Asthma Patient friendly summary

  • Sunlight may reduce the risk of asthma.
  • Vitamin D may reduce the risk of asthma by strengthening the immune system.

Asthma is a common inflammatory disease of the airways.

The disorder is chronic, and symptoms vary. They include wheezing, coughing, chest tightness, and shortness of breath.

Sometimes the airway goes into spasms.

Risk factors

Viral respiratory infections are an important risk factor for asthma. Flair-ups are apt to occur in the fall and winter. During these seasons, respiratory infections are most common. Vitamin D levels are also low.

Sunlight exposure and risk

A study in Sweden found that asthma hospitalization rates are:

  • Lowest in summer for people of all ages
  • Highest in fall for those under 60 years of age
  • Highest in winter for those more than 60 years of age

Younger people encounter more new people each fall when returning to school. This may account for higher asthma rates in the fall for people under the age of 60 years.

Evidence suggests that having respiratory viral infections early in life may lead to asthma later in life. Respiratory infections are more common in winter than in summer. This is in part due to less ultraviolet-B (UVB) light and lower vitamin D levels.

Vitamin D and asthma

Vitamin D levels

High vitamin D intake during pregnancy may reduce the risk of wheezing in infants. However, these infants may have an increased risk for asthma after the age of five years. According to studies:

  • In the United Kingdom, children born to mothers with vitamin D levels greater than 30 ng/mL (75 nmol/L) during pregnancy had five times the risk of asthma after 9 years of age.
  • In Finland, children who received vitamin D in infancy had a 35% higher risk of asthma at age 31.

According to studies, Vitamin D may reduce the severity of asthma attacks:   

  • A trial in Japan found a significantly lower risk of asthma attacks in school children taking 1200 international units (IU)/day of vitamin D supplements.
  • In the United States, analysis of asthma data showed that 35% of the patients had vitamin D levels under 30 ng/mL (75 nmol/L). They also had a 50% increased risk of hospitalizations or emergency room visits.

How vitamin D works

To reduce asthma, vitamin D may:

  • Limit the risk of viral respiratory infections
  • Increase production of interleukin-10, a factor that reduces inflammation
  • Protect smooth muscle cells of the airway
  • Stimulate cytokine production to reduce inflammation


Studies indicate that vitamin D does not prevent the development of asthma in utero or during the first year or two or life.


Based on studies of influenza and asthma, children may benefit by taking 1200–2000 IU/day of vitamin D3 (cholecalciferol). Adults may benefit by having vitamin D levels above 38 ng/mL (95 nmol/L). To reach this level, one should take 1000-5000 IU/day. However, there is considerable variation in how people respond to vitamin D. Vitamin D levels should be checked before supplementation and three months afterwards.

Find out more...

Do you want to find out more and see the research upon which this summary is based?  Read our detailed evidence summary on asthma.

Page last edited: 17 May 2011