Vitamin D supplements offer protection against acute respiratory infections, including colds and flu, a major new study has found.

According to UK researchers, their findings provide the strongest evidence yet that the benefits of vitamin D are not just restricted to bone and muscle health.

Vitamin D is known as the ‘sunshine vitamin' because the body makes it when exposed to the sun. It is also found naturally in some foods, such as oily fish. However, these foods do not tend to be eaten in large amounts by much of the Irish population.

Currently in Ireland, while some foods are fortified with vitamin D, this is not compulsory and is at the discretion of the manufacturer. Supplementation is another option.

The study involved a new analysis of data from almost 11,000 people who were taking part in 25 clinical trials in 14 different countries, including the UK, Italy, Belgium, the US, Canada, Australia and Japan.

When taken individually, these trials offered conflicting results, however this latest analysis addressed this.

"This major collaborative research effort has yielded the first definitive evidence that vitamin D really does protect against respiratory infections. Our analysis of pooled raw data from each of the 10,933 trial participants allowed us to address the thorny question of why vitamin D ‘worked' in some trials, but not in others.

"The bottom line is that the protective effects of vitamin D supplementation are strongest in those who have the lowest vitamin D levels, and when supplementation is given daily or weekly rather than in more widely spaced doses," explained lead researcher, Prof Adrian Martineau, of Queen Mary University of London.

He noted that the fortification of foods with this vitamin provides ‘a steady, low-level of vitamin D that has virtually eliminated vitamin D deficiency in several countries'.

"By demonstrating this new benefit of vitamin D, our study strengthens the case for introducing food fortification to improve vitamin D levels in countries where profound vitamin D deficiency is common," Prof Martineau said.

The researchers believe that the vitamin protects against respiratory infections by boosting levels of antimicrobial peptides in the lungs. These are natural antibiotic-like substances.

They said that their findings fit with the observation that colds and flu are most common in the winter and spring, when vitamin D levels are at their lowest. They may also explain why vitamin D appears to protect against asthma attacks, which are often caused by respiratory viruses.

The study found that people with the lowest vitamin D levels, who were taking daily or weekly supplements, halved their risk of acute respiratory infections.

Those with higher vitamin D levels also benefitted, although not to the same degree. Their risk fell by 10%.

The researchers pointed out that upper respiratory infections, such as colds and flu, are the most common reason for people attending their GP and taking days off work. They added that vitamin D supplementation is safe and inexpensive, so could be a very cost-effective move in the long-term.

Details of these findings are published in the British Medical Journal.