Vitamin C

Vitamin C (ascorbic acid) is a vitamin that dissolves in water. Your body uses it as a helper for enzymes that build collagen and make carnitine and some nerve messengers. It also works as an antioxidant, says the NIH (the US National Institutes of Health).

People are among the few higher animals that cannot make vitamin C. The gene for the last enzyme that makes it is still in our DNA, but it carries many mutations. So all of our vitamin C has to come from food.

Here is the part most people do not know: about 10 mg a day prevents scurvy. Yet the US allowance is 75 to 90 mg. The NIH explains that the allowance is based on what vitamin C does in white blood cells, not on what it takes to avoid scurvy. White blood cells hold at least 14 times the concentration found in blood plasma (the liquid part of blood).

Your body also limits what a bigger dose can add. About 80% of an intake of 100 mg is absorbed, but under 50% above 1 g. The absorbed excess leaves in urine.

Most people in rich countries get enough. EFSA is the EU’s food safety agency. In European surveys, it reports average intakes from food of 69 to 130 mg a day for men and 65 to 138 mg for women.

Deficiency has not disappeared, though. In US survey data from 2003 to 2004, 7.1% of people aged 6 and older had a deficient blood level. Smokers’ average levels were about one-third lower than those of nonsmokers.

Health benefits of vitamin C

• ★★★Did not prevent colds in most peopleIn 29 trials, daily vitamin C (0.2 g or more) left the cold risk almost unchanged in the general community (0.97 times). Daily use made colds 8% shorter in adults.
• ★★★No change in the risk of deathAcross 29 reliable trials of vitamin C, alone or with other vitamins, the risk of death was 1.02 times that of controls. That is no real change.
• ★★★No help against pre-eclampsia or preterm birthAcross 29 trials in 24,300 pregnant women, vitamin C (alone or with other supplements) did not clearly prevent pre-eclampsia (0.92 times the risk). Preterm birth (0.99 times the risk) and stillbirth (1.15) also did not differ clearly; these differences could be chance.
• ★★No fewer heart attacks or strokesIn 14,641 male doctors, vitamin C supplements (500 mg a day) gave 0.99 times the risk of major heart and blood vessel events. In 8,171 high-risk women taking 500 mg a day in combinations with other supplements, the risk was 1.02 times as high.
• ★★Slightly lower blood pressureIn 29 trials of oral vitamin C (median 500 mg a day), the top number fell by 3.84 points (mm Hg systolic). The trials were short, so effects on heart attacks or strokes are unknown.

Foods high in vitamin C

Fruits and vegetables supply most vitamin C, and the leaders in our USDA ranking are not always the ones people expect. Per serving, bell peppers lead with 152 mg in 1 medium pepper (119 g). That is 169% of the 90 mg Daily Value (DV), the amount used on US food labels. Guava follows with 126 mg in one fruit.

Strawberries, papaya, kohlrabi, pineapple, Brussels sprouts, oranges, broccoli and chili peppers complete the top ten. Per 100 g, guava leads with 228 mg, ahead of thyme, chili peppers, parsley and bell peppers. The ranking below shows every amount.

The NIH table lists ½ cup of raw red pepper at 95 mg and ¾ cup of orange juice at 93 mg. One medium orange has 70 mg, one medium kiwifruit 64 mg and ½ cup of cooked broccoli 51 mg. Five varied servings of fruits and vegetables can give more than 200 mg, the NIH says. Citrus and potatoes are major sources in the American diet, though not the densest, and some breakfast cereals are fortified (have vitamin C added).

Vitamin C from food is absorbed about as well as vitamin C from tablets (see the section on forms of vitamin C). Cooking and storage can lower it. Heat and oxygen destroy vitamin C, and it dissolves into cooking water that is thrown away, according to EFSA.

In a test of several cooked vegetables, the share of vitamin C kept ranged from 0.0% to 91.1%. It was highest after microwaving and lowest after boiling. In broccoli, steaming did not clearly lower vitamin C, but microwaving, boiling and stir-frying did. In carrots and courgettes, boiling and steaming kept ascorbic acid better than frying.

