There were a total of 1,930 participants. The participants were children in 6 studies (ages 1 to 14), and adults in 5 studies (ages 24 to 39). In the studies with adults, the participants were typically pregnant women.

The participants were given iron supplements with or without vitamin C. The dosages of vitamin C varied and were given as a set dosage (44–1,260 mg per day) or relative to body weight (19–37 mg per kg of body weight). The interventions were taken for 45 to 180 days.

Compared to iron only, iron plus vitamin C increased (improved) the following:

Hemoglobin, by 0.14 g/dL (the primary outcome, 10 studies)
Ferritin, by 3.23 µg/L (9 studies)
Reticulocyte (immature red blood cell) percentage, by 0.22% (3 studies)

The effect of taking vitamin C with iron on hemoglobin recovery

The relative difference in hemoglobin values reflects the average effect, but the general placement of values on the chart is for representation purposes.

The heterogeneity was high for each outcome. The risk of bias was low in 6 studies, moderate in 3 studies, and high in 2 studies.

The odds of combined adverse effects were 29% lower with vitamin C, but this difference did not reach statistical significance (p=0.05, 3 studies). The odds of gastrointestinal pain were reduced by 60% with vitamin C (2 studies).

Does taking vitamin C with iron improve iron deficiency anemia? Original paper