In one pass The body cannot make vitamin C (ascorbic acid) and does not store much of it, so it has to come from fresh fruit and vegetables every day. Not this — Mega-dose vitamin C prevents colds — Taking 200 mg a day or more does not lower how often you catch colds; it only shortens them by about 8% (Cochrane review, Hemilä 2013, more than 11,000 people).
Educational content, not medical advice — consult a clinician.
Free radical attacks membrane A free radical steals an electron from a polyunsaturated fatty acid in the cell membrane, which becomes a new radical and sets off a chain reaction.
Procollagen · Gly-X-Y Collagen is the most abundant protein in mammals — 25-30% of total protein, the load-bearing scaffold of skin, vessel wall, bone matrix, tendon, and cornea.
Plant Fe³⁺ is locked Iron in plant foods is overwhelmingly Fe³⁺ (ferric) — it is poorly soluble at neutral pH and is often locked up by phytate, tannins, and oxalate.
The body cannot make vitamin C (ascorbic acid) and does not store much of it, so it has to come from fresh fruit and vegetables every day. And it is sensitive to three things: heat, oxygen, and water.
Cut fruit left out in the air for 24 hours loses 30–50%Vegetables (especially leafy ones) boiled for 10 minutes lose about 50%Shredding and then soaking washes the water-soluble C straight out So the most reliable sources are fruit and vegetables eaten raw or heated only briefly. Bell peppers, kiwi, broccoli, strawberries, and citrus all carry plenty, and two or three servings a day are enough.
Numbers · Which foods carry the most C
The richest foods (mg per 100 g):
Bell peppers 130–190 mg (see Bell Pepper)Kiwi 90 mg (see Kiwi)Broccoli about 90 mg, kale about 90–120 mg (raw, see Broccoli · see Kale)Strawberries 60 mg (see Strawberry)Citrus 50 mg (see Orange · see Pomelo & Grapefruit) Among staples and other produce, potatoes, pineapple, lychee, and tomatoes carry a share as well (see Potato · see Pineapple · see Lychee · see Tomato).
Why you have to eat it every day: most mammals have an enzyme in the liver or kidney, L-gulonolactone oxidase, that makes vitamin C directly from glucose. Humans, other monkeys and apes, and guinea pigs are among the few exceptions; in us the gene for it (GULO) has decayed into a nonfunctional pseudogene. So vitamin C has to come from food, and the body's stock is limited. On long voyages without fresh produce, sailors began falling ill with scurvy within a few months: once the stock is used up, there is no buffer left.
In practice · Cooking so the C survives
Vitamin C is one of the vitamins most easily lost in cooking, and keeping an eye on three variables (temperature, time, and water) saves most of it.
Approximate losses for common methods:
Boiling for 10 minutes: about 50% (the C dissolves into the water)Steaming for 5–7 minutes: about 20%Microwaving (with a little water): about 15%Roasting or dry-frying: about 25% (high heat, but no water to dissolve into)Eating raw: 0% Several methods keep more: a quick stir-fry (with oil, over high heat for 60–90 seconds) retains 70–80%; after steaming or a quick boil, drinking the cooking liquid keeps the C that dissolved into it; and refrigerating produce slows the breakdown 3–5 times compared with room temperature.
Some easily overlooked losses:
Shredding and then soaking (some restaurants do this) can lose half in 30 minutesReheating leftovers again and again speeds the lossCanned vegetables, after processing and long storage, are low in vitamin C (the minerals and fiber are still there)Freshly squeezed juice left open at room temperature for 24 hours loses about 50%; the 100% vitamin C on the label of industrial ultra-high-temperature (UHT) juice usually means it was added back after processing The most reliable pattern: two kinds of fresh fruit a day (for example kiwi plus citrus, together about 200 mg of vitamin C), 1–2 servings of raw vegetables a day (salad, bell peppers, tomatoes), and some raw food alongside the cooked vegetables rather than cooking everything. Eat that way and it is almost impossible to fall short of the US recommended intake of 75–90 mg/day.
