The relationship between vitamin C dosage and its effects on the duration of the common cold symptoms
may extend to 6-8 grams per day according to a statistical analysis
published in Nutrients.
Dozens of animal studies using
different animal species have found that vitamin C significantly
prevents and alleviates infections caused by diverse bacteria,
viruses, and protozoa. Given the universal nature of the effect of
vitamin C against various infections in different animal species, it
also seems evident that vitamin C influences the susceptibility to,
and the severity of infections in humans. However, the practical
importance of vitamin C in human infections is not known.
The common cold is the most extensively
studied infection regarding the effects of vitamin C. The majority
of controlled trials have used a modest dosage of only 1 g per day of
vitamin C. The pooled effect of all published studies has shown a
statistically highly significant difference between the vitamin C and
placebo groups, which indicates a genuine biological effect.
However, the optimal doses and the maximal effects of vitamin C on
the common cold are unknown. The trials that used doses higher than
1 g per day usually found greater effects than trials with exactly 1
g per day, which suggests a dose dependent effect. Nevertheless,
definitive conclusions cannot be made from such a comparison because
of numerous confounding differences between the trials. The most
valid examination of dose-response is therefore within a single trial
that has randomly selected trial groups with different vitamin C
doses, so that exposure to viruses is similar and the outcome
definition is identical in the study groups.
Dr. Harri Hemilä from the University
of Helsinki, Finland, analyzed the findings of two randomized trials
each of which investigated the effects of two vitamin C doses on the
duration of the common cold. The first trial administered 3 g/day
vitamin C to two study groups, 6 g/day to a third group, and the
fourth group was administered a placebo. Compared with the placebo
group the 6 g/day dose shortened colds by 17%, twice as much as the
3-g/day doses did.
The second trial administered 4 g/day
and 8 g/day vitamin C, and placebo to different groups, but only on
the first day of the cold. Compared with the placebo group, the 8
g/day dose shortened colds by 19%, twice as much as the 4 g/day dose
did. Both studies revealed a significant dose-response relationship
between the vitamin C dosage and the duration of the common cold.
The dose-response relationship in these two trials was also linear up
to the levels of 6-8 g/day, thus it is possible that even higher
doses may lead to still greater reductions in the duration of common
cold. Dr. Hemilä notes that there have been proposals that vitamin
C doses should be over 15 g/day for the best treatment of colds, but
the highest doses that have so far been investigated in randomized
trials have been much lower.
Dr. Hemilä concludes that "given
the consistent effect of vitamin C on the duration of colds, and its
safety and low cost, it would be worthwhile for individual common
cold patients to test whether therapeutic 8 g/day vitamin C is
beneficial for them. Self-dosing of vitamin C must be started as
soon as possible after the onset of common cold symptoms to be most
effective." Dr Hemilä also states that further therapeutic
trials should be carried out to investigate the dose-response
relation in the region of over 8 g/day of vitamin C.
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