I suggested to ramp up the Vit C slowly. She was more enthisiastic and ramped up fast. The result was that she got a cold. It was a pretty bad one too. She stopped the Vit C and hasn't been neer Vit C since.
This is apparently a fairly common occurence, and Pauling even mentions in HTLLAFB that a "big name" professor of Nutrition reported having the same experience which caused him to "pooh pooh" vitamin C.
This shows that vitamin C is intimately involved with the onset of colds, and the explanation is the same as the rebound effect. By rapidly increasing the level in the body, enzyme systems are activated that use up the "excess" vitamin C. As ascorbate is converted to other substances, if the supply does not stay in step with the new demand, then the blood levels decline leading to the onset of the cold.