Chapter 2
Absorption drops at higher doses
Vitamin C is absorbed in the small intestine by a transporter protein, sodium-dependent vitamin C transporter 1 (SVCT1). This channel has a ceiling: the more you take at once, the smaller the share that gets absorbed:
Single dose
Share absorbed
Amount taken in
200 mg
~ 100%
200 mg
500 mg
~ 70%
350 mg
1000 mg
~ 50%
500 mg
At higher doses the share keeps falling. These figures come from pharmacokinetic studies that gave healthy volunteers step-by-step increasing doses (Levine 1996 and others): up to 200 mg is almost fully absorbed, and from 500 mg on absorption drops noticeably.
The excess that is absorbed leaves in the urine. What is not absorbed stays in the gut, and at very large doses it pulls water into the bowel and causes osmotic diarrhea. So small amounts several times a day are absorbed more fully than one large dose.
SVCT2, SVCT1's sister transporter, sits in white blood cells, the adrenal glands, the brain, and the retina. These tissues actively concentrate vitamin C to 10–100 times the plasma level. One explanation is that these high-oxidative-stress sites are where vitamin C is used most.
Myth · Is liposomal vitamin C absorbed better?
The marketing pitch for liposomal vitamin C goes like this: ordinary vitamin C is poorly absorbed, but wrapping it in liposomes (tiny bubbles made of phospholipid) bypasses SVCT1 and achieves nearly 100% absorption, direct entry into cells, and the equivalent of an intravenous dose, at a price several times higher.
Checking each point:
A few small studies (Davis 2016, Hickey 2008) found somewhat higher peak plasma levels with oral liposomal vitamin C than with ordinary vitamin CNo large randomized trial has compared clinical outcomes (colds, wound healing, exercise)Equivalent to an IV dose is an exaggeration: intravenous dosing can push plasma above 10 mmol/L, which no oral form can reach (the oral ceiling is about 220 µmol/L, tens of times lower)Truly pharmacologic doses of vitamin C (for example in trials of cancer adjunct therapy) have to be given intravenously; there is no oral substitute Some practical judgments:
For everyday supplementation of 200–500 mg/day, ordinary tablets, chewables, and powders are all the same; pick the cheapestTo take in more (1–2 g), splitting it into several doses gets more absorbed than taking it all at onceLiposomal vitamin C is unnecessary for most people: several times the price, with no clear clinical benefitNatural vitamin C (acerola, rose hip) is better than synthetic has no evidence behind it; ascorbic acid is the same molecule A few points on timing:
Adding a vitamin C-rich fruit or vegetable to an iron-containing meal clearly raises non-heme iron absorption (Hallberg 1989); having it in the same meal as the iron matters more than how much you takeTaking high-dose vitamin C plus vitamin E every day for a long period blunted some markers of training adaptation in one randomized endurance-training trial (Paulsen 2014); if you want the training effect, there is no need for extra high-dose antioxidant supplementsSmokers' recommended intake is 35 mg/day higher than other people's, which one orange covers
Chapter 3
How much the body holds, and where
White blood cells, the adrenal glands, and the eyes concentrate vitamin C to tens of times its level in the blood; they are the places in the body most vulnerable to oxidation.
The body's total vitamin C stock is about 1.5 g, and scurvy appears once it falls to about 300 mg (only 1/5 left). Going from almost no vitamin C to scurvy takes roughly 2–3 months. The first controlled comparison showing that citrus prevents and treats scurvy was James Lind's, aboard a Royal Navy ship in 1747: sailors given lemons and oranges recovered fastest, while those given cider or seawater did much worse.
The half-life of the body's vitamin C stock ranges from 8 to 40 days depending on how large the stock is. Intense oxidative stress (a bad cold, surgery, burns) drives blood vitamin C down quickly, which is the chemical basis for the body really does need more C when ill. But that is not the same as 10 g of vitamin C a day prevents everything, which the later chapter on antioxidants and the cold evidence takes apart.
