Vitamin C, also known as ascorbic acid, is a water-soluble vitamin needed by the body for hundreds of vital metabolic reactions. The vitamin C deficiency disease, scurvy, is prevented by adequate intake of ascorbic acid (“ascorbic” literally means “without scurvy”). As a dietary supplement, vitamin C is consumed by more people than any other vitamin, mineral or herbal product. Good food sources of vitamin C include all citrus fruits (oranges, grapefruit, lemons) as well as many other fruits and vegetables such as strawberries, tomatoes, broccoli, Brussels sprouts, peppers and cantaloupe. Vitamin C is a fairly “fragile” vitamin and can be easily destroyed by cooking or exposure of food to oxygen. Dietary supplement containing vitamin C are generally promoted as an antioxidant, and specifically to prevent colds, boost immunity, and reduce the risk of certain cancers.
As a water-soluble vitamin, ascorbic acid performs its antioxidant functions within the aqueous compartments of the blood and inside cells and can help restore the antioxidant potential of vitamin E (a fat-soluble antioxidant). Vitamin C also functions as an essential cofactor for the enzymes involved in the synthesis of collagen – the chief structural protein in connective tissues such as bones, cartilage and skin. As such, vitamin C is often recommended for wound healing and as an added ingredient in supplements designed for healthy skin.
As a preventive against infections such as influenza and other viruses, vitamin C is thought to strengthen cell membranes, thereby preventing entrance of the virus to the interior of the cell. Support of immune cell function is also a key role performed by vitamin C and an effect which may help fight infections in their early stages. The combined effects of cellular strengthening, collagen synthesis and antioxidant protection are thought to account for the multi-faceted approach by which vitamin C helps to maintain health.
As a dietary supplement, vitamin C is the most popular single nutrient supplement. It is typically included in all multivitamin blends, but at widely varying levels. As a single nutrient supplement, typical doses range from 100-500mg per tablet. The body, however, can only absorb and retain about 250mg of vitamin C at one time – the rest is quickly washed out in the urine. This means that the most effective approach to supplementing with vitamin C is to take it in divided doses throughout the day.
Vitamin C is one of the least expensive dietary supplements available – with a monthly supply costing no more than a few dollars. Beware of expensive “natural” forms of ascorbic acid, as there is no reliable evidence to show that these forms (derived from rose hips or acerola and containing varying levels of bioflavonoids in addition to ascorbic acid) are any better absorbed or utilized by the body compared to synthetic vitamin C (but this is not the case with vitamin E, where the natural form is clearly superior to the synthetic form).
As a way to prevent or reduce the symptoms associated with the common cold, over 100 vitamin C studies have been conducted. In several of the largest studies, no effect on common cold incidence is observed, indicating to many scientists that vitamin C has no preventive effects in normally nourished subjects (Douglas et al. 2004). However, a number of smaller targeted studies, conducted in subjects under heavy acute physical stress, show that vitamin C decreases the incidence of the common cold by half (Douglas et al. 2004). In other studies, healthy subjects consuming low levels of vitamin C (below 60mg/day), have a cold incidence that is about one-third lower following vitamin C supplementation (Wolters et al. 2004).
In general, regular vitamin C supplementation at levels at or slightly above 1,000mg/day has consistently reduced the incidence and duration of colds (although the degree of benefit has varied significantly). For example, in some of the larger studies, duration of infection was reduced by only about 5%, but absentee reports were reduced by nearly 20% (Fletcher et al. 2003). At least 3 controlled studies have shown an 80% reduction in the incidence of pneumonia among vitamin C users (Sasazuki et al. 2005). In one large study (over 700 students), vitamin C (1000mg per hour for the first 6 hours followed by 3000mg per day), reduced cold and flu symptoms by 85% (Audera et al. 2001).
In most cases, it appears that while the most important and dramatic preventive effects of vitamin C supplementation will be experienced by individuals with low vitamin C intakes, those with average daily consumption from foods may also benefit from supplemental levels. In support of an elevated vitamin C intake, an expert scientific panel recently recommended increasing the current RDA for vitamin C from 60mg to at least 100-200mg per day. This same panel also cautioned that taking more than 1,000mg of vitamin C daily could have adverse effects (read more under Safety) and recommended that “whenever possible, vitamin C intake should come from fruits and vegetables” (at least five servings of fruits and vegetables daily). The current RDA of 60mg per day was established in 1980, but since the last RDA review (in 1989), scientists have learned a great deal more about the functions of vitamin C in the body and the need for elevated vitamin C consumption.
Safety / Dosage
As a water-soluble vitamin, ascorbic acid C is extremely safe even at relatively high doses (because most of the excess is excreted in the urine). At high doses (over 1,000mg/day), some people can experience gastrointestinal side effects such as stomach cramps, nausea, and diarrhea. In addition, vitamin C intakes above 1,000mg/day may increase the risk of developing kidney stones in some people. Although the RDA for vitamin C has recently been raised from 60mg to 75-90mg (higher for men), it is well established that almost everybody can benefit from higher levels. For example, the vitamin C recommendation for cigarette smokers is 100-200mg per day because smoking destroys vitamin C in the body (Schectman et al. 1991). The vitamin C deficiency disease, scurvy, can be prevented by consumption of as little as 10mg daily of vitamin C, but somewhat higher levels are prudent during exposure to stress (physical or psychological) or infection (like a sick friend of family member).
Although vitamin C is well absorbed, the percent absorbed from supplements decreases with increasing dosages and optimal absorption is achieved by taking several small doses throughout the day (100-200mg/dose for a total daily intake of 200-1,000mg). Full blood and tissue saturation is typically achieved with daily intakes of 200-500mg per day (in 2-3 divided doses).
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EDITOR'S NOTE: This monograph can be found in The Health Professional's Guide to Dietary Supplements (Lippincott, Williams & Wilkins) by Shawn M. Talbott, PhD and Kerry Hughes, MS.