Large doses of Vitamin C - up to eight grammes per day - can significantly reduce the duration of a spell of common cold, scientists have found.
Previous studies on 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.
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 one gramme 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.
Harri Hemila from the University of Helsinki in Finland, analysed the findings of two randomised trials each of which investigated the effects of two vitamin C doses on the duration of the common cold.
The first trial administered three grammes per day vitamin C to two study groups, six grammes per day to a third group, and the fourth group was administered a placebo.
Compared with the placebo group the six grammes per day dose shortened colds by 17 per cent, twice as much as the three grammes per day doses did.
The second trial administered four grammes per day and eight grammes per day vitamin C, and placebo to different groups, but only on the first day of the cold.
Compared with the placebo group, the eight grammes per day dose shortened colds by 19 per cent, twice as much as the four grammes per day dose did.
Both studies showed a significant dose-response relationship between the vitamin C dosage and the duration of the common cold.
It is possible that even higher doses may lead to still greater reductions in the duration of common cold, researchers said.
Hemila noted that there have been proposals that vitamin C doses should be over 15 grammes per day for the best treatment of colds, but the highest doses that have so far been investigated in randomised trials have been much lower.
"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 eight grammes per day vitamin C is beneficial for them," Hemila said.
"Self-dosing of vitamin C must be started as soon as possible after the onset of common cold symptoms to be most effective," she said.