Vitamin C, also known as ascorbic acid, is an essential nutrient in the human diet. It plays a critical role in the synthesis of the neurotransmitter norepinephrine --- a molecule that affects mood and regulates brain function --- as well as the collagen in your skin, cartilage, tendons, ligaments and blood vessels. Vitamin C is also an antioxidant, which is an important molecule that prevents the kind of cellular damage normally acquired from the metabolic processes that sustain living organisms. Like other nutrients, the human body can only handle a certain amount of vitamin C.
Upper Limit
In 2000, the Food and Nutrition Board of the Institute of Medicine set a tolerable upper limit of vitamin C for the first time. Their recommendation was no more than 2 g per day in adults. This is the threshold at which the majority of adults begin experiencing side effects. The most common symptom is the acidification of the urine, which can cause diarrhea and gastrointestinal disturbances. It can also interfere with a normal antioxidant balance and, in patients with the blood disorder thalassemia or hemochromatosis, promote an iron overload. This last symptom is more severe and may disrupt the normal function of organs.
Cancer Risk
Some studies, including one in a 2001 issue of the journal "Science," have brought up the possibility that under certain conditions, excess vitamin C, by reacting unfavorably with other molecules, may harm the DNA in the cells and affect the risk of cancer. However, Balz Frei, Ph.D., a Professor of Biochemistry and Biophysics at the Linus Pauling Institute of Oregon State University, considers this conclusion unwarranted due to the unlikely conditions necessary to seriously affect DNA. No studies have yet proved the claim.
Toxicity
Because vitamin C is water-soluble, high levels tends to be excreted through the urine rather than absorbed into the blood, so the excess consumption of vitamin C will produce few toxic side effects in healthy adults. Doses of at least 10 g per day are not rare and only cause temporary symptoms. In animal models, the LD50 --- the amount necessary to poison 50 percent of a population --- is between 5 and 12 g/kg of body weight, according to the chemistry department of Oxford University. If the same holds true in humans, then for practical purposes there is no risk of consuming more vitamin C than your body can cope with.
Dietary Intake
Vitamin C is present within fruits and vegetables such as strawberries, tomatoes, peppers, broccoli, potatoes, mango, cantaloupe, watermelon and citrus fruits. The recommended dietary allowance is only 90 mg for adult males and 75 mg for adult females a day, although pregnant and breastfeeding women may need 85 and 120 mg per day, respectively. By comparison, five servings of fruits and vegetables, the recommended intake, average out to 200 mg of vitamin C. Therefore, the only practical way to willfully consume more than the upper limit is through the daily use of dietary supplements. If you feel the side effects of excess consumption, then you should merely cut back on the amount of vitamin C you intake.
References
- Oregon State University Linus Pauling Institute; Vitamin C; Jane Higdon; January 2006
- The Merck Manual; Vitamin C; Larry E. Johnson; April 2007
- Oregon State University Linus Pauling Institute; Vitamin C Doesn't Cause Cancer!; Balz Frei; November 2001
- Oxford University; Safety Data for Ascorbic Acid; September 2010
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