Benefits
Immune function support with regular daily intake
Vitamin C collects in white blood cells and supports their normal work. In a Cochrane review of 31 placebo-controlled comparisons, at least 200 mg a day taken regularly shortened colds by 8% in adults and 14% in children, and a later pooled analysis found 1 g a day or more made them about 15% less severe. It did not make colds less frequent in most people, and starting it after symptoms began showed no consistent benefit.
Immune support for people under heavy physical stress
In five trials of 598 marathon runners, skiers and soldiers on subarctic exercises, regular vitamin C roughly halved the chance of catching a cold (risk ratio 0.48). The NIH Office of Dietary Supplements notes that regular intake of at least 200 mg a day may also help people with marginal vitamin C status. This finding comes from a small part of the trial evidence.
Collagen formation for normal skin and wound healing
Vitamin C is needed to make collagen, which holds skin and other connective tissue together. When intake falls very low, scurvy develops, with skin bleeding, poor wound healing, rough skin and corkscrew hairs. Two small studies found supplements raised skin vitamin C, but whether extra vitamin C improves skin in well-fed people has not been tested in trials of vitamin C alone, so that benefit is uncertain.
Antioxidant defense and glutathione support
Vitamin C is a major water-soluble antioxidant and helps regenerate vitamin E. In a study of healthy adults on a low vitamin C diet, 500 mg a day for two weeks raised red blood cell glutathione by nearly 50%, and 2,000 mg gave no further rise. A review by NIH scientists found that trials of extra vitamin C did not change markers of oxidative damage, so benefit above adequate intake is uncertain.
Iron absorption from plant foods
Vitamin C helps the gut absorb non-heme iron, the form found in plant foods, by keeping iron soluble and converting it to the ferrous form. In radio-iron studies in 299 people, added vitamin C raised iron absorption most in meals rich in phytates and tannins, which normally block it, and synthetic and food vitamin C acted alike. The researchers judged about 50 mg per main meal desirable.
Carnitine and neurotransmitter production
The body uses vitamin C to make carnitine, which carries fats into cells to be burned for energy, and to make certain neurotransmitters. Fatigue is one of the first signs of deficiency, probably because carnitine production slows.
Blood pressure support
A pooled analysis of 29 short randomized trials (median 500 mg a day for 8 weeks) found vitamin C lowered systolic blood pressure by 3.84 mmHg and diastolic by 1.48 mmHg. In people with high blood pressure, systolic pressure fell 4.85 mmHg, while the diastolic change was not significant. The effect is small, and long-term trials on blood pressure and heart events are still needed.
Mechanism of action
Electron donor and antioxidant
Vitamin C works by donating electrons, which lets it neutralize reactive oxygen species in the watery parts of blood and tissues and regenerate vitamin E. Its antioxidant action is clear in lab experiments, but trials in people have not shown changes in validated markers of oxidative damage.
Collagen-building cofactor
Vitamin C is required for the body to make collagen. Without it, collagen synthesis is impaired and connective tissue weakens, which causes the skin bleeding, poor wound healing, gum problems and joint pain of scurvy.
Iron absorption
In the gut, vitamin C stops non-heme iron from forming insoluble compounds and reduces ferric iron to ferrous iron, which appears to be needed for uptake into intestinal cells. This counters iron blockers such as phytates and tannins.
Immune cell function
Vitamin C builds up in white blood cells such as neutrophils, where it supports their movement toward germs, their ability to engulf and kill them, and their orderly clearance afterward. Infections and inflammation use up vitamin C, and deficiency weakens immune defenses.
Clinical trials
Systematic review and meta-analysis of placebo-controlled trials of at least 200 mg a day of vitamin C, taken regularly or started at the first symptoms of a cold. (Hemilä and Chalker 2013, Cochrane Database Syst Rev)
29 comparisons with 11,306 participants for the chance of catching a cold; 31 comparisons with 9,745 cold episodes for duration.
Regular vitamin C did not lower the chance of catching a cold in the general population (risk ratio 0.97), but halved it in 598 marathon runners, skiers and soldiers on subarctic exercises (risk ratio 0.48). Colds were 8% shorter in adults and 14% shorter in children, and 18% shorter in children taking 1 to 2 g a day. In seven therapeutic comparisons, starting vitamin C after symptoms began had no consistent effect.
Meta-analysis of randomized, double-blind, placebo-controlled trials of at least 1 g a day of oral vitamin C in people in good health at baseline, limited to trials reporting both cold duration and severity. (Hemilä and Chalker 2023, BMC Public Health)
15 comparisons from 10 randomized trials.
