Since my recent post on the growing health claims for vitamin D (see: Bring on the sunshine!), the 'sunshine vitamin' has continued to appear in the headlines.
Of note is a study from Japan (just published in American Journal of Clinical Nutrition)1 suggesting that the risk of children contracting influenza A may be reduced if they take vitamin D supplements during the winter.
The study, which took place in winter 2008-09, was a randomized, double-blind, placebo-controlled trial comparing vitamin D3 supplements (1200 IU/day) with placebo.
Influenza A occurred in 10.8% of children in the vitamin D3 group compared with 18.6% of children in the placebo group. The reduction in influenza A was even more prominent in children who had not already been taking other vitamin D supplements and who started nursery school later. However, the number of children in the vitamin D3 and placebo groups was small (167 in each) - the study needs to be replicated with larger numbers of children to confirm the benefits of vitamin D supplementation.
Searching our Nutrition and Food Sciences Database, I found a paper published in 2006 hypothesizing that vitamin D levels may help explain the seasonality of epidemic influenza.2 The article in Epidemiology and Infection notes that the active form of vitamin D has 'profound effects on human immunity' and, since vitamin D deficiency is common in winter, this may explain the influenza peak.
[Incidentally, researchers from the University of Copenhagen recently reported that vitamin D is crucial for activating our immune defences - they have discovered that the first stage in the activation of a T cell involves vitamin D.]3
Further references from the Nutrition and Food Sciences Database support the idea that vitamin D is important for respiratory health and that deficiency may increase the risk of respiratory infections.4,5
However, despite all the attention being paid to vitamin D, many of the health benefits are not yet supported by substantial clinical evidence. There is still much debate about optimal levels of the vitamin and the best way to obtain it (sun, diet or supplements?).6
1. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Urashima, M.; Segawa, T.; Okazaki, M.; Kurihara, M.; Wada, Y.; Ida, H;. American Journal of Clinical Nutrition (online, 10 March, 2010)
2. Epidemic influenza and vitamin D. Cannell, J. J.; Vieth, R.; Umhau, J. C.; Holick, M. F.; Grant, W. B.; Madronich, S.; Garland, C. F.; Giovannucci, E.; Epidemiology and Infection, 2006, 134, 6, pp 1129-1140
3. Vitamin D controls T cell antigen receptor signaling and activation of human T cells. Rode von Essen, M.; Kongsbak, M.; Schjerling, P.; Olgaard, K.; Ødum, N.; Geisler, C.; Nature Immunology (online 7 March).
4. Vitamin D and respiratory health. Hughes, D. A.; Norton, R.; Clinical and Experimental Immunology, 2009, 158, 1, pp 20-25
5. Vitamin D, respiratory infections, and asthma. Ginde, A. A.; Mansbach, J. M.; Camargo, C. A., Jr.; Current Allergy and Asthma Reports, 2009, 9, 1, pp 81-87
6. Health impacts of vitamin D: are we getting enough? Ruxton, C. H. S.; Derbyshire, E.; Nutrition Bulletin 2009 Vol. 34 No. 2 pp. 185-197