Table 2

Vitamin C—summary of meta-analyses findings—main and subgroups

MicronutrientSubgroupEffect on occurrence of infectionEffect on duration of symptoms
RR (95% CI)P valuePercent diff (95% CI)P value
Vitamin CMain0.96 (0.93 to 0.99)0.01−9 (−16 to –2)0.014
Common cold outcome*0.96 (0.93 to 0.99)0.022−9 (−16 to –2)0.007
Daily dose, mg
 ≥20000.94 (0.87 to 1.00)0.63−5 (−17 to 7)0.62
 1000 to <20000.95 (0.91 to 0.98)−7 (−14 to –0.1)
 500 to <10000.95 (0.92 to 0.99)−8 (−15 to –0.1)
 <5000.96 (0.91 to 1.00)NR
Loading dose, mg
 >40000.95 (0.92 to 0.98)0.18−10 (−36 to 16)0.92
 >2000–40000.96 (0.93 to 0.99)−9 (−21 to 3)
 >1000–20000.98 (0.94 to 1.02)−9 (−16 to –2)
 <10001.02 (0.93 to 1.11)NR
Mean age, years
 <400.98 (0.93 to 1.03)0.56−5 (−21 to 11)0.81
 40 to <650.91 (0.72 to 1.16)−9 (−39 to 21)
 >65NR−13 (−74 to 48)
Sex
 Female1.11 (0.95 to 1.30)0.049−13 (−28 to 1)0.43
 Male0.82 (0.70 to 0.96)−2 (−21 to 16)
Continent
 Africa0.81 (0.65 to 1.02)0.141 (−16 to 19)0.22
 Asia0.85 (0.72 to 1.00)−2 (−15 to 11)
 Australia0.89 (0.78 to 0.99)−5 (−14 to 3)
 Europe0.92 (0.87 to 0.98)−9 (−16 to 2)
 North America0.96 (0.93 to 0.99)−12 (−21 to 3)
 South AmericaNR−16 (−28 to –3)
Country income class
 High0.96 (0.93 to 0.99)0.017−8 (−16 to –1)0.77
 Middle0.65 (0.47 to 0.89)−11 (−30 to 7)
Length of trial
 <2 weeksNR0.14−16 (−36 to 3)0.30
 2 weeks to <3 months0.93 (0.83 to 1.03)−13 (−29 to 3)
 3 months to <1 year0.77 (0.60 to 0.98)−3 (−22 to 17)
 >1 year0.47 (0.20 to 1.14)26 (–42 to 93)
  • NR: Not relevant; that is, no primary studies that met the criteria.

  • *Sensitivity analysis.