A Cochrane Review was published recently:
This version first published online: October 18. 2004
Date of last subtantive update: May 14. 2007
...on the effectiveness of at least 0.2 grams/day (200mg/day -- US RDA is 60 mg/day) Vitamin C. BBC News ran a mainstream report on the matter, and it's called: Vitamin C 'does not stop colds' . I tried to take a slightly closer look at the Cochrane Review myself -- as Cochrane Reviews are generally taken seriously by the medical community.
For my felllow laymen, I'd like to provide some introductory background information on Cochrane Review. If you're totally unfamiliar with Cochrane Review, I might suggest clicking here to review: "The Cochrane Group's handbook for systematic reviewers of interventions."
Here's some information regarding Cochrane's systematic review from Wikipedia (I think that this information was accurate as of June 25, 2007):
Systematic review
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A Systematic review is a literature review focused on a single question which tries to identify, appraise, select and synthesis all high quality research evidence relevant to that question. Systematic reviews are generally regarded as the highest level of medical evidence by evidence-based medicine professionals. An understanding of systematic reviews and how to implement them in practice is becoming mandatory for all professionals involved in the delivery of health care.
A systematic review is a summary of healthcare research that uses explicit methods to perform a thorough literature search and critical appraisal of individual studies to identify the valid and applicable evidence. It often, but not always, uses appropriate techniques (meta-analysis) to combine these valid studies
While many systematic reviews are based on an explicit quantitative meta-analysis of available data, there are also qualitative reviews which nonetheless adhere to the standards for gathering, analyzing and reporting evidence.
Many healthcare journals now publish systematic reviews, but the best-known source is the Cochrane Collaboration, a group of over 6,000 specialists in health care who systematically review randomised trials of the effects of treatments and, when appropriate, the results of other research. Cochrane reviews are published in the Cochrane Database of Systematic Reviews section of the Cochrane Library, which to date (February 2007) contains 2,893 complete reviews and 1,646 protocols.
The Cochrane Group provides a handbook for systematic reviewers of interventions, where they suggest that each systematic review should contain the following main sections:
o Background
o Objectives
o Methods of the review
o Results
o Conclusion and discussion
There are seven steps for preparing and maintaining a systematic review, as outlined in the Cochrane Handbook:
1. Formulating a problem
2. Locating and selecting studies
3. Critical appraisal of studies
4. Collecting data
5. Analyzing and presenting results
6. Interpreting results
7. Improving and updating reviews
First, here's the Cochrane review summary on this matter -- i.e. this is sort of layman's terms:
Vitamin C for preventing and treating the common cold
This is a Cochrane review abstract and plain language summary, prepared and maintained by The Cochrane Collaboration, currently published in The Cochrane Database of Systematic Reviews 2007 Issue 3, Copyright © 2007 The Cochrane Collaboration. Published by John Wiley and Sons, Ltd.. The full text of the review is available in The Cochrane Library (ISSN 1464-780X).
This record should be cited as: Douglas RM, Hemilä H, Chalker E, Treacy B. Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews 2004, Issue 4. Art. No.: CD000980. DOI: 10.1002/14651858.CD000980.pub3
This version first published online: October 18. 2004
Date of last subtantive update: May 14. 2007
Douglas RM, Hemilä H, Chalker E, Treacy B
Summary
The term 'the common cold' does not denote a precisely defined disease, yet the characteristics of this illness are familiar to most people. It is a major cause of visits to a doctor in Western countries and of absenteeism from work and school. It is usually caused by respiratory viruses for which antibiotics are useless. Other potential treatment options are of substantial public health interest.
Since vitamin C was isolated in the 1930s it has been proposed for respiratory infections, and became particularly popular in the 1970s for the common cold when (Nobel Prize winner) Linus Pauling drew conclusions from earlier placebo-controlled trials of large dose vitamin C on the incidence of colds. New trials were undertaken.
This review is restricted to placebo-controlled trials testing at least 0.2 g per day of vitamin C. Thirty trials involving 11,350 participants suggest that regular ingestion of vitamin C has no effect on common cold incidence in the ordinary population. It reduced the duration and severity of common cold symptoms slightly, although the magnitude of the effect was so small its clinical usefulness is doubtful. Nevertheless, in six trials with participants exposed to short periods of extreme physical or cold stress or both (including marathon runners and skiers) vitamin C reduced the common cold risk by half.
