How much vitamin C do we need, and what’s the real immune-boosting potential of supplements?
Vitamin C, also known as ascorbic acid, is a water-soluble vitamin found in many multivitamin supplements and single-entity nutritional supplements. Vitamin C has an important role in the formation of collagen, carnitine, amino acids and hormones; is an essential component in the healing of wounds and burns; and aids in the absorption of iron. Moreover, vitamin C is classified as an antioxidant, and many clinical studies report that megadoses (500 to 1000 milligrams per day) of ascorbic acid may prevent, or shorten the duration of, the common cold.
While vitamin C is approved by the U.S. Food and Drug Administration for the treatment of scurvy, vitamin C is most often used by consumers to boost the immune system. Results from a 2010 analysis of 29 clinical trials with an estimated 11,000 subjects reported that taking vitamin C routinely (at least 0.2 grams day) did not decrease the likelihood of getting a cold. However, the use of vitamin C supplements was associated with a slight decrease in the severity of cold symptoms and the duration of the common cold, and some studies report a reduction of 8 percent in adults and 14 percent in children.
A review published in 2009 presenting research on the effects of vitamins and minerals for treating colds concluded that supplementation with vitamin C does have some potential benefits for treating the common cold; however, because there are only a few therapeutic trials, more research is needed to ascertain the effective dosages and the treatment guidelines. Many clinical studies suggest that vitamin C is most effective for reducing the duration of upper respiratory tract infection symptoms in children and healthy adults, but it does not appear to be as useful once a patient exhibits symptoms.
In general, vitamin C is considered safe, but adverse effects have been reported when this supplement is taken in megadoses. When ingested in megadoses of four or more grams per day, vitamin C may cause diarrhea, nausea, stomach cramps and nephrolithiasis; therefore, megadoses of the vitamin are typically not recommended. Other potential adverse effects include headache, dizziness, flushing and fatigue.
To prevent the development of kidney stones, you should drink a full glass of water when taking vitamin C. People with diabetes, a history of recurring renal calculi and/or renal dysfunction should avoid prolonged use of megadoses of vitamin C supplements. Although most individuals in the United States obtain adequate vitamin C via dietary means, the NIH reports that certain populations may be at greater risk of not obtaining sufficient recommended daily allowances.
Individuals at risk for vitamin C deficiency:
- Smokers may need 35 milligrams per day more vitamin C because smoke can increase the amount of vitamin C that the body requires to repair damage due to free radical exposure
- Infants who are fed with artificial formula that does not contain vitamin C
- Individuals whose diets are low in vitamin C
- Individuals with severe malabsorption syndrome and individuals with end-stage renal disease undergoing dialysis
Nonprescription vitamin C dietary supplements typically contain ascorbic acid, which is considered to have bioavailability equivalent to that of naturally occurring ascorbic acid in foods such as orange juice and broccoli, but some supplements contain other forms, such as sodium ascorbate, calcium ascorbate, other mineral ascorbates and ascorbic acid with bioflavonoids. Adult multivitamin supplements typically contain doses of 60 to 100 milligrams, which are considered to be adequate if supplements are required. Vitamin C is available in several dosage formulations, including capsule, tablet, lozenge, syrup, chewable tablet, effervescent tablet, oral disintegrating tablet and gummy. According to the Handbook of Nonprescription Drugs, the recommended daily limit of vitamin C is 2 grams.
Examples of nonprescription vitamin C supplements:
- Alive Vitamin C powder
- Emergen C
- Ester C Vitamin C
- Halls Defense Vitamin C Lozenges
- Nature’s Bounty
- Nature Made Vitamin C
- Sunkist Vitamin C
- Vitafusion Power C Adult Gummies
Ideally, you should obtain vitamin C through dietary means. People who choose to use supplements should take only the recommended dosage unless otherwise directed by their primary health care provider. These people should also be screened for potential drug interactions, contraindications and therapeutic duplications. For example, vitamin C supplements may interact with chemotherapeutic agents, so talk to your primary health care provider before using these supplements. Some research indicates that vitamin C supplements may also interact with HMG-CoA reductase inhibitors (statins) when taken in combination with other antioxidants, such as beta carotene and vitamin E. This combination may decrease the effectiveness of statins; however, it is not known whether vitamin C alone impacts the effectiveness of this drug class.
In addition, megadoses of vitamin C may decrease the effectiveness of some agents, such as protease inhibitors, warfarin, estrogens and niacin.