The flu virus wreaks the most havoc in the winter and declines in the summer months.

Influenza risk factors

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.

Vitamin D and influenza risk

Vitamin D3, the true form of vitamin D is a fat-soluble vitamin produced in the skin when skin is exposed to the sun's ultraviolet rays. Vitamin D is also found in food and dietary supplements. 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.

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 international units 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 2009 “swine flu” infections.

How vitamin D boosts the immune system

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.

Optimal Vitamin D level

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 IU per day of vitamin D during the influenza season. On average, 2000-5000 IU/day vitamin D3 may provide protection against influenza. 

In Health and Happiness,

Kelly Harrington, MS, RDN

Registered Dietitian Nutritionist for Healthy Goods

References:

William B. Grant, Ph.D. Sunlight, Nutrition, and Health Research Center (SUNARC) P.O. Box 641603 San Francisco, CA 94164-1603, USA www.sunarc.org

Vitamin D Council. Influenza. March 2014.