, they called for vitamin D to be added to foods such as milk or bread - a policy already in the US, Canada, Sweden, Finland and Australia.
Nearly a third of the British population is deficient in vitamin D, thanks to modern diets, indoor lifestyles and grey weather.
During the spring and summer, the skin makes vitamin D when it is exposed to the sun. But in the autumn and winter, most people have to rely on their diet to get enough. They can do that by eating liver, eggs, red meat and plenty of oily fish - but millions who do not eat enough of these foods should take supplements instead.
The study, published in the British Medical Journal, analysed data from nearly 11,000 participants who took part in 25 clinical trials conducted in 14 countries. Among people who were already deficient in vitamin D, taking supplements reduced the risk of respiratory infection by 50 percent, and among others the risk dropped by up to 20 percent.
Lead researcher Professor Adrian Martineau, of Queen Mary University of London, said, "This major collaborative research effort has yielded the first definitive evidence that vitamin D really does protect against respiratory infections. Our data has provided a whole new reason to back up the advice about vitamin D supplementation."
But he said it was not realistic to expect everyone to take pills. "Either the whole adult population takes an over-the-counter supplement or moves are made to introduce food fortification. Vitamin D really does protect against respiratory infections. By demonstrating this benefit, our study strengthens the case for introducing food fortification to improve vitamin D levels."
His team is now planning a study looking directly at the benefits of food fortification. Dr Benjamin Jacobs, consultant paediatrician at the Royal National Orthopaedic Hospital, said, "The case for universal vitamin D supplements, or food fortification, is now undeniable." A Department of Health spokesman said the government would "remain open" to considering the results of future studies, adding, "Mandatory food fortification is a complex issue, but experts keep evidence under review."
However, ministers have a poor track record on food fortification. Scientists have long called for flour to be fortified with folic acid to reduce birth defects, but the government has so far refused.