Vitamin C – Ascorbic Acid

Vitamin C — also called ascorbic acid — is a water-soluble vitamin and the body’s most important water-soluble antioxidant. It works mainly as an electron donor: ascorbate gives up electrons to neutralize free radicals and to regenerate other antioxidants, turning into dehydroascorbic acid in the process, before being recycled itself with the help of glutathione and NADPH. That same electron-donating ability is what links vitamin C to collagen production, iron absorption, neurotransmitter production, immune defense, and carnitine synthesis — the same chemistry, applied to different enzyme systems.

Unlike most mammals, humans can’t make vitamin C. The enzyme that most animals use to make it from glucose (L-gulonolactone oxidase, or GULO) stopped working in our primate ancestors tens of millions of years ago, probably without much cost, since their fruit-rich diets supplied plenty of vitamin C. As a result, vitamin C is a genuine dietary essential for humans — and with little or no intake, deficiency symptoms can appear within about a month.

Key Mechanisms

Collagen production — the foundational structural role:

Collagen is the most abundant protein in the body — the structural framework of skin, tendons, ligaments, bones, blood vessel walls, cartilage, and connective tissue. Making it requires two enzymes, prolyl hydroxylase and lysyl hydroxylase, which modify the amino acids proline and lysine in new collagen chains. Both enzymes contain iron, and vitamin C keeps that iron in its active form. Without enough vitamin C:

• The enzymes lose activity
• Proline and lysine in new collagen chains can’t be properly modified
• The collagen can’t form a stable triple-helix structure
• The weakened collagen doesn’t cross-link properly and is broken down
• Tissues that depend on collagen for their strength begin to fail

This is the mechanism behind most of the symptoms of scurvy — bleeding gums, fragile blood vessels, painful joints, loose teeth, and poor wound healing are all signs of failing collagen.

Carnitine production:

Vitamin C is needed for two steps in making L-carnitine — the molecule that carries long-chain fatty acids into the mitochondria to be burned for energy. With too little vitamin C, carnitine production falls, which may help explain why fatigue is one of the earliest signs of vitamin C deficiency, appearing before the collagen-related symptoms.

Making noradrenaline from dopamine:

Dopamine β-hydroxylase — the copper-containing enzyme that converts dopamine into noradrenaline (norepinephrine) — needs vitamin C. The adrenal glands, which produce adrenaline and noradrenaline, have one of the highest concentrations of vitamin C in the body, and release vitamin C during stress. This is the link between vitamin C and the body’s stress response.

Iron absorption from plant foods:

In the gut, vitamin C converts ferric iron (Fe³⁺) into ferrous iron (Fe²⁺), the form that can be absorbed through the gut wall, and keeps it soluble. That’s the basis of vitamin C’s well-established boost to iron absorption: in meal studies, around 50–100mg of vitamin C with a meal can roughly double the absorption of non-heme iron, the form found in plant foods and supplements.

Regenerating vitamin E:

When vitamin E neutralizes a free radical in a cell membrane, it becomes a radical itself. Vitamin C can donate an electron to restore it, so the two antioxidants work as a team — vitamin E in the fatty membranes, and vitamin C in the watery fluids around them.

Vitamin C RDA (Recommended Dietary Allowance)

* Adequate Intake (AI) for infants. Source: US National Academies, via the NIH Office of Dietary Supplements. The daily value used on food labels is 90mg.

Smokers: add 35mg a day. Cigarette smoke increases the breakdown of vitamin C, so smokers have lower blood levels at the same intake and need more to keep up.

ODA (Optimal Daily Allowance)

Meet the RDA — 75–90mg a day for adults, plus 35mg for smokers — and aim for around 200mg a day from food, which five or more servings of fruit and vegetables easily provide. Around 200mg a day brings blood levels close to their natural ceiling; above roughly 400mg, most extra vitamin C is simply excreted. There’s no good evidence that very high doses benefit most people. The main exception is people under heavy physical stress, such as endurance athletes, for whom 250mg–1g a day has reduced colds (see below) — though high doses taken long-term around training may blunt some training adaptations.

