The Bioavailability of Different Forms of Vitamin C

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The Bioavailability of Different Forms of Vitamin C

http://lpi.oregonstate.edu/ss01/bioavailability.html

Spring/Summer 2001 Issue

 

Jane Higdon, R.N., Ph.D.

LPI Research Associate

 

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.

 

Calcium ascorbate: Pure calcium ascorbate provides 114 mg of calcium

per 1,000 mg of ascorbic acid. 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®: 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.

 

• 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 small published studies. In one study synthetic ascorbic acid given in

a natural citrus extract containing bioflavonoids (in the ratio of

bioflavonoids to ascorbic acid of 4:1), proteins, and carbohydrates, was

more slowly absorbed and 35% more bioavailable than synthetic ascorbic acid

alone, based on plasma levels of ascorbate over time and 24-hour urinary

excretion of ascorbate. In the other study, there was no difference in the

bio-availability 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.

 

• Ascorbyl palmitate: Ascorbyl palmitate is a fat-soluble antioxidant

sometimes used to increase the shelf life of vegetable oils and potato

chips. 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 vitamin E (a fat-soluble antioxidant)

from oxidation by free radicals. 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. The presence of ascorbyl palmitate in oral

supplements contributes to the ascorbic acid content of the supplement and

probably helps protect lipid-soluble antioxidants during storage. 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.