Vitamin D supplementation and respiratory infections in children
Biological, behavioural and contextual rationale
Acute lower respiratory infections, predominantly pneumonia, kill more children under the age of five in every region of the world1 than any other cause. In 2007, of the estimated nine million child deaths that occurred globally, close to 20% or 1.8 million were attributable to pneumonia. Undernutrition has been shown to both increase the severity and overall prevalence of acute lower respiratory infections and is an important factor in determining the mortality rates from severe forms of respiratory infections1–4.
Poor nutritional status is a well-recognized cause of early childhood susceptibility to acute lower respiratory infections, along with lower socioeconomic status, ethnicity, suboptimal immunization, tobacco exposure, air pollution and other underlying chronic diseases or infant prematurity5–7. Several micronutrient interventions have been proposed to both protect and prevent children from developing acute lower respiratory infections. Vitamin D deficiency in children has been strongly associated with risk of acute lower respiratory infections in a number of settings8–13. In Ethiopia, for example, researchers found that 42% of children in hospital with pneumonia had rickets, or severe vitamin D deficiency14.
Vitamin D is a group of fat-soluble molecules that are important micronutrients for health. Both vitamin D2 and vitamin D3 can be obtained from the diet15, 16 but in relatively low quantity. Only fatty fish such as salmon, tuna, sardines or cod liver oil contain significant amounts of vitamin D3. Most vitamin D3 is synthesized in the skin after exposure to UVB light from the sun. For children, fortified foods such as cereals, cheese and milk represent an important source of vitamin D in some countries, although these items contain low and often fluctuating amounts of vitamin D15, 16, 17. Diet contributes to only 10–20% of vitamin D stores in adults, with this percentage most likely even smaller in children 18.
During the winter months, when vitamin D synthesis is naturally diminished because of the decreasing hours of sunlight, angle of solar radiation and skin exposed, acute lower respiratory infections are more frequent in adults and children19, 20. Vitamin D is thought to play an important role in immune system regulation, and can potentially protect against infections21, 22, in addition to cancer, cardiovascular disease and autoimmune disorders such as type 1 diabetes23–26. Vitamin D supplementation appears to reduce the incidence of and adverse outcomes from these conditions and others such as acute lower respiratory infections, as well as reducing all-cause mortality27, 28.
Nutritional interventions aimed at the treatment or prevention of forms of acute lower respiratory infections have thus far been very few in the published literature. A randomized control trial in Afghanistan showed that one high dose of vitamin D3, combined with antibiotic treatment, given to children aged 1–36 months who were hospitalized for pneumonia, did reduce the reoccurrence of pneumonia among children living in an area of high vitamin D deficiency28.
Future studies, however, need to be undertaken in different settings to confirm these results, especially among populations of children who are not classified as vitamin D deficient living in high-resource settings. Children in more northern latitudes lacking sun exposure, and dark-skinned children, are most at risk of vitamin D deficiency and of developing more severe forms of acute lower respiratory infections29, 30. However, cut-off values for vitamin D sufficiency and recommended daily intake in children are still under debate13, 31, 32. The American Academy of Pediatrics currently recommends supplementation with 400 IU (international units) daily of vitamin D from shortly after birth and continued throughout childhood and adolescence22. Recommendations for increased sun exposure to increase vitamin D3 synthesis in areas where supplementation does not occur must be balanced with concerns over excess exposure to UV radiation33. More data on the role of vitamin D in paediatric infection and immune function are required34.
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