Clinical · Who needs more vitamin C
The US Recommended Dietary Allowance () is 90 mg a day for men and 75 mg for women, but several groups actually need more:
Smokers: the official advice is an extra 35 mg a day on top of the RDA. Oxidants in smoke use up the vitamin C stock, and smokers' plasma vitamin C tends to be lowSecondhand smoke also speeds depletion, though the US Institute of Medicine (IOM) has not issued a separate recommendationHeavy drinking interferes with vitamin C absorption and metabolism, and vitamin C is one of the things commonly short in chronic alcohol-related malnutritionAfter surgery, burns, or severe injury: the stock is used up fast (needed both for making collagen and for antioxidant defense), and burn patients in the ICU often receive 1–3 g/day, which is hospital treatmentChronic inflammation (rheumatoid arthritis, inflammatory bowel disease, ) creates ongoing oxidative stressSepsis: the ICU Marik protocol tried high-dose intravenous vitamin C (1.5 g every 6 hours) with thiamine and hydrocortisone; the randomized trials that followed did not confirm a benefitType 2 diabetes: high blood glucose raises oxidative stress, and plasma vitamin C tends to be lowA small number of people carry SVCT1 gene variants that absorb less efficiently, so their blood vitamin C runs low even on a normal intake Worth remembering: people who combine heavy smoking, heavy drinking, and chronic inflammation tend to have low vitamin C, and for them getting about 200 mg a day from food (one serving of citrus plus one of bell pepper) is worth more than for most people.
But the folk practice of taking 5 g of vitamin C because you are very ill has no randomized-trial support. The clinical evidence reaches only two places: eating enough to meet the recommended intake, and acute high-dose intravenous treatment tested in ICU trials. Taking large oral doses at home for a long time commonly causes diarrhea. In a prospective cohort of men (Thomas 2013), those who took vitamin C supplements long-term had more kidney stones; this is an observed association, and the study included only men.
Chapter 4
Antioxidant
Vitamin C works as an antioxidant by giving away electrons directly, turning various free radicals into stable molecules. Each time it gives one away, it becomes its oxidized form, dehydroascorbic acid (DHA, not the DHA in fish oil).
It has one more key move: recharging used vitamin E. After vitamin E in a cell membrane blocks one round of oxidation, it becomes a tocopheryl radical; in test-tube and cell experiments, vitamin C in the watery surroundings turns it back into tocopherol so that vitamin E can go back to work. Vitamins C and E and glutathione (GSH) are a team of antioxidants that normally run together.
But vitamin C prevents every disease goes too far. In observational studies, people who eat more fruit and vegetables and get more vitamin C have lower rates of cardiovascular disease and some cancers (an association that cannot show vitamin C is the cause), while randomized trials that actually gave high-dose vitamin C supplements have mostly come out negative. One possible explanation: at food doses vitamin C is an antioxidant, but at very large doses, in the test tube, it can turn pro-oxidant. It reduces ferric iron to ferrous iron, which then reacts with hydrogen peroxide to form the highly damaging hydroxyl radical (the Fenton reaction). How much this step matters inside the body is not known.
Evidence · Does vitamin C prevent colds?
Vitamin C prevents colds is the famous 20th-century claim promoted by Linus Pauling (who himself took 18 g a day). 50 years on, the evidence can be sorted out.
The most systematic summary is a Cochrane review (Hemilä and Chalker 2013), which included only placebo-controlled trials using at least 200 mg a day, most of them randomized and double-blind:
Prevention (29 trial comparisons, 11,306 people in total, taking it regularly every day):
In the general population, the rate of colds did not fallBut among people exposed to brief periods of extreme physical strain (marathon runners, skiers, soldiers on subarctic exercises), the risk of a cold fell by about 50% Shortening colds (again, taken regularly before getting ill):
In adults, colds were about 8% shorter (a 7-day cold becomes about half a day shorter, from 7 days to 6.5), barely noticeable to the personIn children, about 14% shorter, somewhat more noticeableSymptoms were also somewhat milder Starting after the cold begins: in trials that began vitamin C only after symptoms appeared, neither duration nor severity improved consistently. Some people advocate taking about 8 g at once on the first day of a cold; doses that large often cause diarrhea, and the benefit has not been confirmed in trials.
