Vitamin D supplements may protect against acute respiratory infections including colds and flu, according to scientists who analysed data from 14 countries including India and Afghanistan.
The study led by Queen Mary University of London (QMUL) provides the most robust evidence yet that vitamin D has benefits beyond bone and muscle health.
The results are based on a new analysis of raw data from around 11,000 participants in 25 clinical trials conducted in 14 countries including the UK, US, Japan, India, Afghanistan, Belgium, Italy, Australia and Canada.
"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," said Professor Adrian Martineau from QMUL. "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," he said.
"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," he added."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," Martineau said.
Vitamin D - the 'sunshine vitamin' - is thought to protect against respiratory infections by boosting antimicrobial peptide levels in the lungs. Results of the study fit with the observation that colds and flu are the most common in winter and spring, when levels of vitamin D are at their lowest. They may also explain why vitamin D protects against asthma attacks, which are commonly triggered by respiratory viruses.
Daily or weekly supplementation halved the risk of acute respiratory infection in people with the lowest baseline vitamin D levels, below 25 nanomoles per litre. However, people with higher baseline vitamin D levels also benefited, although the effect was more modest (10 per cent risk reduction).
The reduction in risk of acute respiratory infection induced by vitamin D was on a par with the protective effect of injectable 'flu vaccine against 'flu-like illnesses. Acute respiratory infections are a major cause of global morbidity and mortality. Upper respiratory infections such as colds and 'flu are the commonest reason for GP consultations and days off work.
Acute lower respiratory infections such as pneumonia are less common, but caused an estimated 2.65 million deaths worldwide in 2013. Vitamin D supplementation is safe and inexpensive, so reductions in acute respiratory infections brought about by vitamin D supplementation could be highly cost-effective. The study was published in the journal BMJ.