The NIH adds that long storage also reduces vitamin C, and that many of the best sources are usually eaten raw.

How vitamin C works

Most of what we know about how vitamin C works comes from biochemistry and controlled feeding studies in small groups of volunteers. It does not come from trials in people with disease. Humans cannot make vitamin C because the gene for its last enzyme has gathered many mutations since it stopped working.

Four ideas cover most of what follows. Vitamin C helps enzymes make collagen and other molecules, and it helps the gut take up iron. The gut and the body limit how much stays, and a drip goes around those limits. Then comes what has been seen in people.

It helps enzymes build collagen, carnitine and nerve messengers

Vitamin C is a helper (cofactor) for enzymes of the monooxygenase and dioxygenase families, according to EFSA. It plays an important role in making collagen, and it is essential for making carnitine and catecholamines (nerve messengers such as adrenaline). It also takes part in turning cholesterol into bile acids.

In water, it readily mops up reactive oxygen and nitrogen molecules, and the NIH says it can renew vitamin E. The highest levels in the body are in white blood cells, the eyes, the adrenal glands, the pituitary gland and the brain.

When vitamin C is missing, collagen is weak. That explains the bleeding gums, bruising and poor wound healing of scurvy. EFSA adds that the early tiredness of deficiency may relate partly to too little carnitine. EFSA says low mood may relate to too little dopamine hydroxylation, a step in making nerve messengers.

It helps the gut take up iron

Iron from plants (nonheme iron) is poorly absorbed. Hallberg and colleagues describe two ways vitamin C helps. It stops iron forming insoluble compounds that cannot be absorbed. It also turns ferric iron into ferrous iron, which seems to be needed for uptake into gut cells.

The effect is largest in meals that contain inhibitors of iron absorption. Teucher and colleagues’ review says a meal with low to medium inhibitors needs a 2 to 1 molar ratio of vitamin C to iron. That means, for example, 20 mg of vitamin C for 3 mg of iron, and more when inhibitors are high.

Across a whole diet the effect is much smaller than in single-meal tests, as the card by Cook and Reddy above shows. The EU authorizes the claim that vitamin C increases iron absorption.

The gut absorbs most of a small dose and less of a large one

Vitamin C is taken up by transport proteins that depend on sodium. A review by Savini and colleagues says SVCT1 helps keep the body’s vitamin C in balance. SVCT2 protects cells with high activity.

The NIH says gut absorption is controlled by at least one active transporter that depends on the dose. About 70% to 90% is absorbed at 30 to 180 mg a day, and under 50% above 1 g a day. EFSA gives about 80% at an intake of about 100 mg.

In a hospital study of seven volunteers, a single 200 mg dose was absorbed completely. At single doses of 500 mg and higher, absorption declined and the absorbed excess left in the urine.

Blood levels rise, then level off

EFSA reports that in people who had run low, 60 to 100 mg a day lifts plasma vitamin C steeply to about 50 µmol/L. The body pool rises to 1.0 to 1.5 g. Above 100 mg the curve flattens, up to a plateau of 70 to 80 µmol/L. Only regular doses above 200 mg can hold that plateau.

Scurvy occurs below 10 µmol/L, when the body pool is under 300 mg, and 10 mg a day can prevent it. In the hospital study by Levine and colleagues, volunteers lived for 4 to 6 months on a diet with under 5 mg of vitamin C. They then took doses from 30 to 2,500 mg a day.

Plasma was completely saturated at 1,000 mg a day, and white blood cells at 100 mg. Oxalate and urate in urine rose at 1,000 mg. The NIH puts the total body content between 300 mg near scurvy and about 2 g.

A drip goes around those limits

Oral doses cannot push plasma levels very high, but a drip into a vein can. In 17 healthy hospitalized volunteers, a 1.25 g dose gave a peak of 134.8 µmol/L by mouth and 885 µmol/L by vein. Modeling predicted 220 µmol/L for 3 g by mouth every 4 hours, and 13,400 µmol/L for a 50 g drip.

