The new results add to the ongoing debate about vitamin C's efficacy on colds.
A recent review by researchers at the University of Helsinki in Finland found no evidence to support reduced risk of colds after analysing 23 studies done in the general population, using doses of up to 2g daily.
However they did find tha people who were given vitamin C and then caught a cold experienced a small reduction in the duration of the cold compared with those taking a placebo.
The authors also found that the vitamin could help prevent colds in people exposed to extreme physical exertion or cold weather.
In the Japanese trial, reported in the 24 August issue of the European Journal of Clinical Nutrition (DOI:10.1038/sj.ejcn.1602261), researchers gave 244 people living in Akita prefecture, one of the regions in Japan with the highest mortality from gastric cancer, a daily supplement of either 50mg (low-dose) or 500 mg vitamin C for five years.
After adjusting for external factors, they found that risk of suffering from a common cold three or more times during the survey period was 0.34 for the high-dose group.
Total number of common colds (per 1,000 person-months) was 21.3 and 17.1 for the low- and high-dose groups, respectively.
In contrast with the Helsinki researchers however, no apparent reduction was seen for the severity and duration of the common cold.
They warned however that "considering several limitations due to protocol amendment, the findings should be interpreted with caution".