Practical conclusions: for ordinary people, 1 g of vitamin C a day to prevent colds has no evidence behind it; for people facing brief periods of extreme physical strain, there is evidence for regular supplementation; starting high doses after a cold begins is not supported by the evidence. Overall, getting enough from a varied diet is a better deal than high-dose supplements, most of which end up in the urine.
Where Pauling went wrong: he mixed up the dose effect when correcting a severe deficiency with the marginal effect of taking more once you already have enough, a classic thinking trap in nutrition.
Collagen makes up about 30% of the body's protein and is the structural base of skin, blood vessels, bone matrix, tendons, and cartilage. Its strength comes from three protein chains twisted into a helix. Only after the proline (Pro) in the chains has a hydroxyl group added does the helix stay stable at body temperature, and only after lysine (Lys) is hydroxylated do the collagen fibers have anchor points for strong cross-links between them.
The hydroxyl groups are added by prolyl hydroxylase (P4H) and lysyl hydroxylase (LH), each of which has a ferrous iron (Fe²⁺) at its active site. When the reaction occasionally runs idle, the Fe²⁺ is oxidized to Fe³⁺ and the enzyme stalls; vitamin C reduces the Fe³⁺ back to Fe²⁺ so the enzyme can work again. That makes vitamin C a cofactor for both enzymes.
Scurvy is what happens when this chain breaks: without vitamin C, hydroxylation stops, newly made collagen helices are loose, and so blood vessel walls leak, gums bleed, old wounds reopen, and bones ache. Scurvy is rare today, but after surgery, while recovering from a sports injury, or with chronic wounds (diabetic foot ulcers, pressure sores), the body has to make more collagen and its need for vitamin C rises with it. That is the chemical reason wound care pays attention to vitamin C. Whether extra vitamin C makes wounds heal faster in people who are not short of it has limited trial evidence.
A CLOSER LOOK
Vitamin C reactivates a collagen-processing enzyme
Here, iron sits in the enzyme's active site. Vitamin C helps restore the enzyme, and the body processes new collagen; vitamin C is not a collagen fiber delivered to skin.
1Restore enzyme-bound iron
An occasional uncoupled reaction can inactivate P4H; vitamin C reduces its iron from Fe³⁺ to Fe²⁺, helping the enzyme work again.
2Process newly made chains
P4H hydroxylates proline in collagen chains, helping stabilize their triple-helical structure.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
Myth · Does drinking collagen refill skin?
Drinking collagen, or collagen peptides, has been one of the hottest beauty and joint supplements of the past 5 years. The claims are a mix of true and false, so here they are one by one.
The part with evidence behind it: oral collagen peptides (2.5–15 g/day) did improve some measures in randomized trials:
Skin elasticity and hydration: a by de Miranda 2021 pooled 19 trials with 1,125 people and found results leaning favorable; the trials mostly saw improvement after 6–12 weeksNail growth rate and brittleness (Hexsel 2017, a small study)Joint pain (a literature review by Bello 2006)Joint comfort in athletes: Lugo 2016 used a different product, undenatured type II collagen (UC-II) The proposed mechanism is that some short peptides reach the blood and may act as signals that switch on fibroblasts (the cells that make collagen), while also supplying raw materials such as glycine (Gly), proline (Pro), and hydroxyproline (Hyp).
The overstated part:
It puts collagen straight back into your skin does not hold up: most swallowed collagen is digested into amino acids and very short fragments (dipeptides and tripeptides). Whole collagen molecules cannot get into the blood, let alone be delivered intact to the skinIt beats ordinary protein: most studies had no ordinary-protein control group, so much of the effect may simply be eating more protein overallCollagen in drinks: most collagen drinks contain < 2.5 g, far below trial doses Cheaper alternatives: eat enough protein (1.2–1.6 g/kg) along with vitamin C and let the body make its own collagen; gelatin, which is partly broken-down collagen, is much cheaper; and other proteins supply glycine and proline too, just less of them than collagen does.