Vitamin C reduced cold severity by 15% (95% CI 9% to 21%) compared with placebo. In the five comparisons that reported both, vitamin C shortened severe symptoms but had no significant effect on the duration of mild symptoms. The review did not test doses below 1 g a day.
Randomized, placebo-controlled trial of intravenous vitamin C (50 mg/kg every 6 hours for up to 96 hours) in adults in intensive care. (Lamontagne et al. 2022, N Engl J Med)
872 adults with sepsis receiving blood pressure support drugs in the ICU.
The primary outcome, death or persistent organ dysfunction at 28 days, was more common with vitamin C than placebo (44.5% vs 38.5%, risk ratio 1.21). Death alone (35.4% vs 31.6%) and organ dysfunction alone did not differ significantly. This tested a hospital intravenous treatment, not an oral supplement.
Randomized, double-blind, placebo-controlled trial in 7 US intensive care units of intravenous vitamin C (50 mg/kg every 6 hours for 96 hours). (Fowler et al. 2019, JAMA)
167 adults with sepsis and acute respiratory distress syndrome.
Vitamin C did not improve the primary outcomes: organ failure scores at 96 hours, C-reactive protein or thrombomodulin. The authors called it a preliminary study. This tested a hospital intravenous treatment, not an oral supplement.
Meta-analysis with trial sequential analysis of randomized trials of vitamin C given around cardiac surgery. (Hu et al. 2017, Int J Surg)
1,060 adults in 8 randomized controlled trials.
Vitamin C was linked to fewer cases of atrial fibrillation after surgery (odds ratio 0.47, evidence rated moderate) but did not shorten ICU or hospital stay. The authors judged short-term use safe and called for more high-quality trials.
Randomized, double-blind, placebo-controlled factorial trial of 500 mg vitamin C daily and vitamin E, run from 1997 to 2007. (Gaziano et al. 2009, JAMA)
14,641 US male physicians aged 50 or older, followed for a mean of 8 years.
Vitamin C did not change total cancer (hazard ratio 1.01) or prostate cancer (hazard ratio 1.02) compared with placebo, and had no significant effect on colorectal, lung or other site-specific cancers.
Meta-analysis of randomized controlled trials of oral vitamin C lasting at least 2 weeks. (Juraschek et al. 2012, Am J Clin Nutr)
29 trials, each with 10 to 120 participants; median dose 500 mg a day for a median of 8 weeks.
Vitamin C lowered systolic blood pressure by 3.84 mmHg and diastolic by 1.48 mmHg. In people with high blood pressure, systolic pressure fell 4.85 mmHg, while the 1.67 mmHg diastolic drop was not significant (P=0.17). Adding 9 trials with estimated data weakened the effects, which stayed significant.
Meta-analysis of randomized controlled trials of oral vitamin C lasting at least 1 week, excluding children and people on dialysis. (Juraschek et al. 2011, Arthritis Care Res)
556 participants in 13 trials, with starting uric acid levels of 2.9 to 7.0 mg/dL.
Vitamin C (median 500 mg a day for a median of 30 days) lowered serum uric acid by 0.35 mg/dL, with large differences between trials. The authors said trials in people with high uric acid were still needed.
Randomized controlled pilot trial comparing 500 mg a day of vitamin C with starting or raising allopurinol for 8 weeks. (Stamp et al. 2013, Arthritis Rheum)
40 adults with gout and serum urate above 6 mg/dL.
Serum urate fell by only 0.23 mg/dL with vitamin C versus 1.9 mg/dL with allopurinol, which the authors called clinically insignificant, even though plasma vitamin C rose. The trial measured serum urate only, not gout flares.
Double-blind study with placebo, 500 mg and 2,000 mg periods in adults eating diets low in vitamin C. (Johnston et al. 1993, Am J Clin Nutr)
Healthy adults on self-selected vitamin C-restricted diets.
Mean red blood cell glutathione rose nearly 50% after two weeks of 500 mg a day, with individual changes from 8% to 84%. The 2,000 mg dose gave no further rise, and levels returned to baseline after the placebo withdrawal week.
Radio-iron absorption studies giving meals with and without ascorbic acid or vitamin C-rich foods to the same people. (Hallberg et al. 1986, Hum Nutr Appl Nutr)
299 people.
Vitamin C increased non-heme iron absorption, most strongly in meals high in iron blockers such as phytates and tannins. Synthetic and food vitamin C acted alike, and about 50 mg per main meal was judged desirable.