Found In (per 100g)

Citrus is far from the only good source: sweet peppers, kiwi, guava, broccoli, and Brussels sprouts all rival or beat oranges. The table also shows how much handling matters. Vitamin C is destroyed by heat and leaches into cooking water — boiled broccoli has about 30% less than raw, and boiled spinach about two-thirds less — so raw or lightly cooked vegetables (steamed, microwaved, or stir-fried) keep far more. Juice processing and storage take a toll too: commercial not-from-concentrate orange juice has about 40% less vitamin C than freshly squeezed.

Vitamin C Helps With

• Making collagen for skin, tendons, ligaments, bones, and blood vessels
• Neutralizing free radicals as the body’s main water-soluble antioxidant
• Regenerating vitamin E
• Making L-carnitine for fat transport and burning
• Making noradrenaline from dopamine
• Absorbing iron from plant foods
• Immune function — vitamin C accumulates in immune cells and supports several of their functions
• Wound healing — every stage of tissue repair needs collagen
• Colds: regular supplements (200mg a day or more) slightly shorten colds — by about 8% in adults — and roughly halve the risk of catching one in people under heavy physical stress; taking vitamin C only after symptoms start hasn’t consistently helped

Vitamin C for Athletes and Active People

Vitamin C is one of the most widely used supplements among athletes, but the evidence comes with important nuances — especially around dose and timing.

Collagen and connective tissue: the vitamin C-dependent enzymes are essential steps in making collagen. Tendons, ligaments, and cartilage renew slowly and have a limited blood supply, and adequate vitamin C is a prerequisite for the collagen production that repairs them after training. A specific protocol has some early research support: in a small 2017 study, taking about 15g of gelatin with roughly 50mg of vitamin C an hour before a short bout of exercise (six minutes of skipping) doubled a blood marker of collagen production. This is a modest, targeted dose — very different from chronic high-dose supplementation — and it’s still early research, measured with markers rather than injury rates.

The stress response: intense exercise is a physical stressor, and the adrenal glands release vitamin C alongside cortisol and adrenaline during hard sessions. Heavy training may modestly raise vitamin C needs — usually easily covered by a diet rich in fruit and vegetables.

Colds during heavy training: very heavy or prolonged exertion temporarily weakens immune defenses, and colds are more common after events such as marathons. In trials of people under heavy physical stress — marathon runners, skiers, and soldiers in subarctic conditions — regular vitamin C (from 250mg to 1g a day) roughly halved the risk of catching a cold. This is one of the few situations where vitamin C supplements have a clear benefit.

The antioxidant–training paradox — the most important nuance for athletes:

Exercise generates reactive oxygen species (ROS), and that’s not entirely bad: ROS act as signals that help trigger training adaptations, such as building new mitochondria, boosting the body’s own antioxidant enzymes, and improving insulin sensitivity. High-dose antioxidant supplements may dampen these signals.

Several controlled trials found that high-dose vitamin C (typically 1,000mg a day, often combined with vitamin E) taken during training blunted:

• Molecular markers of new mitochondria being made (such as PGC-1α)
• Increases in the body’s own antioxidant enzymes
• Improvements in insulin sensitivity, in one well-known study

Effects on actual performance — such as VO₂max and endurance — have been mixed, and mostly unchanged. Still, the practical implication is sensible: vitamin C from whole foods (typically 150–250mg a day in a good diet) doesn’t appear to cause problems. The concern is with chronic high-dose supplements (around 1,000mg a day) during training blocks focused on adaptation. Keeping high doses for specific purposes — such as protecting against colds around a big event — and otherwise relying on food avoids most of the concern.

Carnitine and fat burning: adequate vitamin C supports carnitine production, and therefore fat burning during long aerobic exercise. Extra vitamin C doesn’t boost this in people who already get enough.

