Question

many people take megadoses of vitamin c for the prevention or treatment of the common cold. why are intakes of vitamin c above 100% of the rda generally not recommended?
multiple choice
supplemental sources of vitamin c are not as effective as food sources of vitamin c
vitamin c does not play a role in immunity, so taking megadoses is a waste of money
as intake of vitamin c increases, absorption efficiency decreases, such that most of the excess vitamin is excreted in urine or feces
vitamin c is a fat soluble vitamin and, therefore, toxicity is a concern

Explanation:

• Vitamin C is water - soluble. When intake increases, absorption efficiency decreases. Once the body's needs are met, excess vitamin C is excreted in urine or feces. So taking megadoses (above 100% of RDA) is not beneficial as most of the excess is not utilized.
• Supplemental vitamin C can be as effective as food - sourced in terms of chemical structure and function when in appropriate amounts.
• Vitamin C does play a role in immunity (e.g., it is involved in the function of white blood cells).
• Vitamin C is water - soluble, not fat - soluble. Fat - soluble vitamins (like A, D, E, K) have more potential for toxicity due to storage in adipose tissue, while water - soluble vitamins (like C and B - complex) are more readily excreted.

Answer:

As intake of vitamin C increases, absorption efficiency decreases, such that most of the excess vitamin is excreted in urine or feces.