Full-text evidence summary

InfluenzaVitamin D levels

 Randomized controlled trials have shown that taking 1000-2000 IU/day during the influenza season greatly reduce the risk of influenza and pneumonia.

There are no reports of incidence rates of influenza with respect to serum 25(OH)D levels.  However, there are several indirect ways to estimate the dose-response relation.

One is to compare wintertime serum 25(OH)D levels to summertime levels since solar UVB is the primary source of vitamin D for most people.  

Studies in the U.S. and England found that serum 25(OH)D levels increase  by about 10-15 ng/mL (25-37 nmol/L) in summer for younger people1 2.

A second is through randomized controlled trials of vitamin D supplementation and incidence of influenza.  

A study in Long Island involving black post-menopausal women found a 60% reduction in influenza and common cold cases for those taking 800 IU/d and 90% reduction for those taking 2000 IU/d3.  

A second study, involving 334 school children in Japan, half taking 1200 IU/d of vitamin D3, half taking 200 IU/d of vitamin D3, found a relative risk of 0.36 (95% confidence interval, 0.17, 0.79, p=0.006) for the 280 not taking additional vitamin D, i.e., a 64% reduced risk, for type A influenza4.  

Each 1000 IU/d increases serum 25(OH)D levels by 6-10 ng/mL for average sized adults5.  

The population mean values of serum 25(OH)D levels in the U.S. are 16 ng/mL for Black-Americans, 21 ng/mL for Hispanic-Americans, and 26 ng/mL for White-Americans6.  

 It appears that increasing serum 25(OH)D levels to above 35-40 ng/mL provides reasonable protection against type A influenza.

Thus, it appears that increasing serum 25(OH)D levels to above 35-40 ng/mL provides reasonable protection against type A influenza.  

A recent prospective cohort study involving healthy adults in Greenwich, CT, USA (latitude 41º N) found a serum concentration of 25(OH)D of 38 ng/mL or higher was associated with a two-fold reduction in the risk of acquiring respiratory viral infections and with a marked reduction in the number of days ill with such infections compared to lower values7.

In addition to the well-known seasonality of influenza8 9 10, during the recent A/H1N1 “swine flu” epidemic, those at greatest risk of complications following infection were pregnant women11 12, Australian Aborigines13, and those obese or with diabetes or congestive heart failure12.  See, also, the recent review in the New England Journal of Medicine14. All of these groups have lower serum 25(OH)D levels than that of the average population.

Pneumonia often develops in those who contract influenza15 12.  Pneumonia is also frequently found in children with rickets16, who are severely vitamin D deficient.  

Page last edited: 17 May 2011


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