The NIH notes that levels of that size are toxic to tumor cells in test tubes. It says the question of whether intravenous vitamin C helps people with cancer needs more research. People having chemotherapy or radiation should talk to their oncologist (cancer doctor) first, the NIH says. The two sepsis trials above, which used a drip, did not show benefit, and one found harm.

What has been seen in people

The clearest human findings are the official ones: vitamin C prevents and cures scurvy, and it helps iron absorption from a single meal. In trials of 500 mg a day, effects on heart attacks, cancer and death were null.

The NIH points out why this may be so. If people’s vitamin C levels were already close to saturation (full) when a trial began, extra vitamin C would be expected to change little.

The effects that did show up, all among the study cards above, are small. Colds last about 8% to 14% less in people who take it every day. Blood pressure falls by about 4 mm Hg in short trials. In people under heavy physical stress, such as marathon runners and soldiers, colds were about half as common.

How much vitamin C do you need?

Adults need 75 to 90 mg of vitamin C a day, according to the US Recommended Dietary Allowance (RDA). The RDA covers the needs of nearly all healthy people: 90 mg for men and 75 mg for women. People who smoke need 35 mg more. In the EU, EFSA’s Population Reference Intake (its version of the RDA) is higher: 110 mg for men and 95 mg for women.

In pregnancy, the US allowance rises to 85 mg a day from age 19 (80 mg at ages 14 to 18). For breastfeeding it is 120 mg (115 mg at ages 14 to 18). EFSA adds 10 mg a day in pregnancy, and 60 mg for women who breastfeed only (exclusively) in the first six months.

For older adults, the US allowance stays the same as for younger adults. EFSA found too little data to set different values for them.

EFSA sets no separate value for smokers. But it cites a study of 17 men who had smoked more than 20 cigarettes a day for years. They lost 50.2 to 114.0 mg a day through metabolism (except one man at 29.6 mg), against 13.7 to 47.6 mg in nonsmokers.

The NIH adds that people exposed to secondhand smoke should make sure they meet the allowance. No specific extra amount has been set for them.

Vitamin C deficiency: signs and who is at risk

Deficiency causes scurvy. Signs can appear within about a month of eating less than about 10 mg of vitamin C a day, according to the NIH. The first signs are tiredness, a general unwell feeling and inflamed gums.

Later come tiny skin bleeds, bruises, joint pain, slow wound healing, thickened skin and corkscrew-shaped hairs. Gums swell and bleed, teeth loosen or fall out, and depression can set in. Iron deficiency anemia can follow, children can develop bone disease, and untreated scurvy is fatal.

The NIH lists the groups at risk: people who smoke or breathe secondhand smoke, and infants fed evaporated or boiled cow’s milk. People who eat a very limited range of foods are at risk too. Among them are some older people who cook for themselves, people with alcohol or drug dependence and food faddists. So are people with severe malabsorption, some cancer patients and people on long-term hemodialysis (kidney dialysis).

In a trial of smokers and nonsmokers who ate similar amounts of fruit and vegetables, only vitamin C was lower in smokers. A moderate supplement brought it back up within 3 months. It held 272 mg of vitamin C, in a mix with vitamin E and folic acid.

Scurvy is rare in rich countries but still turns up. One hospital saw 22 adults with scurvy over 8 years, with a mean blood level of 6 µmol/L. More than half had iron deficiency anemia, and more than half were taking proton pump inhibitors (acid-reducing drugs). In children it can mimic autoimmune disease, infection or cancer, which delays the diagnosis.

The usual test is plasma vitamin C. EFSA reads below 10 µmol/L as the scurvy range, 10 to 50 as below ideal and about 50 as adequate. The US survey defined deficiency as below 11.4 µmol/L, so cut-offs differ between agencies. A doctor reads the result together with symptoms and diet.

Forms of vitamin C: absorption and effects

Vitamin C supplements differ in their chemical form and in added ingredients, such as bioflavonoids or other vitamin C breakdown products. They also differ in how they are delivered (liposomes, sustained release). The NIH says most supplements contain ascorbic acid. It is as bioavailable (as well absorbed and used) as the vitamin C in foods such as orange juice and broccoli.

