Table 1.

Summary of the major clinical studies evaluating the relationship between vitamin D status and acute respiratory infections

Source	Study design	Condition	Population (cases)	Main outcome(s)
Ginde et al41	Retrospective study (secondary analysis of the US NHANES III data)	25(OH)D3 levels <30 ng/mL	18,883 participants	Serum 25(OH)D3 levels were inversely associated with recent upper respiratory tract infections (URTIs)
Berry et al39	Retrospective study (secondary analysis of the Nationwide 1958 British Birth Cohort data)	25(OH)D3 levels >10 ng/mL	6,789 participants	Vitamin D status had a linear relationship with respiratory infections and lung function
Sabetta et al40	Cross-sectional (prospective from Tromsø Study)	25(OH)D3 levels >38 ng/mL	198 healthy adult participants	25(OH)D3 levels >38 ng/mL were associated with reduction in risk of viral URI
Laaksi et al42	Cross-sectional (prospective Tromsø Study)	25(OH)D3 levels <40 nmol/L	800 young Finnish men	Serum vitamin D concentrations with acute respiratory tract infection (ARTI) in young Finnish men
Science et al37	Cross-sectional (prospective cohort study)	Median serum 25(OH)D3 level 62.0 nmol/L	743 participants (children aged 3–15 years)	Lower serum 25(OH)D3 levels were associated with increased risk of viral RTI in children
Mohamed and Al-Shehri38	Cross-sectional (prospective Tromsø Study)	Cord blood 25-hydroxyvitamin D levels	206 newborns	Low cord blood 25(OH)D3 levels were associated with increased risk of ARTI in the first 2 years of life
Camargo et al43	Double-blinded randomized clinical trials	Vitamin D3 supplementation (300 IU)	744 school children	Vitamin D supplementation (300 IU/daily) significantly reduced the risk of ARTI in winter among children with vitamin D deficiency
Laaksi et al44	Double-blinded randomized clinical trials	Vitamin D3 supplementation (400 IU)	164 young Finnish men	400 IU vitamin D3 daily significantly decreased the risk of ARTI in young Finnish men