What Is Vitamin C?
Vitamin C (ascorbic acid) is a water-soluble antioxidant vitamin the human body cannot produce or store — unlike most mammals, humans lost the enzyme needed to synthesise it, so it must come from diet or supplements every day. Excess is simply excreted in urine, which is why very high-dose “megadosing” mostly just produces expensive urine rather than added benefit beyond a certain point.
Why Your Body Needs It
EFSA-authorised health claims for vitamin C include:
- Contributes to the normal function of the immune system
- Contributes to the normal function of the immune system during and after intense physical exercise
- Contributes to the protection of cells from oxidative stress
- Contributes to normal collagen formation for the normal function of skin, bones, cartilage, gums, teeth, and blood vessels
- Contributes to normal energy-yielding metabolism and reduction of tiredness and fatigue
- Increases iron absorption
- Contributes to normal functioning of the nervous system and normal psychological function
- Contributes to the regeneration of the reduced form of vitamin E
Signs of Vitamin C Deficiency
True deficiency (scurvy) is rare in the UK/EU but not extinct — it still turns up occasionally, usually linked to very restricted diets, alcohol dependency, or malabsorption. Milder insufficiency is more common and can show as:
- Fatigue
- Bleeding or swollen gums
- Slow wound healing
- Easy bruising
- Dry, rough skin
- Frequent infections
Who’s Most at Risk
- Smokers — smoking increases vitamin C turnover and requirements significantly
- People with very limited fruit and vegetable intake
- Older adults, particularly those living alone or with limited food access
- People with malabsorption conditions or on restrictive diets
- Heavy alcohol users
How Much Do You Need?
UK Reference Nutrient Intake is 40mg/day for adults — a relatively low bar that’s easy to reach through diet. EFSA’s Population Reference Intake is somewhat higher, at roughly 110mg/day for men and 95mg/day for women, with higher amounts recommended for smokers to offset increased turnover.
Food Sources
- Citrus fruits (oranges, lemons)
- Peppers (particularly red and yellow)
- Broccoli and Brussels sprouts
- Strawberries and kiwifruit
- Blackcurrants — among the richest natural sources available in the UK
A varied diet with regular fruit and veg easily meets the RNI; supplementation tends to matter more for smokers, restrictive dieters, or during periods of high immune demand (e.g. cold and flu season).
Vitamin C 1000mg: Side Effects and What to Watch For
At 1000mg daily, vitamin C is well tolerated by most people. It’s water-soluble with no meaningful accumulation risk — what your body doesn’t use is excreted in urine. There’s no formal tolerable upper intake set by EFSA, largely because the main limiting factor is digestive tolerance rather than toxicity.
The side effects that do occur are almost entirely gastrointestinal, and they become more common as the dose rises:
- Stomach cramps, nausea, diarrhoea — the most frequently reported effects, and the reason many people can’t tolerate doses above roughly 2000mg
- Heartburn — ascorbic acid is acidic; buffered forms (calcium or magnesium ascorbate) are gentler if this affects you
- Kidney stones — a consideration for people with a history of oxalate stones, since vitamin C is partly metabolised to oxalate. If that applies to you, discuss high-dose supplementation with your GP rather than self-prescribing
- Iron overload risk — vitamin C substantially increases iron absorption, which is useful for most people but relevant if you have haemochromatosis
A practical note on dosing: absorption efficiency drops sharply as the single dose increases. At 1000mg you absorb a considerably smaller proportion than at 200mg. If you want more than about 500mg daily, splitting it across two doses is more effective than taking it all at once.
- Ascorbic acid is the standard, well-absorbed, and most cost-effective form
- Buffered forms (e.g. calcium ascorbate) are gentler on sensitive stomachs
- Very high doses (2,000mg+) can cause digestive upset in some people and offer diminishing returns — there’s little evidence that more is meaningfully better past typical supplemental ranges
Our pick: Solgar Vitamin C 1000mg — a straightforward, well-absorbed daily option.
Does Vitamin C Help Iron Absorption?
Yes — this is one of the best-established nutrient interactions there is, and it’s specifically recognised in EU law. “Vitamin C increases iron absorption” is an EFSA-authorised health claim under Regulation (EC) 1924/2006.
The effect applies to non-haem iron — the form found in plants, fortified foods, and most iron supplements. Non-haem iron is poorly absorbed on its own, and vitamin C improves this substantially by converting iron to a more absorbable form and by preventing compounds in food (particularly phytates in grains and legumes) from binding it.
Haem iron, from meat and fish, is already well absorbed and gets little additional benefit.
How to use this in practice:
- Take iron supplements with a source of vitamin C — a glass of orange juice, or a vitamin C capsule alongside
- With plant-based iron sources, pair them with vitamin C-rich foods: lentils with peppers, spinach with lemon, beans with tomatoes
- Around 100mg of vitamin C is enough to produce a meaningful effect; you don’t need 1000mg for this purpose
- Avoid tea and coffee within an hour either side of an iron-rich meal — the tannins work in the opposite direction
This is particularly relevant if you’re vegetarian or vegan, since your iron comes almost entirely from non-haem sources.
→ See our full iron guide for who’s at risk of deficiency and how to supplement safely.
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Frequently Asked Questions
How much vitamin C do you need to support your immune system?
Less than most supplement labels imply. Vitamin C’s EFSA-authorised immune claim — that it “contributes to the normal function of the immune system” — applies at everyday intakes, and the UK RNI is only 40mg daily. EFSA’s Population Reference Intake sits higher, around 95–110mg. Supplements typically provide 500–1000mg, which is safe but offers diminishing returns: once your levels are adequate, additional vitamin C doesn’t push immune function higher. The people most likely to genuinely benefit from supplementing are smokers, those eating very little fruit and veg, and people training intensely.
Does vitamin C prevent colds?
For most people, no. Large reviews have consistently found that routine supplementation doesn’t meaningfully reduce how often people catch colds, though it may shorten duration slightly. The clear exception is people under sustained heavy physical stress — endurance athletes, soldiers in extreme conditions — where a protective effect has been documented. That’s reflected in the separate EFSA claim covering immune function during and after intense exercise.
Can you take vitamin C and zinc together?
Yes, with no interaction concerns — they’re commonly sold as a combination. Zinc has a firm upper limit (25mg/day from all sources under EFSA guidance), so check your total if you take several products. See our zinc guide for how the two compare.
Is it better to take vitamin C in the morning or evening?
Timing matters little for vitamin C itself. The one case where it does: if you take iron, take your vitamin C at the same time to improve absorption.
Can you get enough vitamin C from food?
Easily, for most people. The UK RNI of 40mg is roughly half a large orange, or a small handful of red pepper. Blackcurrants, broccoli, kiwifruit and strawberries are all rich sources. Supplementation makes most sense for smokers, whose requirement is substantially higher, and for anyone whose diet contains very little fresh produce.
This page is for general educational purposes only and is not medical advice.
Sources & Further Reading
- NHS — Vitamin C
- NIH Office of Dietary Supplements — Vitamin C Fact Sheet
- EFSA — Health claims on vitamin C
- Linus Pauling Institute — Vitamin C Micronutrient Information
- Examine.com — Vitamin C
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