“…you probably won’t get much protection from common colds by taking a daily supplemental dose of vitamin C, according to an updated review of 30 studies,” so says this article.
Taking vitamin C did nothing for most people except “…they found that people exposed to periods of high stress — such as marathon runners, skiers and soldiers on sub-arctic exercises — were 50 percent less likely to catch a cold if they took a daily dose of vitamin C.”
If you’re a marathon runner, you could read this one of two ways: you’re lucky that you can do something to prevent colds besides washing your hands and being a hermit — or — you’re putting your immune system under a ridiculous amount of stress in pursuit of your sport.
Either way, I’m going to keep popping those C’s.
Races Like a Girl 
i eat lots of tangerines…and only seem to get maybe one big cold a year. then again i’m not running very far at the moment….