The Antioxidant Vitamins A and C
December 1, 2000 4 minutes read
The Antioxidant Vitamins A and C
Vitamin A (Vitamin A, oleovitamin A)
Vitamin a was discovered in 1913 because of its ability to prevent night blind-
ness. As a part of the retinoid class of compounds, vitamin A requires both fats and minerals to be absorbed from the digestive tract. Found only in animal products, vitamin A is necessary for good vision, growth and maintenance of epithelial tissue, and the growth and development of healthy bones.
Recommended Dietary Allowance (RDA):
375 mcg/d (1,250 IU) for children 0-1 y
400 mcg/d (1,333 IU) for children 1-3 y
500 mcg/d (1,667 IU) for children 4-6 y
700 mcg/d (2,333 IU) for children 7-10 y
1,000 mcg/d (3,333 IU) for men 11 y and older
800 mcg/d (2,667 IU) for women 11 y and older
Special Populations
• Patients with pancreatic disease or other malabsorptive syndromes may develop vitamin A deficiencies.
Active Constituents
Vitamin A is a fat-soluble vitamin that occurs naturally in two forms: retinol and dehydro-retinol.
Formulation
Formulation is based on retinol equivalents (1 unit vitamin A = 0.3 mcg retinol).
Food Sources
• Liver, fish, and dairy products.
Mechanism of Action
• Functions as an antioxidant and a free radical scavenger.
• Induces cellular differentiation via nuclear receptors.
Clinical Uses
• To prevent and treat vitamin A deficiency syndromes, including night blindness and xerophthalmia.
• To improve outcomes of acute measles infections.
• To treat several skin conditions, including acne and psoriasis.
• To maintain a healthy immune system.
• To decrease the risk of epithelial-cell cancers (mouth, skin, lungs, bladder, breast, stomach, cervix).
• Although there is no evidence to support a relationship between vitamin A and cardiovascular disease, vitamin A may decrease the risk of certain malignancies.
Adverse Effects/Toxicity
• In doses greater than 10,000 IU/d, vitamin A supplementation during pregnancy increases the risk of CNS, cardiovascular, and facial anomalies.
• Chronic toxicity occurs at doses of 50,000-100,000 IU/d.
• Symptoms of chronic toxicity include irritability, vertigo, lethargy, malaise, fever, headache, dry skin, and alopecia.
• Acute toxicity may occur with one or more doses of 1,000,000 IU/d.
• Symptoms of acute toxicity include neurologic compromise, skin exfoliation, nausea and vomiting, coma, and death.
Interactions/Nutrient Depletion
• Absorption of vitamin A is decreased by cholestyramine (and other drugs resulting in fat malabsorption), colestipol, mineral oil, and neomycin.
Vitamin C (ascorbic acid, ascorbate)
Vitamin C is best known for its ability to cure the vitamin deficiency disease, scurvy. A water-soluble vitamin that is absorbed from by small intestine, vitamin C may be found throughout the body, but the highest concentrations are found in the adrenal glands. Because humans cannot synthesize vitamin C, adequate amounts must be obtained through diet or supplementation.
Dietary Reference Intakes (DRI):
40 mg/d for children 0-6 mo
50 mg/d for children 6 mo-1 y
15 mg/d for children 1-3 y
25 mg/d for children 4-8 y
45 mg/d for children 9-13 y
75 mg/d for men 14-18 y
90 mg/d for men 19 y and older
65 mg/d for women 14-18 y
75 mg/d for women 19 y and older
Special Populations
• Smokers should increase the DRI by 35 mg/d.
• Pregnant women ages 14-18 years should consume 80 mg/d; pregnant women ages 19-50 years should consume 85 mg/d.
• Stress, both physical and emotional, may reduce vitamin C levels.
• People with a history of gout, kidney stones, or kidney disease should not take elevated levels of vitamin C without medical supervision.
Active Constituents
Vitamin C is water soluble; ascorbate is the active form.
Food Sources
Citrus fruits, melons, kiwi, mango, strawberries, tomatoes and tomato juice, asparagus, potatoes, brussel sprouts, cauliflower, broccoli, cabbage, and spinach.
Mechanism of Action
• Functions as an antioxidant by losing electrons.
• Is a cofactor or substrate for eight enzymes.
• Plays a role in collagen synthesis, ATP synthesis (via carnitine), and the synthesis and stability of several hormones, including norepinephrine, thyrotropin-releasing hormone, adrenocorticotropic hormone, vasopressin, oxytocin, and cholecystokinin.
• Enhances intestinal absorption.
• Acidifies the urine at higher doses.
Clinical Uses
• To prevent and treat scurvy, colds, and urinary tract infections.
• To prevent cancer (particularly breast and colon), cardiovascular disease, and cataracts.
• To stimulate wound healing.
Adverse Effects/Toxicity
• Sustained use at doses greater than 1,000 mg/d may result in increased urinary oxalate with subsequent risk of calculi.
• Symptoms of toxicity include flushing, dizziness, headache, fatigue, nausea, vomiting, heartburn, and diarrhea.
• Large doses of vitamin C may interfere with tests to determine occult blood in the stool and tests to monitor blood glucose levels in diabetics.
Interactions/Nutrient Depletion
• Aspirin, bumetanide, choline magnesium trisalicylate, choline salicylate, corticosteroids, ethacrynic acid, furosemide, oral contraceptives, and torsemide can decrease ascorbate levels.
• High vitamin C levels decrease fluphenazine levels and warfarin effect.
• Vitamin C increases iron absorption.
• Smoking decreases vitamin C levels.
Resources
Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academy Press; 2000. Available at http://books.nap.edu/books/0309069351/html/95.html. Accessed November 15, 2000.
Pelton R, et al. Drug-Induced Nutrient Depletion Handbook. Hudson, OH: Lexi-Comp; 1999.
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