Vitamin C Deficiency

Deficiency risk is higher in:

• Smokers, and people regularly exposed to secondhand smoke
• People who eat very few fruits and vegetables — the most common cause
• People with heavy alcohol use, which often goes with poor diets
• People with malabsorption conditions and some chronic diseases
• Older adults with restricted diets
• People on dialysis — vitamin C is removed during treatment
• Babies fed only evaporated or boiled cow’s milk, which contains little vitamin C

Scurvy — the deficiency disease:

Scurvy is one of the most dramatic deficiency diseases in history. A well-nourished adult has about 1,500mg of vitamin C in the body, and with little or no intake, symptoms can begin within about a month — far faster than with B12, whose stores last years. Scurvy is commonly estimated to have killed around two million sailors between 1500 and 1800 — often said to be more than died from combat and shipwrecks combined. It hasn’t disappeared, either: cases still occur today in people with very limited diets.

Symptoms of scurvy, and why they happen:

• Bleeding around hair follicles: small bleeding spots around hair follicles, from capillaries weakened by failing collagen — often one of the earliest visible signs.
• Corkscrew hairs: coiled, brittle body hairs.
• Gum disease: swollen, spongy gums that bleed at the slightest touch.
• Loose teeth: as the collagen that anchors teeth to the jaw breaks down.
• Bleeding under the membrane covering the bones: causing severe bone and joint pain.
• Poor wound healing: wounds fail to close, and old, healed wounds can even reopen.
• Anemia: from bleeding and reduced iron absorption.
• Profound fatigue: appearing early, possibly linked to reduced carnitine production.
• Depression and mood changes: possibly linked to reduced noradrenaline production.

The history: in 1747, the Scottish naval surgeon James Lind carried out what’s often considered one of the first controlled clinical trials, showing that citrus fruit cured scurvy in sailors. The British Navy began issuing lemon juice to its sailors in 1795, largely eliminating scurvy. In the 1860s, it switched to cheaper West Indian lime juice — which contains much less vitamin C, and contributed to scurvy’s return on some later expeditions — but the lime juice gave British sailors the nickname “limeys.” Albert Szent-Györgyi, who isolated the compound later identified as vitamin C, received the 1937 Nobel Prize in Physiology or Medicine.

Early signs of low vitamin C (before full scurvy):

• Fatigue — often the first sign
• Irritability and low mood
• Aching joints and muscles
• Dry, rough skin
• Slow wound healing
• Easy bruising
• Bleeding gums

Excessive Doses of Vitamin C

Upper limit (UL): 2,000mg a day for adults. The limits are lower for children:

Source: US National Academies, via the NIH Office of Dietary Supplements.

Vitamin C is generally safe well above the RDA. The main side effects of too much are digestive, and there are a few groups who need to be careful:

• Digestive upset: diarrhea, cramps, and nausea are the most common side effects of high doses — which is what the upper limit is based on. Many people notice them well below 2,000mg.
• Kidney stones: some excess vitamin C is converted to oxalate, and high-dose supplements raise oxalate in the urine. In a large Swedish study, men taking vitamin C supplements (typically about 1,000mg) had roughly twice the risk of kidney stones. People with a history of kidney stones should avoid high-dose vitamin C.
• Hemochromatosis: because vitamin C boosts iron absorption, people with hereditary hemochromatosis (iron overload) should avoid high-dose vitamin C supplements.
• Effects on lab tests: very high doses of vitamin C — especially intravenous doses — can make some blood glucose meters read falsely high or low, depending on the meter, and vitamin C supplements can cause false-negative results in some stool tests for hidden blood. Tell your doctor if you take high doses before testing.
• Cancer treatment: some cancer treatments partly work by producing ROS, so high-dose antioxidants could theoretically reduce their effectiveness. Anyone having cancer treatment should discuss supplements with their oncologist.

Absorption falls as doses rise: at moderate intakes, about 70–90% of vitamin C is absorbed, but above about 1,000mg, less than half is. Beyond that point, extra vitamin C mostly ends up in the urine. Some small studies have found that liposomal vitamin C produces higher blood levels than the same dose of standard vitamin C.

