We've always been told to venture outside every so often for a dose of vitamin D, particularly in winter when the sun rarely bothers to show itself in the UK. But have you ever stopped to think about why the so-called "sunshine vitamin" is so crucial for your health? Its main function is in maintaining healthy bones, teeth and muscles but, according to new research, it has a far greater impact on our health. Writing in the British Medical Journal, researchers from Queen Mary University of London said the rise in colds and flu in winter is due, at least in part, to a lack of sunlight and therefore a lack of vitamin D in the body. They claim there is a link between the vitamin and respiratory tract infections, including flu, bronchitis and pneumonia, which affect millions of us each year. By analysing and pooling data from 25 clinical trials involving more than 11,000 patients from 14 countries, they were able to highlight the benefits of vitamin D, particularly when taken daily or weekly rather than monthly. Professor Adrian Martineau, one of the researchers, said: "Assuming a UK population of 65 million, and that 70% have at least one acute respiratory infection each year, then daily or weekly vitamin D supplements will mean 3.25 million fewer people would get at least one acute respiratory infection a year," the BBC reported. Taking vitamin D supplements would even be more effective than a flu vaccination, the research suggested, sparing infection for one person in every 33, compared with one in 40 who receive a flu vaccination. Public Health England already recommends people take vitamin D supplements in the winter months for bone and muscle health, but the new research gives us even more reason to pop to the nearest Boots or Superdrug. The researchers are now calling for our food to be fortified with the vitamin, like in the US, where it's added to milk. "Vitamin D fortification of foods provides a steady, low-level intake of vitamin D that has virtually eliminated profound vitamin D deficiency in several countries," Professor Martineau said. "By demonstrating this new benefit of vitamin D, our study strengthens the case for introducing food fortification to improve vitamin D levels in countries such as the UK where profound vitamin D deficiency is common." As well as respiratory infections, a lack of vitamin D can also lead to osteomalacia, a condition involving severe bone pain and muscle aches, and to rickets in children, where the soft and weak bones become misshaped but continue to grow. Too much, however, can lead to an excess of calcium in the bloodstream and may lead to heart and kidney problems.