The Bioavailability of Different Forms of Vitamin C
Higdon, R.N., Ph.D.
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:
Calcium ascorbate: 1,000 mg of calcium ascorbate generally provides 890-910 mg of ascorbic acid and 90-110 mg of calcium. Calcium in this form appears to be reasonably well absorbed. The recommended dietary calcium intake for adults is 1,000 to 1,200 mg/day. Total calcium intake should not exceed the tolerable upper intake level of 2,500 mg/day.
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, so 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 on 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.
Magnesium ascorbate: The recommended dietary allowance (RDA) for magnesium is 400-420 mg/day for adult men and 310-320 mg/day for adult women. The maximum upper level of intake for magnesium from supplements is 350 mg/day.
Zinc ascorbate: The RDA for zinc is 11 mg/day for adult men and 8 mg/day for adult women. The upper intake level of zinc for adults is 40 mg/day.
Molybdenum ascorbate: The RDA for molybdenum is 45 micrograms (mcg)/day for adult men and women. The upper intake level of molybdenum for adults is 2,000 mcg (2 mg)/day.
Chromium ascorbate: The recommended dietary intake for chromium is 30-35 mcg/day for adult men and 20-25 mcg/day for adult women. An upper level of intake has not been determined by the U.S. Food and Nutrition Board.
Manganese ascorbate: The recommended dietary intake for manganese is 2.3 mg/day for adult men and 1.8 mg/day for adult women. The upper level of intakefor manganese is 11 mg/day. Manganese ascorbate is found in some preparations of glucosamine and chondroitin sulfate. Following the recommended dose on the label could result in a daily intake exceeding the upper intake level for manganese.
Ester-C® contains mainly calcium ascorbate, but also contains
small amounts of the vitamin C metabolites dehydroascorbic acid (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 bio-availability of the vitamin C in this product
and that they have performed a study in humans demonstrating 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 excretion of vitamin C.
Last updated May, 2001
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