Some real clinical situations:
After major surgery, burns, or serious wounds: keep up protein, vitamin C, and zinc together; there is no need for collagen in particularTendons and ligaments: in Shaw 2017, 8 healthy men drank vitamin C-enriched gelatin (15 g) 1 hour before skipping rope, and a blood marker of collagen synthesis (PINP) doubled. It was a small crossover trial that measured a synthesis marker, not recovery from injurySkin: the better-supported steps are sun protection, moisturizing, topical retinol, and not smoking, plus enough sleep
Non-heme iron from plants is mostly in the ferric form (Fe³⁺), which dissolves poorly at neutral pH and is often locked up by phytate, tannins (tea, coffee), and oxalate. Only 2–10% of it is absorbed.
Vitamin C does two jobs here:
1. Reduction: it turns Fe³⁺ into Fe²⁺ by handing over one electron. Fe²⁺ dissolves much better in the slightly acidic small intestine and is less easily grabbed by phytate. 2. Chelation: vitamin C forms a soluble complex with the iron and escorts it all the way to divalent metal transporter 1 (DMT1) on the membrane of the gut cells. DMT1 carries only Fe²⁺, not Fe³⁺.
The effect grows with the dose (single-meal absorption studies by Hallberg 1989): the more vitamin C in the same meal, the better non-heme iron is absorbed, and the amount in one kiwi or half a bell pepper already makes a clear difference; beyond that, extra vitamin C adds less and less. These are single-meal effects; across a whole diet they are much smaller.
In practice, vegetarians and people prone to low iron can put an iron-containing meal (spinach, beans, fortified cereals) together with vitamin C foods (citrus, bell peppers, tomatoes) in the same meal. It is the cheapest dietary step there is, but for someone who is already iron deficient it does not replace the iron tablets a doctor prescribes. Move tea and coffee to at least 1 hour after the meal, because tannins work the other way and lock the iron up.
A CLOSER LOOK
Iron changes into a form the gut can carry
This is non-heme iron in the gut lumen. Vitamin C can help absorption; the symbols do not show an amount absorbed from a meal.
1Reduce Fe³⁺ to Fe²⁺
Vitamin C donates an electron to reduce ferric iron to ferrous iron, one step that helps non-heme iron absorption.
2Keep iron soluble, too
Vitamin C also helps prevent insoluble iron compounds that are difficult to absorb; ferrous iron then enters a cell through the gut membrane.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
In practice · Pairing vitamin C with iron meals
For women with heavy periods, vegetarians, and people with iron-deficiency anemia, pairing vitamin C with an iron-containing meal is one of the cheapest dietary steps available.
A day might look like this:
Breakfast: oats with almonds (non-heme iron) and 1 kiwi (about 90 mg of vitamin C), raising iron absorption about × 3Lunch: lentil soup or red-bean rice with bell pepper and tomato, so iron and vitamin C are in the same mealDinner: spinach (oxalate locks up part of its iron, but some is still usable) with a squeeze of lemonKeep tea and coffee more than 1 hour away from meals, before or after, so their tannins do not lock up the iron Some common pairings that work against you:
Spinach with milk: oxalate and calcium both lock up ironRice with tea (common in Chinese and Indian meals): the tannins lock up ironIron and calcium supplements taken together: they compete for absorption When diet is not enough and is < 30, doctors usually prescribe oral iron, commonly 60–200 mg of elemental iron per dose, taken on an empty stomach. Moretti 2015 found in iron-depleted young women that 24 hours after a dose of 60 mg or more, hepcidin (the hormone the body uses to turn iron absorption down) was raised, and absorption of the next dose fell by 35–45%. Going by that rhythm, taking iron every other day may pay off better, though its long-term effects still need study. Recheck after 8–12 weeks. Whether to add vitamin C to iron tablets is not settled.
