Can Vitamin D Protect You From the Flu?

Vitamin D, termed the sunshine vitamin because it is produced by skin exposed to sunlight, has been in the news a lot recently. We are well familiar with the need for vitamin D to develop strong bones, but a rash of recent studies show that vitamin D also functions as an important regulator of immunity. Vitamin D deficiency is seen in many immune-mediated diseases such as Crohn’s disease, type 1 diabetes, multiple sclerosis, rheumatoid arthritis and asthma. Vitamin D deficient states have also been theorized to increase susceptibility to infections. Because vitamin D is mostly obtained from sunlight, blood concentrations of vitamin D decrease in the winter months, a time when influenza infections become more widespread.



A study released ahead of publication in the American Journal of Clinical Nutrition is one of the first clinical trials to report that vitamin D supplementation in school-aged children can decrease the incidence of influenza A. In this study, a randomized group of 334 children in Japan were given either 1200 IUs of vitamin D or a placebo over a four month period (December 2008 trough March 2008). The study showed a significant decrease in influenza A infection in the children who took vitamin D supplements. Vitamin D increases the production of antimicrobial proteins that inhibit some forms of the influenza virus, and vitamin D’s anti-inflammatory properties may also lessen flu symptoms.


Some of the study results were not as distinctly positive. A decrease in incidence of influenza B was not seen in this study. Various strains of flu have different mechanisms of infection within the body, and some may be more resistant to the effects of vitamin D. Children with asthma did not show a decrease in the occurrence of flu, but it is thought that asthma suffers are more prone to infection overall. Interestingly, the study showed that there were significantly less asthma attacks in the vitamin D supplemented group. Flu incidence was not decreased in the first month of supplementation; previous studies have shown that vitamin D supplements need to be taken for three months before blood concentrations reach a steady value. Therefore the optimal time to begin vitamin D supplementation would seem to be before the start of flu season. The skin loses its ability to make vitamin D from sunlight beginning in mid fall. Flu season typically peaks in November and April. Thus it has been recommended to supplement vitamin D from October through April.


Clearly, more research needs to be done to determine whether or not vitamin D supplementation can prevent the flu in larger, more diverse populations. We know that senior adults are more susceptible to influenza. Vitamin D requirements are also increased in this population as older skin is less efficient at making vitamin D. Overweight or obese individuals have also been shown to have low levels of vitamin D; one study showed that 1 in 3 women and 1 in 2 men were deficient in vitamin D. Though it has not yet been definitively determined that vitamin D supplementation prevents the flu, supplementation with up to 2000 IUs of vitamin D has been shown to be safe, and emerging evidence suggests that it may indeed help to prevent infection with some strains of the virus.