Vitamin C has many functions including:
- Prevention of scurvy (vitamin C deficiency). Symptoms include bleeding gums, fatigue, muscle weakness, unhealed wounds, nosebleeds and bruising.
- Formation of collagen (cementing protein that holds cells together) needed for strengthening bones, teeth, skin and blood vessels.
- Wound healing and protection from infections and bruising (immunity).
- Production of amino acids (makes proteins), hormones, hemoglobin (carries oxygen in red blood cells) and neurotransmitters (brain's chemical messengers).
- Metabolism of folic acid (B-vitamin).
- Increased absorption of iron from plant-based foods (e.g., cereal, bread).
- Antioxidant (neutralizes "free radicals" that damage cells) that may reduce risk for cancer (e.g., gastrointestinal, lung, cervix), heart and eye diseases, strokes and other diseases. May reduce asthma symptoms (prevents lung damage).
- Enhancement of action of other antioxidants such as vitamin E.
Food sources: The best food sources for vitamin C are fruits (especially citrus and tropical varieties), vegetables and juices.
Examples include oranges, grapefruits, lemons, limes, strawberries, kiwifruit, melons, mangoes, peppers (red and green), cabbage, tomatoes, potatoes, cauliflower and dark green, leafy vegetables (see chart on this page). Apples (and bananas) are not rich in vitamin C, so serve vitamin C-fortified apple juices.
Fresh-squeezed orange juice usually contains more vitamin C than frozen or canned. But, vitamin C content of all juices declines with time (about 50% in one month) and exposure to air (juice in cartons loses vitamin C). Keep juices refrigerated in tightly closed glass containers, and use quickly.
How much: In 2001, the Daily Reference Intake for vitamin C was increased to 75 mg./day for women and 90 mg./day for men. For smokers, an additional 35 mg./day is advised.
The optimal intake is still unknown. The amount needed to saturate tissues and help prevent diseases (especially in the elderly) is higher than the DRI—about 200 mg. /day.
However, food sources (at least five servings of fruits and vegetables daily) are recommended rather than supplements. These foods also provide fiber, other nutrients (e.g., potassium and beta carotene) and phytochemicals (plant substances) that may interact with vitamin C to reduce risk of chronic diseases.
Possible toxicity: Excess vitamin C from supplements can lead to diarrhea, kidney stones, infections, blood clotting or nutrient deficiencies (e.g., copper). Over 500 mg./day may not be well absorbed. The "Tolerable Upper Intake Level" (UL) is 2,000 mg./day for adults.
Disease prevention: Several large-scale studies are currently investigating how and if antioxidants like vitamin C reduce risk for chronic diseases. Preliminary research with supplements has been inconsistent and disappointing. Here's a summary of some recent findings:
Heart Disease—Vitamin C may slow or prevent the oxidation of "bad" LDL-cholesterol in blood to prevent plaque build-up in arteries. Vitamin C may also increase nitric oxide production which dilates arteries allowing blood to flow freely (prevents clots). Vitamin C may boost "good" HDL-cholesterol (protective) and reduce blood pressure by dilating blood vessels.
Stroke—Men with the lowest blood levels of vitamin C had more than double the risk of stroke (blood clot in the brain) as men with the highest levels.
Cancer—Vitamin C may mop up free radicals, detoxify carcinogens (cancer-causing agents) and boost immunity. Vitamin C may also inhibit growth of H. pylori (bacteria) which causes ulcers and increases risk of stomach cancer. In the stomach, vitamin C can block formation of nitrosamines (carcinogens) from nitrites and nitrates (found in vegetables and salt-cured foods like hotdogs and cold cuts). Nitrosamines may increase risk of stomach cancer.
Eye diseases—Some people taking vitamin C supplements had lower risk of cataracts. For elderly who already had age-related macular degeneration (ARMD), which can cause blindness, supplements containing vitamin C (and other antioxidants like vitamin E plus zinc and copper) slowed the progression of ARMD, but did not lower cataract risk.
Colds—High doses of vitamin C do not prevent colds. In some studies, vitamin C supplements reduced the severity and duration of a cold, but only slightly.
Gallbladder disease—Bile (stored in the gallbladder to help digest fat) is made from cholesterol. Vitamin C helps convert cholesterol into bile to prevent formation of gallstones from cholesterol. In one study, women with high blood levels of vitamin C had less gallbladder disease than women with low levels.
No such connection was seen in men, but they have a lower incidence of gallbladder disease. In another study, postmenopausal women taking vitamin C supplements also developed less gallbladder disease.
Brain diseases—Supplements of antioxidants like vitamins C and E may prevent or slow the progression of Alzheimer's and Parkinson's diseases. In one study, supplements were also associated with less dementia and memory loss in the elderly.