Vitamin C (Ascorbic Acid)

Overview

Vitamin C functions primarily to form collagen, the primary protein used to make connective tissue and form scar tissue, and is needed to form bone, and cartilage. Vitamin C is also an antioxidant which protects all cells from oxidative damage. It also protects other vitamins such as vitamin E and A from oxidation as well. Research has shown that vitamin C protects the body against heart disease and many types of cancer. It is essential to proper wound healing, and has shown benefit for use in treating exercise-induced asthma. Most importantly, it is now known that vitamin C is critical to proper immune system function. Infection and inflammation rapidly deplete vitamin C from the body. Constant replacement is needed in order to maximize immune function and overcome infection.

Probably the biggest controversy surrounding vitamin C in the past few decades is whether or not it prevents colds. Research has shown that vitamin C does reduce the formation of histamine, which may help reduce cold symptoms. Clinical studies have shown that although vitamin C can reduce the length and severity of cold symptoms, it has not yet been proven to prevent colds. However, maintaining a strong and healthy immune system which increases resistance to all types of infection and disease is a good reason for consuming adequate amounts of vitamin C. The latest research on vitamin C shows that it is protective against sunburn and reduces the development of nitrate tolerance during nitroglycerin therapy in congestive heart failure patients.

Dietary Sources

Vitamin C is present in many fruits and vegetables. The following foods are excellent sources of vitamin C: orange juice, green peppers, watermelon, papaya, grapefruit juice, grapefruit, cantaloupe, strawberries, mango, broccoli, tomato juice, brussels sprouts, cauliflower, and cabbage. Vitamin C is also found in raw and cooked leafy greens, (turnip greens, spinach), canned or fresh tomatoes, potatoes, winter squash, raspberries, and pineapple. Vitamin C is sensitive to light, air, and heat. Eating vegetables raw, or minimally cooked, increases their vitamin C content.

Constituents/Composition

Vitamin C is ascorbic acid. It becomes dehydroascorbic acid when it is oxidized by the body. Ascorbic acid is the form that is used for supplements and as a food additive to protect against oxidative spoilage.

Commercial Preparations

You can purchase either natural or synthetic ascorbic acid in a wide variety of supplement forms. Tablets, capsules, and chewable tablets are probably the most popular, but vitamin C also comes in powdered crystalline, effervescent tablet and liquid form. You can purchase dosages from 25 mg to 1,000 mg per tablet; the most common are 100 mg, 250 mg, 500 mg, and 1,000 mg. "Buffered" C is available for those who find regular ascorbic acid is too harsh for their stomachs. "Ester-C" is a form of vitamin C composed of esters, which is promoted to be better absorbed by the body. Laboratory testing has challenged this claim, showing that absorption did not differ significantly from regular vitamin C.

Therapeutic Uses

• Boosts immune functions
• Protects against cancer
• Necessary for wound healing
• Helps prevent cataract development
• Increases HDL (good) cholesterol
• Decreases risk of cardiovascular disease
• Reduces blood pressure
• Decreases histamine levels (useful in treating allergies)
• Reduces capillary fragility
• Counteracts asthma (reduces airway spasms)
• Helps overcome male infertility
• Used in treatment of cervical dysplasia
• Helps protect diabetics against long-term complications by lowering sorbitol levels and reducing glycosylation of proteins
• Protects against sunburn and its effects
• Increases integrity of connective tissue, assisting treatment of bleeding gums, bruising, and arthritis
• Is anti-inflammatory
• Protects fat-soluble antioxidants (e.g., vitamin E) from oxidation

Dosage Ranges and Duration of Administration

The range of safe vitamin C intake appears to be very broad, and research has proved that even very high daily doses over 10 g (10,000 mg) for extended periods of time are usually well tolerated. Although the minimum daily requirement is 60 mg/day, physical stresses such as exertion, illness, infection, surgery, wound healing, exposure to toxic chemicals and metals, high or low temperatures, smoking, and chronic use of medication all increase the need for vitamin C. The amount needed varies with the severity of the stress, but may be as high as 1,000 mg/day or even higher. While consuming 60 mg/day of vitamin C may be sufficient to prevent deficiency symptoms, at least 100 to 250 mg is needed to saturate the tissues. And, since vitamin C is water-soluble and not stored in the body, the tissue supply requires constant replacement. Eating foods rich in vitamin C and/or dividing a supplement dose to be taken bid or tid is optimal. General recommendations for vitamin C intake is 250 to 500 mg/day for the average healthy adult. Some nutritionists recommend 500 to 1,000 mg tid (with each meal) for recovery after illness, surgery, or severe injury.

