A huge amount of animal data has found significant effects for vitamin C in the prevention and alleviation of symptoms of infections, including the common cold. However, data for such effects in humans often said to be as ‘mixed’ or unreliable.
Writing in Nutrients, Dr Harri Hemilä from the University of Helsinki, Finland, reviews the evidence for vitamin C in a range of infections – adding that for now, the potential for vitamin C ‘is not known’.
Citing data from previous clinical trials, Hemilä notes that the majority of controlled trials have used a ‘modest dosage’ of only 1 gram per day of vitamin C, but that trials looking at a wider range of doses indicate that 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.
“Two controlled trials found a statistically significant dose–response, for the duration of common cold symptoms, with up to 6–8 g/day of vitamin C,” writes Hemilä. “Thus, the negative findings of some therapeutic common cold studies might be explained by the low doses.”
“Vitamin C is safe and costs only pennies per gram, and therefore even modest effects may be worth exploiting.”
While the Finnish researcher suggests higher doses of vitamin C are safe and could provide benefits, data from the European Food Safety Authority (EFSA) regarding the tolerable upper intake levels for vitamin C potentially counters such claims.
“Despite the extensive use of vitamin C supplements (up to 10 g/day) for the prevention of colds and other conditions, the tolerability of such intakes has not been subject to systematic assessment,” reads the EFSA document – adding that “there are few data to support the widely held view that high intakes of vitamin C are safe.”
Hemilä added that although previous trial data suggests doses of 6 to 8 grams per day can reduce the duration of a cold by almost 20%, the fact that both trials showed a strong dose-response relationship up to the highest tested dose means an ‘optimal dose’ for maximal effect of vitamin C on the common cold is currently unknown.
The study notes that definitive conclusions cannot be made from comparisons of existing studies because of numerous confounding differences between the trials – meaning that although trials of doses higher than 1 g/d have generally shown a better response than those of exactly 1 g/d, the most valid examination of dose-response remains within a single trial.
The Finnish research 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 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.
A 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. In this trial, compared with the placebo group, the 8 g/day dose shortened colds by 19%, twice as much as the 4 g/day dose did, noted Hemilä.
For both trials, Hemilä said that the dose-response relationship was quite linear up to the maximum doses given - meaning that it is possible that even higher doses may lead to still greater reductions in cold duration.
He added that while some have suggested that doses up to 15 g/d have been suggested (but not tested), further therapeutic trials should be carried out to investigate the dose-response relation in the region of over 8 g/day of vitamin C.
GI issues and saturated absorption
In its document on tolerable upper intake levels (p295 for vitamin C) EFSA notes that high doses of vitamin C have been linked to gastrointestinal effects including stomach pains, flatulence and diarrhoea.
“Gastrointestinal effects are the most common adverse clinical events associated with acute, high doses of vitamin C (above 1 g daily), but these can be reduced by taking the vitamin after meals,” it notes – adding that available data suggest that supplemental doses of up to 1 g, in addition to normal dietary intakes, are not associated with adverse gastrointestinal effects, but that acute gastrointestinal effects may occur at higher intakes (3-4 g/day).
Furthermore, the EFSA report notes that the absorption of vitamin C also becomes saturated at high doses, “and therefore intakes above 1 g/day would be associated with negligible increased uptake and tissue levels, but an increased risk of adverse gastrointestinal effects.”
Despite such suggestions, Hemilä suggests that previous trial data has suggested a significant benefit for very high doses of vitamin C for reducing the duration of colds and other infections - and that only by performing new randomised trials that investigate doses up to 8 or 10 grams per day can efficacy and safety be tested.
Volume 9, Issue 4, Page 339; doi:10.3390/nu9040339
“Vitamin C and Infections”
Author: Harri Hemilä