Respiratory tract infectionsPatient friendly summary

  • Sunlight exposure has not been shown to reduce the risk of upper respiratory infections. However, some upper respiratory infections are more common in winter, when there is less sunlight.
  • Vitamin D may reduce the risk and severity of some upper respiratory infections.

An upper respiratory infection (URI) occurs in the main airways above the lungs. This includes the windpipe, throat, nose and pharynx (area behind the mouth), and sinuses. It may be caused by either a viral or bacterial infection.

Risk factors

Upper respiratory infections occur more frequently in winter because:

  • Temperatures are cold: The body is less able to fight infection.
  • Humidity levels are generally low: Viruses can live outside of the body longer.
  • There is less sunlight: Ultraviolet-B (UVB) light is less intense, and vitamin D levels are low.

Sunlight exposure and upper respiratory infection risk

There does not appear to be any direct evidence that sunlight affects the risk of upper respiratory infections.

However, the role of solar UVB light cannot be ruled out. Sunlight affects temperature and humidity in addition to vitamin D production.

Vitamin D and upper respiratory infections

Vitamin D levels

There have been several studies on how vitamin D affects upper respiratory infections:

  • In the United States, people with lower vitamin D levels were more likely to have recent upper respiratory infections. People with low vitamin D levels and asthma or chronic obstructive pulmonary disease had the highest rates of upper respiratory infections.
  • Two other randomized controlled trials did not find a link between vitamin D supplements and reduced risk of upper respiratory infections.

How vitamin D works

Studies have not proven that vitamin D mechanisms may lower the risk of upper respiratory infections. However, vitamin D:

  • Produces cathelicidin and defensins: These proteins have antiviral effects to combat viruses and antibacterial effects to fight URIs. They may neutralize bacterial toxins.
  • Reduces inflammation: As a result, the lining of the lungs is less disturbed. This makes it harder for viruses and bacteria to grow. In addition, the immune response to viral infections is reduced.


Based on studies of vitamin D and infections and the mechanisms of vitamin D, higher vitamin D levels might reduce the risk of some upper respiratory infections. Based on findings for other diseases, vitamin D blood levels above 40 ng/mL (100 nmol/L) might provide some protection.


Although vitamin D has not been proven to treat upper respiratory infections, increasing vitamin D blood levels to 40 ng/mL (100 nmol/L) might provide some relief. Low levels can be raised rapidly using high doses of vitamin D such as 50,000 international units (IU) (1250 mcg)/day for 1–3 days. Follow this with 1000-5000 IU (25–125 mcg)/day.

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    We will be adding a detailed evidence summary on this topic in the near future.  Please check back soon to find out more.

    Page last edited: 17 May 2011