Table 1.

Evidence to support the association between 25(OH)D levels and acute respiratory illness

Source	Study details	Main outcome(s)
Ginde et al. [46]	Design: Secondary analysis of the US NHANES III data (n = 18 883 adults). Objective: Investigated association between 25(OH)D levels and risk of URI. Reference group: 25(OH)D levels ≥30 ng/ml.	25(OH)D levels <10ng/ml were associated with increased risk of URI (OR 1.36; 95% CI 1.01 – 1.84). 25(OH)D levels ≥10 to <30 ng/ml were associated with increased risk of URI (OR 1.24; 95% CI 1.07–1.43).
Berry et al. [47■■]	Design: Secondary analysis of the Nationwide 1958 British Birth Cohort data (n =6789 adults). Objective: Investigated association between 25(OH)D levels and risk of URI. Reference group: 25(OH)D levels <10ng/ml	Each 4ng/ml increase in 25(OH)D level was associated with reduction in risk of URIs (OR 0.93; 95% CI 0.89–0.97).
Sabetta et al. [48]	Design: Prospective cohort study (n =198 healthy adults). Objective: Investigated association between 25(OH)D levels and risk of URI during a single fall and winter season in New England. Reference group: 25(OH)D levels <38 ng/ml.	25(OH)D levels >38 ng/ml were associated with reduction in risk of viral URI (OR 1.49; 95% CI 1.25–1.84).
Laaksi et al. [49]	Design: Prospective cohort study (n = 756 young males). Objective: Investigated association between 25(OH)D levels and risk of missed work days from URIs over a 6-month period in Finland. Reference group: 25(OH)D levels ≥16 ng/ml.	25(OH)D levels <16ng/ml were associated with increased risk of missed work days from URIs (OR 1.63; 95% CI 1.15–2.24).

25(OH)D, 25-hydroxyvitamin D; CI, confidence interval; OR, odds ratio; URI, upper respiratory infection; US NHANES, United States National Health and Nutrition Examination Survey.