Proponents of evidence-based medicine argue that conclusions of treatment effects should be based on controlled trials and not on observational studies. Nevertheless, based on the correlation between serum vitamin D level and infections, Ginde et al1 proposed that vitamin D supplementation may reduce the incidence of upper respiratory tract infections.
Nutrition is associated with a wide range of lifestyle variables, and there is a great risk of residual confounding caused by inadequate adjustments for complex social and environmental exposures acting over a person’s life.2 Residual confounding may explain, for example, the divergent findings in observational studies and controlled trials on vitamin E.2 Therefore, great caution should be exercised when interpreting correlations between serum vitamin D levels and health outcomes.
Hemilä H. Vitamin D Level, Respiratory Tract Infections, and Controlled Trials. Arch Intern Med. 2009;169(15):1443. doi:10.1001/archinternmed.2009.246