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False Data, spread by hidden sources to make you fear vitamins. People who want to believe lies will believe them, no matter what I write.
Dr. Linus Pauling? Is he a sufficient authority on Vitamin C?
His legacy is carried forward by the Linus Pauling Institute, where this was published:
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The Bioavailability of
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It is possible to find vitamin C (ascorbic acid) in many different forms with any number of claims regarding its efficacy or bioavailability. Bioavailability refers to the degree to which a nutrient becomes available to the target tissue after it has been administered. Our conclusions about the various forms of vitamin C based on the available scientific evidence follow. More information and references are posted on the LPI Micronutrient Information Center. • Natural vs. synthetic ascorbic acid: Natural and synthetic ascorbic acid are chemically identical. As assessed by at least two studies, there appears to be no clinically significant difference in the bioavailability and bioactivity of natural ascorbic acid and synthetic ascorbic acid. • Different forms of ascorbic acid (powders, tablets, etc.): The gastrointestinal absorption of ascorbic acid occurs through an active transport process, as well as through passive diffusion. At low gastrointestinal concentrations of ascorbic acid active transport predominates, while at high gastrointestinal concentrations active transport becomes saturated, leaving only passive diffusion. In theory, slowing down the rate of gastric emptying (e.g. by taking ascorbic acid with food or taking a slow-release form of ascorbic acid) should increase its absorption. The bioavailability of ascorbic acid appears equivalent whether it is in the form of powder, chewable tablets, or non-chewable tablets. Moreover, bioavailability of ascorbic acid from slow-release preparations has not been found to be greater than that of plain ascorbic acid. • Mineral ascorbates: Mineral salts of ascorbic acid (mineral ascorbates) are buffered and therefore less acidic. Thus, mineral ascorbates are often recommended to people who experience gastrointestinal problems (abdominal pain or diarrhea) with plain ascorbic acid. There appears to be little scientific research to support or refute the claim that mineral ascorbates are less irritating to the gastrointestinal tract. When mineral salts of ascorbic acid are taken, both the ascorbic acid and the mineral appear to be well-absorbed, so it is important to take into consideration the dose of the mineral accompanying the ascorbic acid when taking large doses of mineral ascorbates. For the following discussion, it should be noted that 1 gram = 1,000 milligrams (mg) and 1 milligram (mg) = 1,000 micrograms (mcg). Mineral ascorbates are available in the following forms: Sodium ascorbate:
Sodium ascorbate generally provides 131 mg of
sodium per 1,000 mg of ascorbic acid (1,000 mg of
sodium ascorbate contains 889 mg of ascorbic acid
and 111 mg of sodium). Individuals following
low-sodium diets(e.g. for high blood pressure) are
generally advised to keep their total dietary
sodium intake to less than 2,500 mg/day. Megadoses
of sodium ascorbate could significantly increase
sodium intake.
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Honoring a Scientific Giant with Nutritional Research
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Last Update: Tuesday, 22-May-2001 10:50:50 PDT
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The Bioavailability of Different Preparations of Supplemental Vitamin C (Ascorbic Acid) In the rapidly expanding market of dietary supplements, it is possible to find vitamin C in many different forms with any number of claims regarding its efficacy or bioavailability. Bioavailability refers to the degree to which a nutrient (or drug) becomes available to the target tissue after it has been administered. We reviewed the literature for the results of scientific research on the bioavailability of different forms of vitamin C. Natural vs. synthetic ascorbic acid: Natural and synthetic L-ascorbic acid are chemically identical and there are no known differences in their biological activity. The possibility that the bioavailability of L-ascorbic acid from natural sources might differ from that of synthetic ascorbic acid was investigated in at least two human studies and no clinically significant differences were observed. A study of 12 males (6 smokers and 6 nonsmokers) found the bioavailability of synthetic ascorbic acid (powder administered in water) to be slightly superior to that of orange juice, based on blood levels of ascorbic acid, and not different based on ascorbic acid in leukocytes (white blood cells) (1). A study in 68 male nonsmokers found that ascorbic acid consumed as cooked broccoli, orange juice, orange slices, and synthetic ascorbic acid tablets are equally bioavailable, as measured by plasma ascorbic acid levels (2,3).
Different forms of ascorbic acid
(powders, tablets, etc.): A study of 3 men and 1 woman found 1 gram of ascorbic acid to be equally well absorbed from solution, tablets, and chewable tablets, but the absorption from a timed-release capsule was 50% lower. Absorption was assessed by measuring urinary excretion of ascorbic acid after an IV dose of ascorbic acid and comparing it to urinary excretions after the oral dosage forms (4). A more recent study examined the plasma levels of ascorbic acid in 59 male smokers supplemented for 2 months with either 500 mg/day of slow-release ascorbic acid, 500 mg/day of plain ascorbic acid, or a placebo. After 2 months of supplementation no significant differences in plasma ascorbic acid levels between the slow-release and plain ascorbic acid groups were found (5). Mineral ascorbates: Mineral salts of ascorbic acid (mineral ascorbates) are buffered, and therefore, less acidic. Thus, mineral ascorbates are often recommended to people who experience gastrointestinal problems (abdominal pain or diarrhea) with plain ascorbic acid. There appears to be little scientific research to support or refute the claim that mineral ascorbates are less irritating to the gastrointestinal tract. When mineral salts of ascorbic acid are taken, both the ascorbic acid and the mineral appear to be well absorbed, so it is important to take into consideration the dose of the mineral accompanying the ascorbic acid when taking large doses of mineral ascorbates. For the following discussion, it should be noted that 1 gram = 1000 milligrams (mg) and 1 milligram (mg) = 1000 micrograms (mcg). Mineral ascorbates are available in the following forms:
Sodium
ascorbate: Sodium ascorbate generally provides
131 mg of sodium per 1000 mg of ascorbic acid.
