Earlier studies in the American Journal of Clinical Nutrition showed that taking vitamin C supplements in the short-term reduced both systolic blood pressure (SBP) and diastolic blood pressure (DBP) all without any side effects making it an excellent natural alternative to dangerous medications.
A huge amount of data has found significant effects for vitamin C in the prevention and alleviation of symptoms of infections, including the common cold.
Writing in Nutrients, Dr Harri Hemila 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, Hemila 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 Hemila. "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."
Hemila 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 Hemila.
For both trials, Hemila 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, Hemila 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.