The June 2013 issue of the journal PLOSONE (Public Library of Science One) published the results of a meta-analysis conducted by researchers at Sweden's Karolinska Institutet which found a protective role for supplementing with D to reduce the risk of respiratory tract infection.
For their analysis, Jonatan D. Lindh of the Karolinska Institutet's Division of Clinical Pharmacy and his associates selected 11 randomized, placebo-controlled trials that included a total of 5,660 participants averaging 16 years of age. The average dose of vitamin D was 1,600 international units (IU) administered orally. Follow-up periods ranged for three months to three years.
Supplementation with vitamin D resulted in a 36% lower risk of respiratory tract infection compared to placebo in a summary of all 11 trials. When studies using daily dosing regimens were examined, vitamin D supplementation resulted in a 49% lower risk of infection in comparison with the placebo, while large bolus doses administered monthly or less frequently were not as effective. The researchers attribute this phenomenon to an immunosuppressive effect associated with very high doses of vitamin D, which renders high bolus doses of the vitamin useful for the treatment of inflammatory disorders, including multiple sclerosis. "This could explain why many of the studies using bolus doses have provided null effect and is also important information when designing future interventional studies," the authors remark.
"Aggregated evidence from 11 randomized controlled trials indicates that supplementation with vitamin D could be an effective means of preventing respiratory tract infection," they conclude. "However, due to heterogeneity of included studies and possible publication bias in the field, these results should be interpreted with caution. Thus, additional studies addressing the impact of dosing regimen and choice of target population are warranted before definite conclusions can be drawn."
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