Other forms are sodium ascorbate, calcium ascorbate, other mineral ascorbates, and ascorbic acid with bioflavonoids. There are also blends such as Ester-C, which holds calcium ascorbate, dehydroascorbate, calcium threonate, xylonate and lyxonate.

EFSA noted in 2013 that L-ascorbic acid and its sodium, calcium and potassium salts may be added to foods and food supplements in the EU. Magnesium and zinc ascorbate may be added to food supplements only.

The honest bottom line: the human comparisons measure how vitamin C rises in blood and white blood cells after a dose, not health outcomes. Food and tablets raise blood levels alike. The alternatives show modest and inconsistent differences in small studies. The one systematic review of them was written by authors with industry ties.

We found no trial that compared forms for colds, scurvy or any other health outcome. The one cold study in that review compared a calcium ascorbate blend with placebo, not with plain ascorbic acid.

For “natural is better”, the whole-food trials with oranges, broccoli and kiwifruit found vitamin C that matched tablets. Only a small 1988 test of a citrus extract with bioflavonoids found more absorption. For “buffered” vitamin C, the only evidence on stomach comfort we found is that review. It reports fewer stomach side effects with one calcium ascorbate blend.

For ascorbic acid, a single 200 mg dose is absorbed completely, and larger single doses less so. The trials in the benefits section either named ascorbic acid or did not state a form in the abstract.

Form	Activity or conversion	What studies show	Source
Ascorbic acid (most supplements)	Same molecule as in food; bioavailability equal to food vitamin C (NIH)	Raised plasma vitamin C as well as orange segments, orange juice and cooked broccoli in 68 men (raw broccoli 20% lower). A single 200 mg dose is absorbed completely; at 500 mg and above, bioavailability falls and the excess is excreted.	1, 2, 13
Vitamin C from foods (kiwifruit, oranges, broccoli)	Same molecule; no conversion factor stated	Half a gold kiwifruit a day matched a 50 mg tablet in plasma, white blood cells and muscle in 36 men over 6 weeks. A separate 9-man crossover study found comparable plasma curves for 200 mg.	2, 3, 4
With citrus bioflavonoids	Not stated	A citrus extract gave 35% more absorption than ascorbic acid alone in 8 people after one 500 mg dose (1988). A randomized study cited by the NIH found no difference in plasma levels among ascorbic acid, an Ester-C blend and ascorbic acid with bioflavonoids.	1, 5
Sodium, calcium and other mineral ascorbates (“buffered”)	Not stated	In 93 healthy adults, 250 mg of calcium ascorbate and ascorbic acid did not differ; at 500 mg the calcium form changed some immune and blood measures, in a preliminary study. A 2025 review reports fewer stomach side effects with one calcium ascorbate blend; its authors have industry ties.	7, 8
Ester-C type blends	Not stated	In 15 men, 1,000 mg of calcium ascorbate with vitamin C metabolites raised white blood cell vitamin C at 24 hours (P < 0.0001) but not plasma. Seven of the 13 studies in the 2025 review were on one such blend.	1, 6, 8
Liposomal	Not stated	In 27 adults, one liposomal product gave a 27% higher peak and 21% more plasma exposure over 24 hours than standard vitamin C. A 2021 open-label crossover of another product found 1.77 times the bioavailability. No health outcomes were measured.	9, 10
Sustained release (“time release”)	Not stated	A 1,000 mg capsule peaked later (12 hours in whole blood against 4 hours); calculated bioavailability was 149% to 180% of a standard capsule in a 1977 study, with the number of people not given in the abstract.	11
Intravenous (clinical use only)	Not stated	A 1.25 g dose gave a peak of 885 µmol/L by vein against 134.8 µmol/L by mouth in 17 volunteers. The LOVIT sepsis trial found harm with a drip.	12

Here are the 7 studies that compare forms or test a single form, strongest evidence first.

Is vitamin C safe? Supplements, dose and interactions

Vitamin C has low toxicity, and the NIH says high intakes do not cause serious harm in healthy people. The US Tolerable Upper Intake Level (UL), the highest daily amount unlikely to cause harm, covers vitamin C from food and supplements together. It is 2,000 mg a day for adults, including in pregnancy and breastfeeding.

