Artistic rendition of influenza virus.
Influenza is a viral infection of the lungs. There are many symptoms:
- body aches and muscle pain
- dry cough
- runny nose
- dry or sore throat
The lining around the lungs may become inflamed. This can lead to bacterial pneumonia.
Influenza is most common in winter, a time when:
- Solar ultraviolet-B (UVB) doses are low.
- The weather is cold. This prevents white blood cells from reaching the lining of the respiratory tract and fighting the virus.
- The humidity is low. Dry air allows the virus to live longer outside of the body.
Sunlight exposure and influenza risk
Influenza is more common in winter, when reduced sunlight causes vitamin D levels to fall.
Influenza rates peak in winter. There is less solar UVB light in winter, especially in areas farther from the equator. Thus, vitamin D levels are at their lowest.
Vitamin D and influenza
Vitamin D from sunlight or supplements reduces the risk of influenza.
Two randomized controlled trials found reduced incidence of influenza for those taking higher doses of vitamin D. A study involving African-American postmenopausal women in New York found a 60% reduced risk of colds and influenza for those taking 800 IU/d vitamin D3 and 90% reduced risk for those taking 2000 IU/d.
Another study in Japan, involving school children taking 1200 IU/d vitamin D3 vs. 200 IU/d, found a 67% reduction in Type A influenza, but no effect for Type B influenza. Type A influenza includes H1N1 varieties, which was the type involved in the 1918-19 pandemic influenza and the 2009 “swine flu” infections.
According to an observational study, vitamin D provides protection against influenza. This occurs when vitamin D levels in the blood are more than 38 ng/mL (95 nmol/L).
How vitamin D works
To enhance the body’s immune system, vitamin D:
- Produces cathelicidin and defensins—These proteins have antiviral effects to combat viruses.
- Reduces inflammation—As a result, body temperature does not rise as much, and the lining of the lungs is less disturbed. This makes it harder for bacteria to give rise to pneumonia.
High levels of vitamin D may prevent or lower the risk of influenza. Vitamin D may also reduce symptoms of influenza and reduce the risk of developing pneumonia following influenza. Vaccines strengthen the body’s ability to fight infection. Therefore, combining high levels of vitamin D and anti-influenza vaccines provide the best protection.
Based on several studies, raising vitamin D blood levels to 40 ng/ml (100 nmol/l) may reduce the risk of influenza. For most people, this involves taking 1000–5000 international units (IU) (25–125 mcg)/day of vitamin D during the influenza season.
On average, 2000-5000 IU/day vitamin D3 may provide protection against influenza. Vitamin D3, the true form of vitamin D, is produced in the skin. Larger doses of vitamin D taken for a short time strengthen the immune system. This allows the body to fight infection.
Need to know more? Read on with our detailed evidence summary on Influenza.
Page last edited: 28 August 2011