Types of Vitamin C

• Ascorbic acid (L-ascorbic acid): the standard form — inexpensive, well studied, and the reference form for comparison. Slightly acidic, so it may irritate a sensitive stomach.
• Sodium ascorbate: a buffered, less acidic form that’s gentler on the stomach. It contains sodium, which matters on a sodium-restricted diet.
• Calcium ascorbate: buffered and very gentle on the stomach, and provides a little calcium. A common choice for people with a sensitive stomach.
• Magnesium ascorbate: a buffered form that also provides magnesium.
• Liposomal vitamin C: vitamin C enclosed in tiny fat-based bubbles (liposomes). Small studies suggest it produces higher blood levels than the same dose of standard vitamin C, though how it’s absorbed isn’t fully understood. More expensive.
• Ester-C (calcium ascorbate with vitamin C metabolites): marketed as longer-lasting and gentler, but studies show its absorption is broadly similar to standard ascorbic acid, with no clear advantage for the higher cost.
• Ascorbyl palmitate: a fat-soluble form of vitamin C, widely used in skincare and as a food antioxidant because it’s more stable in oils. As an oral supplement, it isn’t a better choice than ascorbic acid.
• Whole-food vitamin C (acerola, camu camu, rose hips): provides vitamin C along with bioflavonoids and other plant compounds. Studies haven’t consistently shown better absorption than ascorbic acid, but some people prefer a food-based source.

Which form for your goal:

How to Take Vitamin C

Split larger doses: absorption is limited at any one time, so 500mg twice a day delivers more in total than 1,000mg at once. For higher intakes, several smaller doses through the day work better than one large one.

Take it with food: this reduces the stomach upset that higher doses can cause.

For iron absorption: have around 50–100mg of vitamin C — from a supplement or vitamin C-rich foods — with plant-based, iron-rich meals (such as legumes, leafy greens, and fortified cereals) or with an iron supplement.

For the tendon and collagen protocol: about 15g of gelatin or collagen with around 50mg of vitamin C, taken an hour before a training session.

Timing and training: during training blocks focused on adaptation, consider avoiding high-dose supplements (around 1,000mg) taken daily around your sessions, given the antioxidant–training evidence. Vitamin C from whole foods isn’t a concern.

Storage: vitamin C breaks down with heat, light, and air. Keep supplements somewhere cool and dark, and eat cut fruit and fresh juice soon after preparing them.

Upper limit: 2,000mg a day for adults — though most people reach the limit of their digestive comfort well below this.

Interactions

High doses of vitamin C can affect some lab tests, including certain glucose meters and stool tests for hidden blood. High-dose supplements during chemotherapy or radiotherapy should be discussed with an oncologist. In one trial, an antioxidant combination including 1,000mg of vitamin C blunted the rise in HDL (“good”) cholesterol from combined statin and niacin treatment. Rare reports suggest very high doses might affect warfarin. Oral contraceptives may modestly lower blood vitamin C levels.

Synergy

• Regenerates vitamin E — the two antioxidants work as a team in the body’s watery fluids and fatty membranes.
• Boosts absorption of iron from plant foods by converting it into the absorbable form.
• Works with collagen building blocks (such as proline and glycine from gelatin or collagen) — vitamin C powers the enzymes that build them into collagen.
• Works with copper in connective tissue — copper’s enzyme, lysyl oxidase, cross-links the collagen that vitamin C helps build.
• Works with zinc in immune function and wound healing.
• Helps protect folate in food from breaking down.

Antagonistic Interactions

An old concern that high-dose vitamin C destroys vitamin B12 came from a lab-testing artifact in the 1970s and hasn’t held up.

High-dose vitamin C strongly boosts iron absorption — a real concern for people with hemochromatosis.

Chronic high-dose supplements taken around training may blunt some training adaptations.

High doses raise urinary oxalate and kidney stone risk in susceptible people.

Food values: USDA FoodData Central — broccoli, raw (Foundation Foods, FDC ID 747447); banana peppers, raw (SR Legacy, 169394); broccoli, boiled (SR Legacy, 169967); Brussels sprouts, boiled (SR Legacy, 169971); strawberries, raw (Foundation Foods, 2346409); orange juice, raw (SR Legacy, 169098); orange juice, not from concentrate, not fortified (Foundation Foods, 2003597); spinach, mature, raw (Foundation Foods, 1999633); potato, baked, flesh (SR Legacy, 170112); spinach, boiled (SR Legacy, 168463). RDA and upper limits from the US National Academies, via the NIH Office of Dietary Supplements. % Daily Values use the FDA’s current Daily Value of 90mg.