Vitamin C: Benefits, Dosage, and Safety

What vitamin c is

Vitamin C, also known as L-ascorbic acid, is a water-soluble vitamin that is naturally present in some foods (mainly fruits and vegetables), added to others, and available as a dietary supplement. Humans, unlike most animals, cannot synthesize vitamin C endogenously, so it is an essential dietary component. Intestinal absorption is regulated by a dose-dependent active transporter: roughly 70% to 90% is absorbed at moderate intakes of 30 to 180 mg/day, but at doses above 1 g/day absorption falls to less than 50% and unmetabolized ascorbic acid is excreted in the urine.

What it does

• Required for the biosynthesis of collagen, an essential component of connective tissue that plays a vital role in wound healing
• Needed to make L-carnitine and certain neurotransmitters, and is involved in protein metabolism
• Acts as an important physiological antioxidant that limits damage from free radicals and helps regenerate other antioxidants, including alpha-tocopherol (vitamin E)
• Plays an important role in immune function
• Improves the absorption of nonheme iron, the form of iron present in plant-based foods

How much vitamin c per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

Vitamin C deficiency causes scurvy. Signs and symptoms can appear within about 1 month of consuming less than about 10 mg/day. Initial symptoms include fatigue (probably from impaired carnitine biosynthesis), malaise, and inflammation of the gums. As deficiency progresses and collagen synthesis becomes impaired, connective tissues weaken, causing petechiae, ecchymoses, purpura, joint pain, poor wound healing, hyperkeratosis, and corkscrew hairs. Additional signs include depression, swollen and bleeding gums, and loosening or loss of teeth. Iron deficiency anemia can occur, and in children bone disease can be present. Left untreated, scurvy is fatal. Deficiency and scurvy are rare in developed countries.

Who may need more

• People who smoke or are regularly exposed to secondhand smoke (require an extra 35 mg/day)
• Infants fed evaporated or boiled milk rather than breastmilk or fortified formula
• People with a limited variety of food in their diet
• People with severe intestinal malabsorption, cachexia, or some cancers
• People with end-stage renal disease on chronic hemodialysis

Food sources

Fruits and vegetables are the best sources, especially citrus fruits, red and green peppers, kiwifruit, broccoli, strawberries, Brussels sprouts, and cantaloupe. Citrus fruits, tomatoes and tomato juice, and potatoes are major contributors to the American diet. Some breakfast cereals are fortified with vitamin C. Because ascorbic acid is water soluble and destroyed by heat, prolonged storage and cooking can reduce food content; steaming or microwaving lessens losses. Five varied servings of fruits and vegetables a day can provide more than 200 mg.

Forms and absorption

Ascorbic acid is the most common supplement form and has bioavailability equivalent to naturally occurring ascorbic acid in foods such as orange juice and broccoli. Other forms include sodium ascorbate, calcium ascorbate, other mineral ascorbates, ascorbic acid with bioflavonoids, and combination products such as Ester-C (calcium ascorbate plus dehydroascorbate, calcium threonate, xylonate, and lyxonate). Human studies found no meaningful difference in plasma vitamin C or urinary excretion among ascorbic acid, Ester-C, and ascorbic acid with bioflavonoids; given the lower cost, simple ascorbic acid is generally considered the preferred supplemental source.

Safety, excess and interactions

Too much

In general vitamin C has low toxicity and high intakes do not cause serious adverse effects. The most common adverse effects of high doses are diarrhea, nausea, abdominal cramps, and other gastrointestinal disturbances, thought to result from the osmotic effect of unabsorbed vitamin C in the gut. High intakes have the potential to increase urinary oxalate and uric acid excretion, which could contribute to kidney stones, especially in people with pre-existing hyperoxaluria or renal disorders, although evidence is conflicting. Other reported effects of high intakes, such as reduced vitamin B12 and copper levels, erosion of dental enamel, and allergic responses, have not been confirmed and some appear to be assay artifacts.

Interactions to know

• Chemotherapy and radiation therapy: it is uncertain whether high-dose vitamin C and other antioxidants protect tumor cells, protect normal tissue, or alter treatment effectiveness. People undergoing chemotherapy or radiation should consult their oncologist before taking vitamin C or other antioxidant supplements, especially in high doses.
• Statin and niacin therapy: vitamin C, in combination with other antioxidants, may attenuate the rise in HDL (high-density lipoprotein) cholesterol produced by combination niacin plus simvastatin (Zocor). It is not known whether this occurs with other lipid-altering regimens; clinicians should monitor lipid levels in people taking both statins and antioxidant supplements.

How status is measured

Vitamin C status is typically assessed by measuring plasma (or serum) vitamin C (ascorbate) concentration, because other measures are more difficult to assess and may be less reliable. Leukocyte ascorbate reflects tissue stores but is harder to measure routinely.

Regulated by the U.S. FDA as a dietary supplement ingredient under DSHEA, not as a drug; supplements are not FDA-approved to diagnose, treat, cure, or prevent any disease. The FDA Daily Value (DV) used on Supplement and Nutrition Facts labels is 90 mg for adults and children age 4 years and older.

What the evidence supports

We grade the strength of evidence behind each common reason people take vitamin c, mirroring how the NIH describes it.