Vitamin C and Immune Function: What is the Latest?
The Question
Does vitamin C supplementation improve immune function, including preventing or treating the common cold?
The Recommendation
Habitual vitamin C supplementation may reduce cold severity and duration and exercise-induced bronchoconstriction symptoms. For the general population, it does not reduce cold incidence or improve asthma outcomes, but an exception may be decreased cold incidence in individuals undergoing brief periods of heavy physical stress (soldiers and athletes). It is unclear whether vitamin C supplements play a role in the prevention of pneumonia.
Treatment with vitamin C after the onset of symptoms does not reduce cold severity or duration and does not improve acute pancreatitis outcomes. For pneumonia, due to the uncertainty of the evidence, it is unclear whether vitamin C supplements provide any additional support in the treatment of pneumonia in children.
Vitamin C supplements of ≤2 g/day have not been associated with adverse effects, except in individuals with severe acute pancreatitis.
The Evidence
Pneumonia
Results from a 2021 Cochrane review reported that due to limited and very low certainty of evidence, it was unclear whether vitamin C supplementation (2 g/day) prevented pneumonia in young adults. For the adjunct treatment of pneumonia in children under five years of age with vitamin C supplements (200 mg/day), results showed a small improvement in the time it took to improve oxygen saturation and respiration rates compared to controls. However, these results should be viewed with a high degree of caution as the evidence was considered of very low certainty due to study limitations (unclear or high risk of bias in most domains), small sample size and imprecise results (wide confidence intervals). Only one prevention study reported on adverse events with vitamin C supplementation and it was found to have uncertain effects on urticaria. More and better studies are required.
Grade of Evidence C
Common Cold
In a Cochrane review, the routine consumption of ≥200 mg/day vitamin C did not reduce common cold incidence in the general population but may have slightly reduced cold duration and severity. However, for athletes and soldiers undergoing brief periods of heavy physical stress, vitamin C supplementation reduced common cold incidence by about half (RR=0.48; 95%CI, 0.35 to 0.64). In adults, high doses of vitamin C (1-8 g/day) after the onset of a cold did not decrease cold duration or severity compared to placebo.
Grade of Evidence B
Asthma/Bronchoconstriction
In a Cochrane review using GRADE analysis, vitamin C supplementation of 0.5 to 5 g/day in individuals with asthma had no effect on:
There was no evidence of adverse effects with a dose of 1 g/day. For individuals with exercise-induced bronchoconstriction, 1.5 g/day vitamin C improved asthma symptom scores.
Grade of Evidence C
Acute Pancreatitis
In a small RCT of individuals treated for severe acute pancreatitis, there were no significant differences in organ dysfunction or length of hospital stay between those supplemented with a combination of IV antioxidants (vitamin C, selenium and n-acetylcysteine) and a placebo. The trial ended early when it was discovered that all four individuals who had died during the trial were from the intervention group.
Grade of Evidence C
Remarks
For individuals wishing to experiment, vitamin C supplements are low cost and have no adverse effects (except in severe acute pancreatitis). A minimum dose of 2 g/day used in the trials is unlikely to be achieved by dietary sources alone.
To see the full practice question, including the Evidence Statements, Remarks, Comments and References, click here.
April 2022 ·
Volume 12
(4)