Vitamin C is a strong water-soluble antioxidant protecting cells and cellular components from free radicals by donating electrons. Vitamin C seems to regenerate other antioxidants such as vitamin E.113 Because of its reducing power, the vitamin functions primarily as a cofactor for reactions requiring a reduced iron or copper metalloenzyme.112 Vitamin C also has a role in the transport of non-heme iron. Vitamin C is necessary for the synthesis of collagen, which is a structural component of cartilage, ligaments, tendons, and other connective tissue. The vitamin is required for the synthesis of neurotransmitters, norepinephrine, and epinephrine and for the synthesis of carnitine, which is essential for the transport of fatty acids into the mitochondria for conversion to energy. Vitamin C functions in the conversion of cholesterol to bile acids and probably in the reduction of folic acid intermediates and the proper metabolism of the stress hormone cortisol.
The vitamin C deficiency disease is scurvy, characterized by symptoms related to connective tissue defects. Symptoms of scurvy include follicular hyperkeratosis, swollen or bleeding gums, perifollicular hemorrhages, joint effusions, arthralgia, fatigue, depression, and impaired wound healing. Scurvy is rare in developed countries, but does appear in children and the elderly on restricted diets and in alcoholics. The RDA for adults is 90 mg/d for men and 75 mg/d for women. For smokers, intakes of 35 mg vitamin C higher than for nonsmokers are recommended daily as smokers generally have lower blood vitamin C concentrations and are under increased oxidative stress from the toxins in cigarette smoke. The vitamin C content of selected fruits and vegetables is found in Table 4.7. Dietary sources of vitamin C include fruits and vegetables. Some foods such as breakfast cereals, sports drinks, and various nutrition bars, are fortified with vitamin C. Vitamin C in supplements is typically freeascorbic acid, calcium ascascorbate, sodium ascorbate, and ascorbyl palmitate.
Adverse effects from vitamin C intakes have been reported primarily with large doses (> 3000 mg/d), which may include diarrhea and other gastrointestinal disturbances. Possible adverse effects include increased oxalate excretion and kidney stone formation, increased uric acid excretion, pro-oxidant effects, systemic conditioning, increased iron absorption leading to iron overload, reduced vitamin B12 and copper status, increase oxygen demand, and erosion of dental enamel; however, the relationship between these effects and excess vitamin C intakes have not been clearly confirmed. Therefore, the UL for vitamin C established by IOM is 2000 mg/d for adults to prevent diarrhea and gastrointestinal disturbances.
Vitamin C has certain biological functions that can influence physical performance. It is needed for synthesis of collagen for ligament and tendon formation, synthesis of carnitine to use fatty acids as an energy source during exercise, synthesis of neurotransmitters for metabolic responses to exercise, and iron metabolism related to anemia and fatigue with consequential decreases in aerobic performance. Thus, adequate status of vitamin C is of importance for athletes. Physically active adults generally have been reported to have adequate amounts of vitamin C intakes and plasma vitamin C concentrations in the range of normal values. However, suboptimal intakes of vitamin C were determined in some athletic groups, such as male wrestlers and female basketball player and gymnasts. Approximately 12% of a group of athletes had lower levels of plasma vitamin C concentrations than the normal range.70 Because physical activity can increase plasma vitamin C concentrations for up to 24 hours, caution should be taken when interpreting lasma ascorbic acid values of athletes.
