By David Csonka
While a number of studies demonstrate a small percentage reduction in occurrences [1,2,3] the only credible benefit for supplementation in normal situations is for a slight decrease in illness duration. Still, a reduction of 10% on the duration of your cold symptoms ultimately doesn’t amount to very much. Maybe a day?
However, there does seem to be more conclusive evidence that vitamin C supplementation can be beneficial for runners and other athletes who are exercising at high intensities and cold weather.
A collection of studies examining groups participating in ski schools, Canadian winter military training, and long distance running events demonstrated that experimental cohorts receiving approximately 1g of vitamin C per day had a much reduced rate of infection (almost 50%) versus placebo groups. 
Intense exercise can have a profoundly negative effect on immune function, with cold weather further exacerbating this problem. Depression of the immune system by corticosteroids or oxygen radicals, generated while the body is under extreme physical stress, is one set of explanations for immuno-compromise.
Recovery from exercise and muscle damage in general, is primarily an immune response. Inflammation and transposition of macrophages to repair sites are similar for wounds and exercise-mediated tissue damage alike. As such, it is interesting to see how L-ascorbic acid (vitamin C) is theorized to interact with the immune system.
Vitamin C is found in high concentrations in immune cells, and is consumed quickly during infections. It is not certain how vitamin C interacts with the immune system; it has been hypothesized to modulate the activities of phagocytes, the production of cytokines and lymphocytes, and the number of cell adhesion molecules in monocytes. 
Vitamin C has been reported to increase the proliferative responses in T-lymphocytes, to prevent the defects in neutrophils caused by corticosteroids, and it is also a major biological antioxidant. Thus vitamin C possibly could aid the immune system in subjects under heavy physical stress. 
Under normal physiological circumstances, the human body consumes L-ascorbic acid at very low rates. It could take several months for depletion symptoms (scurvy) to set it. Normal dietary consumption in most countries is enough to meet daily needs. However, when injured, sick, or under stress the body depletes its stores of vitamin C at a much quicker rate.
While working out this winter, it might be wise to increase the number of servings of fruit or peppers that you normally consume. Some people don’t realize that bell peppers contain much more vitamin C than oranges.  Or at the least, consider taking a cheap 1g vitamin C supplement. Chances are, as a runner or athlete you’re much more at risk of catching a cold than the average person.