Clinical trails with varying doses of ascorbic acid showed that ascorbic acid does not have significant prophylactic effect, but reduced the severity and duration of symptoms of cold during the period of infection.
Consumption of ascorbic acid as high as 1 g a day for several winter months, had no consistent beneficial effect on the incidence of common cold.
There was a consistent beneficial but generally modest therapeutic effect on duration of cold symptoms.
In trials that tested vitamin C after cold symptoms occurred, there was some evidence of greater benefits with large dose than with lower doses.
Zinc is a nutritionally essential mineral needed for catalytic, structural, and regulatory functions in the body.
The RDA (recommended daily allowance) for adult women and men is 8 mg a day and 11 mg a day of zinc, respectively.
Severe zinc deficiency is a rare, genetic or acquired condition. Dietary zinc deficiency, often called marginal zinc deficiency, is quite common in the developing world, affecting an estimated 2 billion people. Zinc deficiency can cause impaired growth and development in children, pregnancy complications, immune dysfunction, and increased susceptibility to infections. Long-term consumption of zinc in excess of the tolerable upper intake level of 40 mg a day for adults can result in copper deficiency.
Zinc bioavailability is relatively high in meat, eggs, and seafood. Zinc is less bioavailable from whole grains and legumes due to the inhibitory effects of phytic acid on absorption of the mineral.