Re vitamin C's role in the body:
Vitamin C
WP
The presence of ascorbate is required for a range of essential metabolic reactions in all animals and in plants and is made internally by almost all organisms, humans being one notable exception....The pharmacophore of vitamin C is the ascorbate ion. In living organisms, ascorbate is an antioxidant, as it protects the body against oxidative stress,[4] and is a cofactor in several vital enzymatic reactions.[5]
And the effects on respiratory infections:
Review Vitamin C as an Antioxidant: Evaluation of Its Role in Disease Prevention
Sebastian J. Padayatty, MRCP, PhD, Arie Katz, MD, Yaohui Wang, MD, Peter Eck, PhD, Oran Kwon, PhD, Je-Hyuk Lee, PhD, Shenglin Chen, PhD, Christopher Corpe, PhD, Anand Dutta, BS, Sudhir K Dutta, MD, FACN, and Mark Levine, MD, FACN
Other than preventing scurvy, vitamin C has no proven benefits. In humans, vitamin C treatment has not resulted in changes in biomarkers of oxidation or in clinical outcome.
This statement from
Medline Plus on Vitamin C,
Common cold prevention (extreme environments)
Scientific studies generally suggest that vitamin C does not prevent the onset of cold symptoms. However, in a subset of studies in people living in extreme circumstances, including soldiers in sub-arctic exercises, skiers, and marathon runners, significant reductions in the risk of developing colds by approximately 50% have been reported. This area merits additional study, and may be of particular interest to elite athletes or military personnel.
is consistent with this dissertation/meta-analysis (warning, very large PDF file) on the subject:
Do vitamins C and E affect respiratory infections; Harri Hemilä; Department of Public Health; University of Helsinki
Finland?
The new trials did not find reduction of common cold incidence in the ordinary Western population with vitamin C supplementation. Nevertheless, in this thesis it is shown that there may be sub-populations, such as people undergoing heavy acute physical stress and young males with low dietary vitamin C intakes, in which regular vitamin C supplementation may reduce the incidence of the common cold.
But while that author is extremely critical of the negative findings, at least some of the studies cited were fairly small. For example in
Vitamin C Supplementation and Respiratory Infections: a Systematic Review you find:
In this review, the vitamin C trials with military personnel and with other subjects living under conditions comparable to those of military recruits are analyzed to find out whether vitamin C supplementation affects respiratory infections. For this systematic review, we identified seven trials with military personnel, three trials with students in crowded lodgings, and two trials with marathon runners. Eight of these trials were double blind and placebo controlled and seven were randomized. Five small trials found a statistically significant 45 to 91% reduction in common cold incidence in the vitamin C group. These trials were short and the participants were under heavy exertion during the trial. Furthermore, three other trials found a statistically significant 80 to 100% reduction in the incidence of pneumonia in the vitamin C group. The large number of positive findings seems to warrant further consideration of the role of vitamin C in respiratory infections, particularly in military recruits.
The same author of the dissertation also co-wrote the following review:
Vitamin C and acute respiratory infections
So far over 60 studies have examined the effects of vitamin C on the common cold. No effect on common cold incidence was observed in the six largest studies, indicating that vitamin C has no preventive effects in normally nourished subjects in the Western countries. There are, however, smaller studies reporting benefit. In three trials of subjects under heavy acute physical stress, common cold incidence decreased by on average 50%, and in four trials of British males common cold incidence decreased by on average 30% in the vitamin C groups. The dietary vitamin C intake in the UK is low, and consequently the benefit may be due to the correction of marginal deficiency, rather than high vitamin doses. Regular vitamin C supplementation (ge1 g/day) has quite consistently reduced the duration of colds, but the size of the benefit has varied greatly. In the four largest studies the duration of colds was reduced only by 5%. In two of these studies, however, absence from school and work was reduced by 14–21% per episode, which may have practical importance. Three controlled studies recorded a reduction of at least 80% in the incidence of pneumonia in the vitamin C group, and one randomised trial reported substantial treatment benefit from vitamin C in elderly UK patients hospitalized with pneumonia or bronchitis. It seems that the preventive effects of supplementation are mainly limited to subjects with low dietary vitamin C intake, but therapeutic effects may occur in wider population groups. Further carefully designed trials are needed to explore the effects of vitamin C.
In my reading it seems a bit tricky to filter out the bias going into the reviews. If you 'believe' then smaller studies become more significant. If you doubt then inconsistent findings become less significant. Until some mechanism is suggested why the results should be so different in a group such as marathon runners or the military under stress in the Arctic, I tend to discard such findings as inconclusive. For the general population, large studies have shown vitamin C has essentially no effect on numbers of colds or duration of colds. Unless you have young children, adults don't necessarily catch that many colds over a short duration of time. So the fact a small study over a short time span found effects really seems more prone to error than a large study over a longer time span.
A reduction of 5% duration seems pretty minimal. 20% less time loss from school or work might be more significant. You have to know how good the blinding of the study was. If your cold lasted as long with and without but you went back to work anyway, either it was milder or you just believed you were better sooner. That one is hard to know without looking at how well the study was blinded.
As for the elderly benefiting, there are other studies with similar populations showing no benefit. And look at the actual cited study of the benefit in the elderly:
Vitamin C in respiratory infections - adapted from the International Journal of Vitamin and Nutrition Research, 64,3:212-219, 1994
Extensive controlled trials have demonstrated that vitamin C supplementation can decrease the severity and duration of the common cold, although it does not appear to prevent colds from occurring...
...Fifty-seven elderly patients who were hospitalized for acute bronchitis or pneumonia were randomly assigned to receive either 200 mg/day of vitamin C or an inactive placebo. The patients' progress was assessed using a clinical scoring system based on the major symptoms of respiratory infection.
The clinical scoring system showed that patients who received vitamin C fared significantly better than those who received placebo. This was particularly true for those patients who were most severely ill at the start of the study. Among the severely ill patients, many had blood vitamin C levels low enough to suggest that they were at least marginally deficient in this vitamin when they were admitted to the hospital. Six patients died during the study: five in the placebo group and one in the group that received vitamin C. The difference in death rates between the vitamin C and placebo groups was not statistically significant.
So they got better according to some scores, but again, we don't know how well the study was blinded. The patients may have been deficient so their supplements were indicated anyway. It doesn't mean extra C helped. And if the difference in death rate was not statistically significant, you have to wonder what the objective measures were which were "significantly better".
It also seems that in comparing these trials, little is paid to the fact some show decreased rates of infection while others show decreased duration. Such findings are not consistent yet in the meta-analysis they are scored equally as if the results correlate.
But I have not looked at all of the studies involved in these reviews, so I maintain an open but skeptical mind on the benefits of vitamin C for infection.
I do however, take a smaller vitamin C supplement along with vitamin E (studies indicate together matters) for the potential cardiac benefits. It's one of those decisions based on the fact that it is inexpensive, I am not substituting these for a better alternative, the risk of harm is low (though at least one study showed a worse outcome with only one of these two than no supplement), and the potential benefit worthwhile. The results of the combination also suggest the results of either vitamin alone might not show a benefit because the combination is required for some as yet unknown reason.