Trials of high doses of vitamin C administered therapeutically (starting after the onset of symptoms), showed no consistent effect on either duration or severity of symptoms. However, there were only a few therapeutic trials and their quality was variable. One large trial reported equivocal benefit from an 8 g therapeutic dose at the onset of symptoms, and two trials using five-day supplementation reported benefit. More therapeutic trials are necessary to settle the question, especially in children who have not entered these trials.
So here's the Cochrane review abstract itself on Vitamin C for preventing and treating the common cold:
Abstract
Background
The role of vitamin C (ascorbic acid) in the prevention and treatment of the common cold has been a subject of controversy for 60 years, but is widely sold and used as both a preventive and therapeutic agent.
Objectives
To discover whether oral doses of 0.2 g or more daily of vitamin C reduces the incidence, duration or severity of the common cold when used either as continuous prophylaxis or after the onset of symptoms.
Search strategy
We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 4, 2006); MEDLINE (1966 to December 2006); and EMBASE (1990 to December 2006).
Selection criteria
Papers were excluded if a dose less than 0.2 g per day of vitamin C was used, or if there was no placebo comparison.
Data collection and analysis
Two review authors independently extracted data and assessed trial quality. 'Incidence' of colds during prophylaxis was assessed as the proportion of participants experiencing one or more colds during the study period. 'Duration' was the mean days of illness of cold episodes.
Main results
Thirty trial comparisons involving 11,350 study participants contributed to the meta-analysis on the relative risk (RR) of developing a cold whilst taking prophylactic vitamin C. The pooled RR was 0.96 (95% confidence intervals (CI) 0.92 to 1.00). A subgroup of six trials involving a total of 642 marathon runners, skiers, and soldiers on sub-arctic exercises reported a pooled RR of 0.50 (95% CI 0.38 to 0.66).
Thirty comparisons involving 9676 respiratory episodes contributed to a meta-analysis on common cold duration during prophylaxis. A consistent benefit was observed, representing a reduction in cold duration of 8% (95% CI 3% to 13%) for adults and 13.6% (95% CI 5% to 22%) for children.
Seven trial comparisons involving 3294 respiratory episodes contributed to the meta-analysis of cold duration during therapy with vitamin C initiated after the onset of symptoms. No significant differences from placebo were seen. Four trial comparisons involving 2753 respiratory episodes contributed to the meta-analysis of cold severity during therapy and no significant differences from placebo were seen.
Authors' conclusions
The failure of vitamin C supplementation to reduce the incidence of colds in the normal population indicates that routine mega-dose prophylaxis is not rationally justified for community use. But evidence suggests that it could be justified in people exposed to brief periods of severe physical exercise or cold environments.
Personally, when I saw the BBC news headline: "Vitamin C 'does not stop colds,'" I didn't assume that's all the info to be learned from this review. I will re-copy what else can be learned besides what seems to be pretty inconclusive evidence to suggest that Vitamin C is effective to reduce incidence of common cold:
Authors' conclusions
The failure of vitamin C supplementation to reduce the incidence of colds in the normal population indicates that routine mega-dose prophylaxis is not rationally justified for community use. But evidence suggests that it could be justified in people exposed to brief periods of severe physical exercise or cold environments.
So I'm not going to stop taking vitamin C. I'll definitely keep trying to remind myself to take some Vitamin C before I work out or enter a cold environment!
Also, you may note from the summary:
Nevertheless, in six trials with participants exposed to short periods of extreme physical or cold stress or both (including marathon runners and skiers) vitamin C reduced the common cold risk by half
Also, you may note that the studies included in this meta analysis were at least 0.2g/day, and 500mg Vitamin C is in fact ~833% of US RDA. So more or less than 0.2 grams (200mg) a day -- or over 3 times more that US RDA might yield different results. Someone please correct me (or my math) if I made a mistake. Edit: math error fixed.
This may indicate that these six studies were flawed (or that the others without parallel results may be), used a different dose, maybe different forms of Vitamin C -- further examination of these six studies may be warranted; however, I see no evidence to suggest taking 0.2 grams/day (200mg/day) of Vitamin C may be unsafe. Does anyone else?
Thoughts or comments?
Take care.
Edited by adam_kamil, 19 July 2007 - 12:31 AM.