Vitamin D and Influenza

What is Influenza ?

Influenza, more commonly known as the flu, is a respiratory infection that is caused by a virus infecting the nose, throat, and lungs. Influenza is most common during winter and can cause fever, chills, sore throat, cough, body aches, and fatigue 

What is vitamin D ? 

Vitamin D is an important part of the immune system. Some studies have shown that there is a link between vitamin D levels and the risk of getting influenza. People who have low vitamin D levels may have a higher chance of developing influenza.
Influenza epidemics occur in the winter, and vitamin D levels are dramatically lower in the winter as well. Since influenza is seasonal, it is thought that vitamin D might be a factor that can affect your chances of getting the flu.

Many studies that have been done about influenza have shown that people who have lower levels of vitamin D are more likely to get influenza. Not many studies have been done about treating influenza with vitamin D, but some research has shown a relationship between higher levels of vitamin D and shorter duration of the influenza infection.
On the other hand, some experiments have shown that taking vitamin D supplements can reduce your chances of getting influenza in the first place. Some researchers recommend getting more vitamin D to protect against influenza, but more experiments are needed to say whether or not taking a vitamin D supplement can for sure prevent influenza.

If you want to take vitamin D to prevent influenza, it is unlikely to cause you any harm, as long as you take less than 10,000 IU per day. However, it’s not proven that taking vitamin D will help to prevent or treat influenza.
If you have influenza, you shouldn’t take vitamin D in place of your treatment medications. Talk to your physician for more advice about taking supplements (4).

What does recent research say about vitamin D and influenza?

An experiment done in the United States gave African-American women either 800 IU vitamin D per day for 2 years, then 2,000 IU per day for the 3rd year, or a dummy pill. The researchers looked at how many times those women got influenza over the 3 years. They found that (3):
  • The vitamin D group had fewer influenza symptoms compared to the dummy pill group.
  • Only one person in the vitamin D group had influenza when the dose was at 2,000 IU per day.
  • The dummy pill group had influenza symptoms mostly in the winter, whereas the people who got influenza in the vitamin D group had symptoms year round.
    This experiment suggests that vitamin D, especially at higher doses, may help to protect against seasonal influenza. The researchers conclude that vitamin D supplements might be useful to prevent the flu, but that more experiments are needed.
    An experiment done with Japanese schoolchildren looked at the effects of vitamin D supplements on their chances of getting influenza. The researchers gave children either 1,200 IU vitamin D per day for 3 months during the winter, or a dummy pill. They found that (2):
  • More children in the dummy pill group got influenza A than children in the vitamin D group.
  • There was a preventive effect of 1,200 IU vitamin D per day on children getting influenza A.
    The researchers conclude that taking 1,200 IU of vitamin D in children can help to protect against seasonal influenza A.
    In this study, there was no effect of vitamin D on influenza B, possibly because vitamin D may respond in different ways to the inflammatory proteins in the viruses.
    A study done in 2011 looked at vitamin D levels and respiratory infections, like influenza, in a large group of British adults. The researchers found that (1):
  • For each 4 ng/ml increase in vitamin D levels in the body, there was a 7% lower chance of developing influenza.

  • There was a seasonal pattern of influenza which was the same as the seasonal pattern of vitamin D levels. Influenza infections decreased when vitamin D levels increased.
    However, since this study was observational, the researchers couldn’t conclude for certain if higher vitamin D levels protected against the flu.
    Key points from the research
  • People who get influenza are more likely to have low levels of vitamin D in their body
  • Vitamin D can help reduce inflammation caused by the influenza virus and increase the number of proteins that fight against viruses.
  • Influenza infections increase during the winter, which is when vitamin D levels are known to decrease in the population
  • Some experiments have shown that taking vitamin D supplements can reduce the chances of getting influenza.
  • Having high levels of vitamin D may help you to recover faster from an influenza infection, although we don’t know for sure yet if they do.
  • Some researchers recommend getting more vitamin D to protect against influenza. Still, more experiments are needed for scientists and doctors to clearly understand whether or not taking a vitamin D supplement can prevent influenza.

  • Berry D, Hesketh K, Power C, et al. Vitamin D status has a linear association with seasonal infections and lung function in British adults. British Journal of Nutrition 2011;106:1433-1440.
  • Urashima M, Segawa T, Okazaki M, et al. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. American Journal of Clinical Nutrition 2010;91:1255-60.
  • Aloia J & Li-Ng M. Re: epidemic influenza and vitamin D. Epidemiology 2007;135(7)1095-6
  • Vitamin council org. 
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