vitamin c

Vitamin C (ascorbic acid) is a water-soluble vitamin. The human body cannot make it, so it has to come from food or supplements. According to the NIH Office of Dietary Supplements, it works as an antioxidant, is needed to build collagen for wound healing, improves the absorption of iron from plant-based foods, and helps the immune system function normally.

Vitamin C reliably prevents and treats scurvy, but the evidence for most other popular uses is limited or mixed. For the common cold, the NIH states that for most people vitamin C supplements do not reduce the risk of catching a cold; people who take it regularly may have slightly shorter colds or somewhat milder symptoms, but starting it after symptoms appear does not appear to help. The NIH also reports that vitamin C supplements do not seem to protect against cancer, that scientists are not sure whether vitamin C protects against cardiovascular disease, and that its relationship to cataracts is unclear. For age-related macular degeneration, a specific high-dose antioxidant-and-zinc combination (the AREDS formula, which in the cited study supplied 500 mg vitamin C alongside 80 mg zinc, 400 IU vitamin E, 15 mg beta-carotene, and 2 mg copper) was associated with a lower chance of progressing to advanced disease among people already at high risk; it is not evidence that vitamin C alone prevents AMD.

The Recommended Dietary Allowance for adults is 90 mg per day for men and 75 mg per day for women; people who smoke need an additional 35 mg per day. The NIH notes that most people in the US already get enough vitamin C from food and beverages. The Tolerable Upper Intake Level for adults, from food, beverages, and supplements combined, is 2,000 mg per day. These are general reference values, not personal medical advice, so talk with your doctor, pharmacist, or a registered dietitian before starting a supplement, especially at high doses or if you have a health condition.

Vitamin C is generally considered to have low toxicity at recommended intakes, and because it is water-soluble, absorbed amounts the body does not use are largely excreted in urine. The NIH cautions that people with hemochromatosis (a condition that causes the body to store too much iron) could worsen iron overload and tissue damage with chronic high doses. The NIH also notes that high intakes can raise urinary oxalate, which may contribute to kidney stones, with the clearest signal in people who already have a tendency toward stones or pre-existing hyperoxaluria. Anyone being treated for cancer should check with their oncologist before taking vitamin C, especially in high doses.

Taking too much vitamin C can cause diarrhea, nausea, and stomach cramps, according to the NIH. These gastrointestinal effects are associated with high intakes; the NIH sets the upper limit for adults at 2,000 mg per day from all sources.

The NIH notes that vitamin C and other antioxidant supplements may interact with chemotherapy and radiation therapy, and it is not clear whether they might shield tumor cells from treatment or instead protect healthy tissue, so people being treated for cancer should consult their provider before using them. In one study, vitamin C taken with other antioxidants blunted the heart-protective effect of a statin-plus-niacin combination used to control cholesterol; whether this happens with other statins is not known, and clinicians may monitor lipid levels in people taking both statins and antioxidant supplements. Always tell your doctor and pharmacist about every supplement and medication you take.