It is a common belief that antioxidants (compounds that combat free radicals) become available to our bodies once we ingest the foods that contains them. It turns out that this phenomenon is only partially correct. Yes, antioxidants protect our bodies, but not all antioxidants are easily metabolized, which diminishes their ability to safeguard human health.
Dr. Prior and his team investigated the bioavailability of certain antioxidants. To achieve this goal, they fed the study subjects different fruits to eat: blueberries, cherries, dried plums, dried plum juice, grapes, kiwifruit, and strawberries. Then they measured the participants' plasma antioxidant capacity (AOC) levels. They discovered that despite their high antioxidant levels, plums did not raise the plasma AOC levels of the volunteers. The reason? Chorogenic acid - the major phytochemical (plant chemical and antioxidant) in plums is not readily absorbed in humans.
By comparison, consumption of blueberries did produce a rise in plasma AOC, but only after the subjects ate at least a half-cup serving of the fruit. Grape and kiwifruit consumption by the volunteers showed similar plasma level increases of AOC, but it was unclear which compounds were responsible for the antioxidant level increments. (There are more than 8,000 known plant chemicals. About 4,000 have known antioxidant properties). The study appeared in the 2007 issue of the Journal of the American College of Nutrition.
Vitamin C or Ascorbic Acid (AA): The Ever- Ready Protector of the Human Body
Vitamin C or ascorbic acid is considered the primary antioxidant vitamin in the human body because it readily donates electrons to prevent other compounds from being oxidized. As an antioxidant, it can interact with unstable, highly reactive molecules that can pose a danger to health. These reactive compounds include:
- molecules with unpaired electrons such as: reactive oxygen species (superoxide, hydroxyl radicals, peroxyl radicals), sulfur radicals, and reactive nitrogen species.
- reactive compounds that are not radicals, including hypochlorous acid, nitrosamines, and other nitrosating compounds, nitrous acid related substances, and ozone.
- compounds formed by reaction either of the first class and then react with vitamin C. Example: alpha-tocopherol radical is formed when vitamin E interacts with oxidized low-density lipoprotein cholesterol (LDL). The alpha-tocopherol radical is converted back to its non-radical active form by vitamin C.
- transition metal reactions involving iron and copper. Oxidation of iron and copper can lead to the formation of radical molecules.
As an antioxidant, vitamin C can mitigate the development of chronic diseases by preventing free radicals from attacking individual nucleotides in the DNA. Guanine is the DNA base most susceptible to oxidative attacks. AA diminishes DNA damage by reducing radical species directly, thereby reducing lipid peroxidation and by preventing free radicals from attacking proteins that repair DNA.
Human beings cannot synthesize AA so it must be supplied by the diet. In the intestines, vitamin C is oxidized to dehydroascorbic acid which is quickly transported across all cell membranes. The degree of absorption decreases as vitamin C intake increases. For example; intakes of 1-1.5 g results in 50% absorption, but intakes higher than 12 g/day results in only 16% vitamin absorption. Pectin, zinc, inhibits vitamin C absorption. Also excess iron in the gastrointestinal tract degrades the vitamin, reducing its availability.
Bioavailability of Vitamin C
Vitamin C supplements taken with food, or vitamin C from food (fruits, vegetables, etc.) increases its absorption. Adequate cooking also increases its availability. For example, vitamin C in cooked broccoli is more readily available than vitamin C in raw broccoli. Ascorbic acid from oranges and orange juice is also highly available to the body, but vitamin C bioavailability is reduced in a mixed diet.
Significant Food Sources of Vitamin C
The following foods provide significant amounts of vitamin C:
- Fruits: acerola cherries, canteloupe, oranges, grapefruit, honeydew, kiwifruit, mango, papaya, strawberries, tangelo, tangerin, watermelon,
- Vegetables:asparagus, broccoli, brussels sprouts, cabbabe, cauliflower, kale, mustard greens, red and green peppers, plantains, potatoes, snow peas, sweet potatoes, tomatoes, tomato juice.
Daily Recommendations for Healthy Adults
The daily recommendations of vitamin C for adults are as follows:
- men (non-smokers): 90 mg/day
- women (non-smokers): 75 mg/day
- men (smokers): 125 mg/day
- women (smokers): 100 mg/day
- upper tolerable levels: 2000 mg/day
References:
Prior RL et al. "Plasma Antioxidant Capacity Changes Following a Meal as a Measure of the Ability of a Food to Alter in Vivo Antioxidant Status" J Am.Coll Nutr. 2007;26(2):171-181
Padayatty SJ et al. "Vitamin C as an antioxidant: Evolution of Its Role in Disease Prevention" J Am Coll of Nutr.
2003;(1):18-35
Jacob RA "Vitamin C" in Modern Nutrition in Health and Disease Ninth Edition Williams & Wilkins 1999,pp:467-482
Groff JL, Gropper SS & Hunt SM "The Water-Soluble Vitamins" in: Advanced Nutrition and Human Metabolism
Minneapolis West Publishing Co, MN 1995;pp. 222-237
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