Side Effects/Toxicology

Vitamin C is considered nontoxic; excesses are excreted by the body. High doses of vitamin C can cause diarrhea, and nutritionists state that this side effect is useful in assessing an individual's tolerance and optimal daily allowance for proper tissue saturation. Diarrhea, gas, or other intestinal disturbances are harmless and reversible with lower dosing. Although the literature is full of speculated warnings, no harmful effects even from long-term "megadosing" have been observed.

Warnings/Contraindications/Precautions

Individuals with a history of kidney problems of any kind may not be candidates for vitamin C supplements due to increased urinary oxalate formation in oxalate kidney stone formers. Infants born to mothers taking 6 g or more of vitamin C may develop rebound scurvy after birth, due to the sudden drop in intake.

Interactions

Acetaminophen

Oral administration of vitamin C (3 gm po) 90 minutes after acetaminophen dosing has been shown to cause a rapid and pronounced decrease in the excretion rate of acetaminophen sulfate (Houston and Levy 1976).

Aspirin

In a double-blind, randomized, crossover study designed to evaluate the effects of aspirin on gastroduodenal injury, 14 healthy volunteers received aspirin (900 mg bid) and either allopurinol (100 mg bid), sulphasalazine (1 g bid), vitamin C (1 g bid), or placebo (McAlindon et al. 1996). Vitamin C reduced aspirin-induced duodenal injury. Renal clearance of aspirin and other acidic medications may also be reduced by high doses of vitamin C (500 mg/day or more), especially in older patients (Schumann 1999).

Furosemide

In dogs, the combination of furosemide (20 mg IV or 40 mg po) and ascorbic acid (150 mg or 500 mg po) increased both urinary output and excretion of unchanged furosemide (Lee and Chiou 1998). Similar results were observed in rats treated with both furosemide (6 mg po) and ascorbic acid (up to 100 mg po).

Isosorbide Dinitrate; Isosorbide Mononitrate; Nitroglycerin

Administration of vitamin C during long-term nonintermittent administration of glycerolnitrate eliminated vascular tolerance in healthy subjects (Bassenge et al. 1998). In a double-blind, placebo-controlled study with 24 healthy volunteers and 24 patients with ischemic heart disease, coadministration of vitamin C (2 g tid) and nitroglycerin prevented nitrate tolerance (Watanabe et al. 1998a). Concomitant administration of intravenous vitamin C and nitroglycerin also prevented nitrate tolerance in patients with congestive heart failure (Watanabe et al. 1998b).

Nonsteroidal Anti-Inflammatory Drugs (NSAIDs)

Vitamin C (1 g bid) protects against aspirin-induced duodenal injury; it may also protect against gastroduodenal injury induced by other NSAIDs such as ibuprofen (McAlindon et al. 1996).

Oral Contraceptives

Ascorbic acid (1 g) can interfere with oral contraceptive metabolism, leading to higher blood levels of estradiol (Stockley 1999). However, daily use of ascorbic acid and a combination oral contraceptive (30 µg ethinyl estradiol and 150 µg levonorgestrel) in 37 women for two consecutive cycles did not affect Cmax and AUC values for ethinyl estradiol (Zamah et al. 1993). A similar study also found that vitamin C did not affect Cmax and AUC values for levonorgestrel (Kuhnz et al. 1995). In addition, vitamin C exhibited antioxidant effects when supplied with 17beta-estradiol close to physiological levels (Huang et al. 1999). It protected LDL from oxidation in vitro; 17beta-estradiol monotherapy had no antioxidant effect.

Propranolol

Administration of propranolol (80 mg po) to five healthy volunteers pretreated with ascorbic acid (2 g) affected the absorption and metabolism of the drug (Gonzalez et al. 1995). This combination decreased maximum plasma concentrations and urinary excretion of the drug, and increased the time to reach Cmax.

Tetracycline

In one study, the bioavailability of tetracycline hydrochloride was increased by concomitant administration of vitamin C (100 mg/day) in healthy subjects (Omray 1981). More research is needed to confirm these effects.

Warfarin

There are case reports of decreased prothrombin in patients taking vitamin C and warfarin (Harris 1995; Smith 1972). In one case, the patient was taking high doses of vitamin C (16 g/day) (Smith 1972). In follow-up studies, no association was found between vitamin C (1 g/day) and warfarin in humans (Harris 1995). Patients taking warfarin should not exceed the recommended dietary allowance for vitamin C (60 mg/day for adults).

References

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