Vitamin C boosts treatment but does not replace it. With severe anemia (Hb < 90) or heavy periods that cannot be controlled, you still need a medical evaluation, oral or intravenous iron, and an assessment of the periods. When iron deficiency appears after menopause or in a man, bleeding from the gut (colorectal cancer, peptic ulcer) must be ruled out; adding vitamin C and iron will not solve that.
Levine, M., et al. (1996). Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. PNAS, 93(8), 3704–3709. In-hospital depletion-repletion study of 7 healthy volunteers (4-6 months on a diet with less than 5 mg/day), seven daily doses from 30 to 2500 mg. Bioavailability was complete for a 200 mg single dose; at single doses of 500 mg and higher bioavailability declined and the absorbed amount was excreted; plasma saturated at 1000 mg daily, white cells at 100 mg. The abstract gives no absorption percentage for 500 or 1250 mg (abstract, PMID 8623000). 10.1073/pnas.93.8.3704
Thomas, L. D. K., Elinder, C.-G., Tiselius, H.-G., Wolk, A., & Åkesson, A. (2013). Ascorbic acid supplements and kidney stone incidence among men: a prospective study. JAMA Internal Medicine, 173(5), 386–388. 10.1001/jamainternmed.2013.2296
Hemilä, H., & Chalker, E. (2013). Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews, (1), CD000980. Regular vitamin C does not reduce cold incidence in the general population; it modestly shortens duration (~8% in adults), with no benefit when started at symptom onset. 10.1002/14651858.CD000980.pub4
Institute of Medicine. (2000). Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. National Academies Press. www.ncbi.nlm.nih.gov/books/NBK225483
Shaw, G., Lee-Barthel, A., Ross, M. L. R., Wang, B., & Baar, K. (2017). Vitamin C-enriched gelatin supplementation before intermittent activity augments collagen synthesis. The American Journal of Clinical Nutrition, 105(1), 136-143. 8 healthy men, randomized double-blind crossover: 5 g or 15 g vitamin C-enriched gelatin or placebo 1 h before 6 min of rope-skipping, 3 times a day for 3 days. Blood glycine, proline, hydroxyproline and hydroxylysine peaked 1 h after the drink; with 15 g gelatin, blood amino-terminal propeptide of collagen I (PINP) doubled; serum drawn after gelatin raised collagen content of engineered ligaments in vitro. A collagen-synthesis marker; no injury or recovery outcome was measured (abstract, PMID 27852613). 10.3945/ajcn.116.138594
de Miranda, R. B., Weimer, P., & Rossi, R. C. (2021). Effects of hydrolyzed collagen supplementation on skin aging: a systematic review and meta-analysis. International Journal of Dermatology, 60(12), 1449-1461. Pooled 19 trials / 1,125 participants; favorable for skin hydration and elasticity. 10.1111/ijd.15518
Hallberg, L., Brune, M., & Rossander, L. (1989). The role of vitamin C in iron absorption. International Journal for Vitamin and Nutrition Research. Supplement, 30, 103–108. Short review: ascorbic acid has a key role in non-heme iron absorption by preventing insoluble, unabsorbable iron compounds and by reducing ferric to ferrous iron. The abstract gives no dose-response numbers (e.g. no mg of vitamin C or fold increase) (abstract, PMID 2507689).
Moretti, D., Goede, J. S., Zeder, C., et al. (2015). Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women. Blood, 126(17), 1981–1989. 54 nonanemic young women with plasma ferritin <= 20 µg/L, isotope-labelled ferrous sulfate 40-240 mg at 8:00 fasting. 24 h after doses >= 60 mg, hepcidin was raised and fractional absorption of the next dose fell by 35-45%. A sixfold dose increase (40-240 mg) gave only a threefold rise in iron absorbed (6.7-18.1 mg). Three 60 mg doses (2 mornings and an afternoon) were not significantly better absorbed in total than 2 morning doses. Authors: lower doses (40-80 mg) and avoiding twice-daily dosing maximize fractional absorption; the hepcidin time course supports alternate-day dosing, whose longer-term effects need study (abstract, PMID 26289639). 10.1182/blood-2015-05-642223