Vitamin C supplementation may be even more beneficial for people who perform heavy physical activity. In five trials with 598 total participants, who were exposed to short periods of extreme physical stress, vitamin C reduced common cold risk by nearly 50%
Most animals and plants can synthesize ascorbic acid, but humans are unable to synthesize it due to non-functional enzyme L-gulono-1,4,-lactone oxi- dase, which catalyzes the final step in the ascorbic acid biosynthesis in animals (Naidu 2003). Therefore, humans require it as an essential supplement in their diet.
Many animals can synthesize ascorbic acid in vivo; however, others such as humans and guinea pigs have lost the ability to make ascorbic acid be- cause of mutations in the L-gulono-g-lactone oxidase gene, which is essential for ascorbic acid synthesis in vivo.11) Therefore, animals without the enzyme activity of L-gulono- g-lactone oxidase must obtain ascorbic acid from dietary sources.
Vitamin C is readily absorbed when the intake is up to 100 mg/day; and at elevated levels of intake (500 mg/day), the efficiency of absorption of ascorbic acid swiftly declines (Naidu 2003).
Toward a new recommended dietary allowance for vitamin C based on antioxidant and health effects in humans.