Individuals following low-sodium diets (e.g. for
high blood pressure) are generally advised to
keep their total dietary sodium intake to less
than 2500 mg/day. Thus, megadoses of vitamin C
could significantly increase sodium intake (see
Sodium Chloride). The following mineral ascorbates are more likely to be found in combination with other mineral ascorbates, as well as other minerals. It's a good idea to check the labels of dietary supplements for the ascorbic acid dose as well as the dose of each mineral. Recommended dietary intakes and maximum upper levels of intake (when available) are listed after the individual mineral ascorbates below:
Potassium ascorbate: The minimal requirement for
potassium is thought to be between 1.6 and 2.0
grams/day. Fruits and vegetables are rich sources
of potassium, and a diet rich in fruits and
vegetables may provide as much as 8 to 11
grams/day. Acute and potentially fatal potassium
toxicity (hyperkalemia) is thought to occur at a
daily intake of about 18 grams of potassium/day
in adults. Individuals taking potassium-sparing
diuretics and those with renal insufficiency
(kidney failure) should avoid significant intake
of potassium ascorbate. The purest form of
commercially available potassium ascorbate
contains 0.175 grams (175 mg) of potassium per
gram of ascorbic acid (see
Potassium). Vitamin C with bioflavonoids: Bioflavonoids are a class of water-soluble plant pigments. Vitamin C-rich fruits and vegetables, especially citrus fruits, are often rich sources of bioflavonoids as well. The effect of bioflavonoids on the bioavailability of ascorbic acid has been examined in two published studies. A study of 5 men and 3 women found that 500 mg of synthetic ascorbic acid given in a natural citrus extract containing bioflavonoids, proteins, and carbohydrates, were more slowly absorbed and 35% more bioavailable than synthetic ascorbic acid alone, based on plasma levels of ascorbic acid over time and 24-hr urinary excretion of ascorbic acid (7). In that study, the ratio of bioflavonoids to ascorbic acid (weight per weight) was 4:1, which is much higher than most commercially available products. Another study in 7 women and 1 man found no difference between the bioavailability of 500 mg of synthetic ascorbic acid and that of a commercially available vitamin C preparation with added bioflavonoids, where the ratio of bioflavonoids to ascorbic acid was 0.05:1 (6). Ascorbate and vitamin C metabolites (Ester-C®): Ester-C® contains mainly calcium ascorbate, but also contains small amounts of the vitamin C metabolites dehydroascorbate (oxidized ascorbic acid), calcium threonate, and trace levels of xylonate and lyxonate. In their literature, the manufacturers state that the metabolites, especially threonate, increase the bioavailability of the vitamin C in this product and they indicate that they have performed a study in humans that demonstrates the increased bioavailability of vitamin C in Ester-C®. This study has not been published in a peer-reviewed journal. A small published study of vitamin C bioavailability in 8 women and 1 man found no difference between Ester-C® and commercially available ascorbic acid tablets with respect to the absorption and urinary excretion of vitamin C (6). Ester-C® should not be confused with ascorbyl palmitate, which is also marketed as "vitamin C ester" (see below). Ascorbyl palmitate: Ascorbyl palmitate is a fat-soluble antioxidant used to increase the shelf life of vegetable oils and potato chips (8). It is an amphipathic molecule, meaning one end is water-soluble and the other end is fat-soluble. This dual solubility allows it to be incorporated into cell membranes. When incorporated into the cell membranes of human red blood cells, ascorbyl palmitate has been found to protect them from oxidative damage and to protect alpha-tocopherol (a fat-soluble antioxidant) from oxidation by free radicals (9). However, the protective effects of ascorbyl palmitate on cell membranes have only been demonstrated in the test tube. Taking ascorbyl palmitate orally probably doesn't result in any significant incorporation into cell membranes because most of it appears to be hydrolyzed (broken apart into palmitate and ascorbic acid) in the human digestive tract before it is absorbed. The ascorbic acid released by the hydrolysis of ascorbyl palmitate appears to be as bioavailable as ascorbic acid alone (10). The presence of ascorbyl palmitate in oral supplements contributes to the ascorbic acid content of the supplement and probably helps protect fat-soluble antioxidants in the supplement. The role of vitamin C in promoting collagen synthesis and its antioxidant properties have generated interest in its use on the skin. Ascorbyl palmitate is frequently used in topical preparations because it is more stable than some aqueous (water-soluble) forms of vitamin C (11). Ascorbyl palmitate is also marketed as, "vitamin C