The UL is 1,800 mg at ages 14 to 18 and 1,200 mg at ages 9 to 13. It is 650 mg at ages 4 to 8 and 400 mg at ages 1 to 3. It could not be set for infants, who should get vitamin C only from formula and food. It does not apply to people who take vitamin C as medical treatment under a doctor’s care.

EFSA did not set any upper level for vitamin C. It judged acute toxicity low, and saw reversible stomach upset or diarrhea at 3 to 4 g a day as the best-defined adverse effect. It found the data on dose and effect too thin to set a level.

• Side effects. The most common complaints at high doses are diarrhea, nausea and stomach cramps, the NIH says. Unabsorbed vitamin C pulls water into the gut.
• Kidney stones. Vitamin C can raise oxalate and uric acid in urine. The NIH says the evidence is conflicting, with the best evidence in people who already have high urinary oxalate. In the large cohorts on the card above, men taking 1,000 mg or more from supplements had 1.19 times the risk. Women and food-only vitamin C showed no clear link. People who form stones should ask a doctor first.
• Iron overload. The NIH says high vitamin C intakes are not a concern for healthy people. But in people with hereditary hemochromatosis (an inherited iron overload), long-term high doses could make it worse.
• Cancer treatment. The NIH calls the safety and benefit of vitamin C and other antioxidants during chemotherapy or radiation controversial. It advises talking to your oncologist before taking them, especially at high doses.
• Statins and niacin. In a trial of 160 patients, an antioxidant mix that included vitamin C blunted the rise in protective HDL2 cholesterol from simvastatin plus niacin. The NIH says blood fat levels should be checked in people who take both.
• Glucose meters. In a lab test of 30 drugs on 6 handheld glucose meters and a portable analyzer, vitamin C (ascorbic acid) interfered with every device. A later study found vitamin C pushed readings up on several of 7 meters. People with diabetes who take high-dose vitamin C, or get it by drip, should ask how their meter responds.
• Pregnancy. In 2,410 women, high doses with vitamin E (1,000 mg of vitamin C with 400 IU of vitamin E) did not prevent pre-eclampsia. They came with more low-birthweight babies (28% against 24%). The Cochrane review also links vitamin C with vitamin E to waters breaking at term more often.
• Intravenous vitamin C. The LOVIT trial found a higher risk of death or lasting organ failure in sepsis, and a drip belongs in hospital care only.

How to get more vitamin C

• Best foods. In our ranking, one medium bell pepper gives 152 mg, one guava 126 mg and a cup of strawberries 89.4 mg. An orange has 69.7 mg. Eat some raw, since in one test cooking left anywhere from 0% to 91% of the vitamin C, depending on the vegetable and method.
• Cook gently. Steaming kept broccoli’s vitamin C in one test, and microwaving kept the most overall in another, while boiling kept the least. Because vitamin C dissolves into cooking water, use little water or put the water in soup.
• Pair it with plant iron. Vitamin C raises iron absorption from a meal, mainly when the meal contains inhibitors, so beans or grains with peppers or citrus make sense. Across a whole diet the effect is smaller. Food gives about as much vitamin C as a tablet. Blood levels level off at 70 to 80 µmol/L on doses above 200 mg a day, so larger doses add little.

History of vitamin C

Scurvy was a scourge of sailors. An estimated 2 million of them were affected over the years. In 1747 the Royal Navy surgeon James Lind tested six treatments in 12 sailors with scurvy, and only oranges and lemons worked.

His report of a controlled trial came half a century before the British Navy used citrus fruits to prevent and cure scurvy. When lime juice was swapped in for lemon juice around 1860, without anyone realizing the difference, the disease returned, especially among sailors on polar expeditions.

The cause took another two centuries to find. The Norwegian researchers Axel Holst and Theodor Frölich produced scurvy in guinea pigs by chance. Like people, guinea pigs need vitamin C from food.

