The super-nutrient vitamin D, also known as the sunshine vitamin, is a key nutrient in the fight against disease and for better health. In recent years, some scientists have suggested that the upsurge in infections seen in the winter is linked to lower levels of vitamin D. vitamin D, among other things, facilitates the production of 'anti-microbial peptides'. As their name suggests, these natural substances work like natural antibiotics to counter infecting organisms.

An interesting study recently looked at the relationship between vitamin D levels and 'respiratory infections' in 6,789 British adults [1]. The researchers, from University College London, found that vitamin D levels and infection risk had a linear relationship, which in this case meant that the higher levels of vitamin D were, the lower the risk of infection.

• For each 10.0 nmol/l (4.0 ng/ml) increase in vitamin D (25-hydroxyvitamin D), risk of infection was 7 per cent lower.

• There was a similar relationship with lung function too, with higher vitamin D levels being associated with better lung function.

Another study, published in 2010 [2] found higher levels of vitamin D were associated with reduced risk of flu and reduced duration of the illness should they get it.

This research looked at the relationship between vitamin D levels and risk of viral respiratory tract infection such as cold and flu. It assessed blood levels of vitamin D and viral infection in almost 200 American men and women throughout last autumn (fall) and winter.

Some interesting findings from this study were:

• Compared to individuals with vitamin D levels <38 ng/ml (95 mmol/l), individuals with levels of 38 ng/ml or above were about half as likely to suffer from a viral respiratory infection during the study period.

• Of those with higher vitamin D levels (as defined above), 83 per cent had no infections at all during the study period, compared to 55 per cent of those with lower levels.

• Those with higher levels of vitamin D who succumbed to flu were ill for an average of 2 days per infection.

• Those with lower levels of vitamin D who succumbed to flu were ill for an average of 9 days per infection.

This study showed that as vitamin D levels rose, so did resistance to infection. However, the benefit appeared to level off at about 38 ng/ml (which is why this figure was chosen as a cut-off in the analysis).

These studies are 'epidemiological' in nature, so they cannot be used to prove 'causality' (i.e. that higher vitamin D levels protect against viral infection), only that the higher vitamin D levels are associated with improved resistance to infection. However, there is at least a plausible mechanism through which vitamin D might do this, as the authors of this study point out:

Vitamin D has known effects on the immune system. The production of the antimicrobial peptides cathelicidin by macrophages and β-defensin by endothelial cells is upregulated by vitamin D. These peptides may be involved in the direct inactivation of viruses.

Everyone should try to optimize their vitamin D levels to protect against infection and preserve health. Regular exposure to sunlight in the ideal way to do this, but when this is not possible, for example during the winter, then supplementation with a high quality vitamin D3 supplement is the next best thing.

Research Paper Details:

1. Berry DJ, et al. Vitamin D status has a linear association with seasonal infections and lung function in British adults. Br J Nutr. 2011 Jun 6:1-8.

2. Sabetta JR, et al. Serum 25-hydroxyvitamin d and the incidence of acute viral respiratory tract infections in healthy adults. PLoS One. 2010 Jun 14;5(6):e11088.