The team from the University of Western Ontario and the University of Alberta found that when people who usually suffered colds took a standardized extract of the roots of North American ginseng, they had significantly less colds than a placebo group, and less severe symptoms.
Only one in 10 of those given daily doses of the supplement suffered two or more colds during four months including winter, compared with almost a quarter of those taking placebos.
"Our results can be compared with those for many of the common antiviral drugs such as rimantadine, amantadine, zanamivir or oseltamivir for the prevention or treatment of influenza," said the researchers in today's issue of the Canadian Medical Association Journal (173, issue 9).
The new findings follow a warning from the UK's Meteorological Office last week that this winter is likely to be significantly colder than average. And the charity Age Concern has warned that 20,000 people over 65 across Britain will have died by the time the cold season is over in March.
While it has previously been shown that extracts of the root of North American ginseng (Panax quinquefolium) have the potential to modulate both natural and acquired immune responses, as well as fighting colds and flu, many of these studies have been criticized for poor quality.
The new randomized, double-blind study followed 170 people aged between 18-65 years of age with a history of at least two colds in the previous year. They were instructed to take two capsules per day of either ginseng or a placebo for four months, beginning at the start of the flu season.
The ginseng supplement, called Cold-fX, is a standardized extract developed by a spin-off company of the University of Alberta, CV Technologies. It contains 80 per cent poly-furanosyl-pyranosyl-saccharides and 10 per cent protein from the ginseng roots. It is freeze-dried and encapsulated to contain 200 mg/capsule.
Only 10 per cent of the ginseng group had two or more colds during the test period compared with 22.8 per cent in the placebo group.
Symptom score was also significantly higher in the placebo group and the number of days cold symptoms were reported was higher too - 16.5 days compared with only 10.8 in the ginseng group.
The authors say the supplement could be a safe, effective "prophylactic treatment for upper respiratory tract infections", and recommended further studies on its efficacy and safety for children and immunocompromised populations (such as the elderly).
However in a commentary in the same journal, Ronald Turner, from the University of Virginia School of Medicine, warned of the many pitfalls faced by clinical studies of natural remedies for viral infections.
"The absence of information about active constituents and the mechanism of action of most natural remedies prevents the development of specific hypotheses to guide the design of clinical trials and increases the risk that statistically significant effects may be found by chance," he writes.
Since the proposed mechanism of action of ginseng on colds is unclear, and the active compounds have not been identified, even though the extract was standardized "it is possible that there is lot-to-lot variability for important phytochemical components that are not measured", he added.
The authors do point to prior research showing how the extract is capable of enhancing lymphocyte function and initiating acquired immune responses. It may also boost production of natural killer cell activity, thought to decrease susceptibility to frequent colds.
But Turners says it is not clear how these relate to viral respiratory infection.
"Further studies that evaluate the effect of well-characterized and standardized preparations of ginseng in virologically proven influenza infections or more typical common cold illnesses will be needed to confirm and extend the results of the study reported in this issue," he concludes.