ester," which should not be confused with Ester-C® (see above). Erythorbic acid (isoascorbic acid): Erythorbic acid is an isomer of ascorbic acid. Isomers are compounds that have the same kinds and numbers of atoms, but different molecular arrangements. The difference in molecular arrangement among isomers may result in differences in chemical properties. Erythorbic acid is used in the U.S. as a food additive and is generally recognized as safe. It has been estimated that more than 200 mg erythorbic acid per capita is introduced daily into the U.S. food system. Unlike ascorbic acid, erythorbic acid does not appear to exert vitamin C activity, for example, it did not prevent scurvy in guinea pigs (one of the few animal species other than humans that does not synthesize ascorbic acid). However, guinea pig studies also indicated that increased erythorbic acid intake reduced the bioavailability of ascorbic acid by up to 50%. In contrast, a series of studies in young women found that up to 1000 mg/day of erythorbic acid for as long as 40 days was rapidly cleared from the body and had little effect on the bioavailability of ascorbic acid, indicating that erythorbic acid does not diminish the bioavailability of ascorbic acid in humans at nutritionally relevant levels of intake (12). Vitamin C and tooth enamel: Although a great deal of consumer information cautions against chewing large amounts of ascorbic acid due to its acidity and its erosive effects on tooth enamel, few scientific studies have addressed this issue. One study found that dissolving a 500 mg chewable vitamin C tablet in one’s mouth, like a mint, could result in sufficient salivary acidity for a long enough time period to cause enamel erosion. Chewable vitamin C tablets of 250 mg and 60 mg did not cause saliva to become acid enough for enough time to cause enamel erosion when dissolved in the mouth. The study did not examine the effect of chewing the tablets on salivary acidity. Although some chewable tablets are buffered with sodium, the authors felt that the sodium might be of insufficient quantity to prevent dental erosion. Because the application of vitamin C directly to the teeth and gums has no known therapeutic value they recommended swallowing vitamin C tablets (13). Some vitamin C supplement manufacturers recommend rinsing one’s mouth with water after chewing an ascorbic acid tablet to prevent enamel erosion. REFERENCES 1. Pelletier, O. & Keith, M.O. Bioavailability of synthetic and natural ascorbic acid. Journal of the American Dietetic Association. 1974; 64: 271-275 2. Mangels, A.R. et al. The bioavailability to humans of ascorbic acid from oranges, orange juice, and cooked broccoli is similar to that of synthetic ascorbic acid. Journal of Nutrition. 1993; volume 123: pages 1054-1061. (PubMed) 3. Gregory, J.F. Ascorbic acid bioavailability in foods and supplements. Nutrition Reviews. 1993; volume 51: pages 301-309. (PubMed) 4. Yung, S. et al. Ascorbic acid absorption in humans: a comparison among several dosage forms. Journal of Pharmaceutical Sciences. 1982; volume 71: pages 282-285. (PubMed) 5. Nyyssonen, K. et al. Effect of supplementation of smoking men with plain or slow release ascorbic acid on lipoprotein oxidation. European Journal of Clinical Nutrition. 1997; volume 51: pages 154-163. (PubMed) 6. Johnston, C.S. & Luo, B. Comparison of the absorption and excretion of three commercially available sources of vitamin C. Journal of the American Dietetic Association. 1994; volume 94: pages 779-781. 7. Vinson, J.A. & Bose, P. Comparative bioavailability to humans of ascorbic acid alone or in a citrus extract. American Journal of Clinical Nutrition. 1988; volume 48: pages 501-604. (PubMed) 8. Cort, W.M. Antioxidant activity of tocopherols, ascorbyl palmitate, and ascorbic acid and their mode of action. Journal of the American Oil Chemists’ Society. 1974; volume 51: pages 321-325. 9. Ross, D. et al. Ascorbate 6-palmitate protects human erythrocytes from oxidative damage. Free Radical Biology and Medicine. 1999; volume 26: pages 81-89. (PubMed) 10. DeRitter, E. et al. Physiologic availability of dehydro-L-ascorbic acid and palmitoyl-L-ascorbic acid. Science. 1951; volume 113: pages 628-631. 11. Austria R. et al. Stability of vitamin C derivatives in solution and in topical formulations. Journal of Pharmacology and Biomedical Analysis. 1997; volume 15: pages 795-801. (PubMed) 12. Sauberlich, H.E. et al. Effects of erythorbic acid on vitamin C metabolism in young women. American Journal of Clinical Nutrition. 1996; volume 64: pages 336-346. (PubMed) 13. Hays, G.L. et al. Salivary pH while dissolving vitamin C-containing tablets. American Journal of Dentistry. 1992; volume 5: pages 269-271. (PubMed) |
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Reviewed
by:
Balz Frei, Ph.D., Director
The Linus Pauling Institute
Oregon State University
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