Vitamin C

Vitamin C, also known as L-ascorbic acid, is a water-soluble vitamin that humans cannot synthesize endogenously and must obtain from the diet. Most other mammals can produce vitamin C internally; the human inability to do so is due to a non-functional gene for the enzyme L-gulonolactone oxidase.¹ Vitamin C is required for the biosynthesis of collagen — a structural protein essential to skin, blood vessels, tendons, and other connective tissues — as well as for the synthesis of carnitine and certain neurotransmitters. Vitamin C also functions as a potent water-soluble antioxidant and regenerates other antioxidants, including alpha-tocopherol (vitamin E), from their oxidized forms.¹

Intestinal absorption of vitamin C is dose-dependent: at moderate intakes of 30–180 mg/day, approximately 70–90% is absorbed, but absorption efficiency decreases at higher single doses, and excess vitamin C is excreted unchanged in the urine.¹

Adequate vitamin C intake prevents scurvy, the classical deficiency disease characterized by fatigue, gum disease, bruising, impaired wound healing, and joint pain.¹ Scurvy is now uncommon in developed countries but does occur in individuals with severely restricted diets, certain medical conditions, or chronic alcohol or drug use.

Vitamin C is required for normal immune function, and supplementation has been most extensively studied in the context of the common cold. A Cochrane systematic review of 29 trial comparisons (involving more than 11,000 participants) found that regular vitamin C supplementation did not reduce the incidence of colds in the general adult population, but it did modestly reduce duration (by 8% in adults, 14% in children) and severity. In subgroups exposed to brief periods of severe physical stress (such as marathon runners and soldiers in subarctic exercises), vitamin C supplementation cut the incidence of colds approximately in half.²

Vitamin C enhances the absorption of non-heme iron from plant foods when consumed in the same meal, an effect of practical relevance for individuals at risk of iron deficiency, particularly those following plant-based diets.¹

Vitamin C has been studied for many additional applications including cardiovascular disease prevention, cancer prevention, age-related macular degeneration (as part of the AREDS-2 antioxidant combination), and high-dose intravenous use in oncology research. The strength of evidence varies considerably across these applications, and NIH ODS notes that several large prospective studies and systematic reviews have not found clear protective effects of vitamin C supplementation against cardiovascular disease or most cancer types.¹

Recommended Dietary Allowances for adults:¹

• Men 19 years and older: 90 mg/day
• Women 19 years and older: 75 mg/day
• Pregnancy: 85 mg/day
• Lactation: 120 mg/day
• Smokers: an additional 35 mg/day above the standard adult RDA, due to higher metabolic turnover of vitamin C

The Tolerable Upper Intake Level for adults is 2,000 mg/day; intakes above this level are associated with increased risk of gastrointestinal side effects and, in susceptible individuals, kidney stones.¹

Common dietary supplement doses range from 250 mg to 1,000 mg per day; the marginal benefit of doses above the RDA in adequately-nourished individuals is limited, since absorption efficiency decreases at higher single doses and the body excretes the excess.¹

Oral, as tablets, capsules, chewables, or powder. Pharmaceutical-grade ascorbic acid is also available as an intravenous formulation for specific clinical applications including treatment of severe scurvy and certain oncology research protocols; IV vitamin C achieves plasma concentrations not attainable with oral dosing due to the saturation kinetics of intestinal absorption.¹

Standard storage in a cool, dry place out of direct light. Vitamin C is sensitive to heat, oxygen, and light in solution; powdered supplements stored in opaque containers are stable for the duration of typical shelf life.

Vitamin C is among the safest vitamin supplements when used at typical doses. Side effects with oral intake are generally limited to gastrointestinal symptoms — nausea, diarrhea, abdominal cramps — and become more likely at doses above 1,000–2,000 mg/day.¹

In susceptible individuals, high-dose vitamin C supplementation may increase urinary oxalate excretion and the risk of calcium oxalate kidney stones. Individuals with a history of kidney stones, oxalate nephropathy, or significantly impaired kidney function should exercise caution with high-dose supplementation.¹

Vitamin C can interact with several medications and laboratory tests:¹

• Anticoagulants: very high doses of vitamin C may modestly reduce the anticoagulant effect of warfarin in some reports, though the clinical significance is generally small
• Statins and niacin: some antioxidant combinations including high-dose vitamin C have been associated with attenuated lipid responses to statin or niacin therapy in some trials
• Glucose meters: high-dose vitamin C can produce false readings on certain home glucose meters
• Stool occult blood tests: vitamin C can produce false-negative results on guaiac-based stool tests

Individuals with hemochromatosis (iron overload) should avoid high-dose vitamin C supplementation, since vitamin C enhances iron absorption and may contribute to additional iron loading.¹