In 1928 the Hungarian biochemist Albert Szent-Györgyi isolated a substance from adrenal glands that he called hexuronic acid. Four years later Charles Glen King isolated vitamin C in his own laboratory and concluded that it was the same substance. In 1932 Szent-Györgyi and his coworker Svirbely published a paper titled “The chemical nature of vitamin C”.

The chemist Norman Haworth worked out its structure in 1933. In 1937 Szent-Györgyi received the Nobel Prize in Physiology or Medicine, with a citation that names vitamin C. That year Haworth shared the Nobel Prize in Chemistry for his work on carbohydrates and vitamin C.

Linus Pauling then made vitamin C famous. In 1970 he argued that the best daily intake was about 2.3 g or more. In 1976 he and Ewan Cameron reported on 100 terminal cancer patients given extra ascorbate. They lived more than four times as long as 1,000 comparison patients, they reported.

At the Mayo Clinic, randomized trials (where chance decides who is treated) in 1979 and 1985 found no benefit from 10 g a day. Later work on how oral and intravenous doses differ in blood levels reopened the question without settling it. The Cochrane review calls the cold debate one that has run for 70 years.

Frequently asked questions about vitamin C

Does vitamin C prevent colds?

Not in most people. In a Cochrane review of 29 trials, daily vitamin C did not change how many colds most people caught (0.97 times the risk). But it roughly halved them (0.48 times) in 598 marathon runners, skiers and soldiers on subarctic exercises.

Colds lasted 8% less in adults and 14% less in children who took it daily. Taking it only after symptoms began had no consistent effect.

Is it better to get vitamin C from food or supplements?

Food is the better default, because vitamin C from food raises blood levels as well as tablets do. In 68 men, orange segments, orange juice and cooked broccoli matched tablets. Large trials of 500 mg a day found no fewer heart attacks (0.99 times the risk) or cancers (1.01). The NIH says a supplement makes sense when food cannot meet needs, such as with a very restricted diet, smoking or a proven deficiency.

Does cooking destroy vitamin C?

Some of it, depending on the vegetable and the method. In one test, the share kept ranged from 0.0% to 91.1%, highest after microwaving and lowest after boiling. In broccoli, steaming did not clearly lower it, while boiling, microwaving and stir-frying did. EFSA notes that heat and oxygen destroy vitamin C, and that it dissolves into cooking water.

Which foods are highest in vitamin C?

In our USDA ranking per serving, bell peppers lead with 152 mg in 1 medium pepper. Guava follows with 126 mg in one fruit, then strawberries (89.4 mg per cup), papaya (88.3 mg per cup) and kohlrabi (83.7 mg per cup). An orange has 69.7 mg and ranks eighth. Per 100 g, guava leads with 228 mg.

Can too much vitamin C cause kidney stones?

Possibly, in men who take supplements. In three large cohorts, men with a total intake of 1,000 mg or more a day had 1.43 times the risk of kidney stones. Supplements of 1,000 mg or more gave 1.19 times the risk.

Women showed no clear link, and vitamin C from food alone was not linked in either sex. The NIH calls the evidence conflicting and says people with kidney problems or a history of stones are the main concern.

Is 1,000 mg of vitamin C a day too much?

It is below the US upper level of 2,000 mg a day, but it adds little. Above 1 g, absorption falls to under 50%, and plasma was already completely saturated at 1,000 mg in a hospital study. The usual effects of high doses are diarrhea, nausea and cramps. EFSA set no upper level and found stomach upset at 3 to 4 g a day.

Does vitamin C help you absorb iron?

Yes, from a single meal, and less across a whole diet. Vitamin C turns iron into the form the gut can take up and stops it forming insoluble compounds. The EU authorizes the claim that it increases iron absorption.

In 12 people eating a full diet for 5 days, iron absorption did not clearly differ between periods. Their vitamin C intake ranged from 51 to 247 mg a day.

Is liposomal vitamin C better?

It has not been shown to be better for health. In 27 adults, a single 500 mg dose of one liposomal product gave 27% higher peak plasma levels than standard vitamin C. It also gave 21% more plasma exposure over 24 hours. No trial we found tested colds, scurvy or any other outcome.

The NIH cites authors who concluded that plain ascorbic acid is the preferred form, given its low cost.