Bushwhacked – ask him for the evidence – this has been debunked I believe.
It is another one of those odd “nutritionist” ideas that have become accepted despite all the evidence pointing the other way
“”It is now fairly clear that for preventing colds, vitamin C has no worthwhile effect,”
“The many studies done in the last 30 years clearly prove that daily vitamin C supplements, whether 100 mg or 5,000 mg, do not prevent colds and provide, but only for some people, only a slight reduction in duration and severity of colds. “
http://www.quackwatch.org/01QuackeryRelatedTopics/DSH/colds.html
“Despite the popular belief that vitamin C can cure the common cold, the scientific evidence for this is conflicting. A few studies have suggested that taking vitamin C supplements when you are exposed to a cold virus or when you first have symptoms can shorten a cold or even prevent one. Other research has shown that vitamin C supplementation has no impact on a cold’s severity or length, but it can significantly reduce how often a person catches a cold.
Vitamin C may only be useful in case of a cold if you have low levels of this nutrient to begin with. The likelihood of success may be very individual — some people improve, while others do not.”
http://www.nlm.nih.gov/medlineplus/ency/article/002145.htm
“Taking vitamin C every day does not prevent colds for most people. People will not recover from a cold more quickly if they start to take vitamin C when the symptoms of a cold start. Taking vitamin C every day could shorten the amount of time you are sick with colds by a very small amount.”
http://www.informedhealthonline.org/sid0ab9e707ce599f6d095fa0d46d2736d1/common-colds-can-vitamin-c-prevent-or-relieve-them.29.174.en.html
“Authors’ conclusions
The failure of vitamin C supplementation to reduce the incidence of colds in the normal population indicates that routine mega-dose prophylaxis is not rationally justified for community use. But evidence suggests that it could be justified in people exposed to brief periods of severe physical exercise or cold environments.”
http://www.cochrane.org/reviews/en/ab000980.html