What vitamin C is: A water-soluble vitamin, vitamin C (also called ascorbic acid) prevents and cures scurvy, a disease that is hardly seen any more. It is an antioxidant and is essential for healthy skin and connective tissue, wound healing, iron absorption and other functions. The Recommended Dietary Allowance (RDA) is 75 milligrams daily for women and 90 milligrams for men— the amount in about six ounces of orange juice. Smokers need an extra 35 milligrams a day, as do those exposed to tobacco smoke. The vitamin gained popularity in the 1970s when Nobel laureate Linus Pauling claimed it could prevent and treat colds as well as treat cancer.
Claims, purported benefits: Prevents or cures cancer, the common cold, osteoporosis, hypertension, Alzheimer’s disease, cataracts and many other diseases. Boosts immunity and improves arterial function. Protects against the adverse effects of strenuous exercise and improves performance.
Vitamin C: what the studies show
Colds. Many studies have failed to find that vitamin C can prevent colds. According to extensive reviews of studies by the Cochrane Collaboration in 2010 and 2013, vitamin C supplements do not prevent colds, except perhaps in people exposed to severe physical stress, such as marathon runners and skiers. Research on the vitamin’s potential role in reducing the severity and/or duration of cold symptoms when taken at their onset has yielded inconsistent results. Any such effect, if there is one, is apparently small.
Cancer. Although numerous studies have linked high dietary intakes of vitamin C (as well as other antioxidants) to reduced risk of certain cancers, results of studies on C supplements as a possible cancer preventive or treatment have been disappointing. Some researchers theorize that intravenously delivered vitamin C, providing levels far higher than possible with oral dosing, may help treat cancer. However, several studies suggest that high doses of vitamin C may actually harm cancer patients—for example, by interfering with chemotherapy and radiation therapy.
According to the Natural Standard, there is no convincing evidence that vitamin C prevents or cures cancer, cataracts, or heart disease. And a 2007 analysis of studies on antioxidant supplements in the Journal of the American Medical Association concluded that vitamin C pills do not help people live longer.
Cataracts. In 2005, Italian researchers found that high blood levels of the vitamin were associated with a reduced risk of cataracts, and the 2008 Blue Mountain Eyes Study linked higher intakes of C with a reduced risk. However, a 2009 Swedish study suggested that high-dose supplements, especially when taken for a decade or longer, may actually increase cataract risk. A 2012 review by the Cochrane Collaboration concluded that antioxidant supplements, including vitamin C and E, do not prevent or slow the progression of cataracts.
Blood pressure. A 2012 meta-analysis published in the American Journal of Clinical Nutrition found that vitamin C supplements may reduce blood pressure by one to five points in people with or without hypertension. The accompanying editorial pointed out that this effect, if true, “is undoubtedly too small clinically” to justify recommending the vitamin to people with hypertension. A 2012 study in Hypertension found that high-dose vitamin C (combined with vitamin E and alpha lipoic acid) improved arterial function in people over 70 but, surprisingly, disrupted it in people in their twenties, showing once again how unpredictable the effects of antioxidant supplements can be.
Combining C and E. Many studies have looked at vitamin C in combination with vitamin E, since they may have a synergistic effect against oxidative stress. Notably, a study in Archives of Neurology in 2005 found that people over 65 given high doses of C and E had a reduced risk of developing Alzheimer’s disease. This finding has not been replicated by other studies.
In 2009, a German study on vitamins C and E, in the Proceedings of the National Academy of Sciences, suggested that instead of protecting athletes, the supplements may block some of the benefits of exercise, notably its ability to improve insulin sensitivity and boost the body’s natural antioxidant system.
Side effects: The safe upper limit is 2,000 milligrams, according to government guidelines. Higher doses may cause diarrhea as well as kidney stones. The latter was confirmed by a 2013 Swedish study in JAMA Internal Medicine, which found that men who took vitamin C (typically 1,000 milligrams a day) were twice as likely to develop kidney stones.
Bottom line: Vitamin C supplements are not necessary or recommended. An intake of 75 to 90 milligrams daily appears to be all you need, and more does not seem to be better, except for smokers. Still, it’s a good idea to eat lots of fruits and vegetables rich in vitamin C.