Researchers looked at studies of vitamin C where people took at least 200 milligrams of vitamin C daily in the form of a supplement and people were randomly placed in a placebo or vitamin C group. They then pooled the studies using a technique known as “meta-analysis.” Here’s what they found:
In 29 studies involving 11,077 people, the risk of developing cold symptoms during the study period was the same regardless of whether people were taking a daily supplement of vitamin C or a placebo. This was true for almost every group of people studied. There were a few exceptions:
- People undergoing extreme physical exercise (marathon runners) or people exposed to extreme cold (skiers and soldiers in arctic weather) were half as likely to come down with cold symptoms if they were taking a vitamin C supplement.
- Children taking a vitamin C supplement had shorter colds by 13% and adults taking a vitamin C supplement had shorter colds by 8%.
Source: Douglas RM, Hemilä H (2005) Vitamin C for Preventing and Treating the Common Cold. PLoS Med 2(6): e168