Vitamin C: what it actually does, the dose, and whether it's worth it

What vitamin C really does in the body, how much you need per day, when a supplement is worth taking and when it is wasted money, with concrete doses.

Vitamin C is one of the best-selling supplements in the world and also one of the most oversold. That doesn't make it useless — there are a few things it does very well and the evidence there is solid. The problem is that it's mostly marketed for the things it doesn't do.

What it actually does

Three functions are not up for debate:

Collagen synthesis. Without vitamin C, the enzymes that assemble collagen simply stop working. That covers skin, tendons, ligaments, blood vessel walls and wound healing.
Iron absorption from plant foods. It converts non-heme iron into a form your gut takes up far more easily.
Immune cell function. Neutrophils concentrate it at 50 to 100 times the level found in plasma.

What it does not do: prevent colds, burn fat, "detox" anything, or speed up recovery from hard training in the way the labels imply.

How much you actually need

The recommended intake is 90 mg a day for men and 75 mg for women. Smokers turn it over faster, so add 35 mg. To prevent scurvy you need roughly 10 mg a day — that's half a kiwi.

Plasma saturates at around 200 mg per day. Above that, absorption drops off a cliff: from a 200 mg dose you take up 80-90%, from a single 1,000 mg dose about 50%, from 1,250 mg about a third. Your kidneys dump the rest.

A 3,000 mg megadose doesn't give you anything close to a 3,000 mg effect. It gives you roughly the effect of 400 mg and expensive urine.

Colds: what the data says

The largest review on this covers nearly 30 trials and more than 11,000 people. The conclusion: taking 200 mg or more daily does not reduce your chance of catching a cold if you're an average person. It shortens the duration by about 8% in adults — roughly half a day off a week-long cold — and about 14% in children.

The one clear exception is people under extreme physical or cold stress: marathon runners, skiers, soldiers in the field. In that group, incidence drops by around 50%. If you only start taking it once your throat is already sore, the benefit is close to zero.

Training: when a megadose works against you

This is the part most people haven't heard. In controlled trials, 1,000 mg of vitamin C plus vitamin E taken daily for 10-12 weeks blunted some training adaptations. In endurance work the rise in mitochondrial markers was suppressed, and in a strength protocol the supplemented group gained less strength for a similar amount of hypertrophy.

The practical read: oxidative stress from training is a signal, not damage you need to mop up. If you supplement, stay at 200-500 mg and take it with a meal 2-3 hours away from your session. Fruit and vegetables aren't an issue — those doses are small and spread across the day.

Iron: the most concrete benefit

Taking 50-100 mg of vitamin C with a meal high in plant iron increases absorption two- to threefold. Concretely: 100 g of red pepper or one kiwi alongside a plate of lentils or beans. This genuinely matters for vegetarians, vegans and women with heavy periods. Coffee and black tea with the same meal do the opposite, so move them an hour away.

Food before capsules

Red bell pepper, 100 g raw — about 125-150 mg
Blackcurrants, 100 g — about 180 mg
Kiwi, 1 fruit (75 g) — about 65 mg
Orange, medium — about 70 mg
Broccoli, 100 g cooked — about 60 mg
Strawberries, 100 g — about 58 mg
Sauerkraut, 100 g — 15-20 mg

Boiling in water costs you 30-50%, since vitamin C is water-soluble and heat-sensitive; steaming or a quick stir-fry costs far less. Two portions of fruit plus 300 g of vegetables a day puts you at 150-250 mg without a single capsule.

When a supplement makes sense

You eat fewer than two portions of fruit and veg a day and that isn't changing.
You smoke — you need roughly 35 mg more.
You're on a plant-based diet with low ferritin and need it for iron uptake.
A stretch of extreme load: camp, competition prep, outdoor work in the cold — 200 mg daily for 2-4 weeks.
Healing a significant wound or recovering from surgery, as agreed with your doctor.

The dose that covers all of the above is 200-500 mg a day, with food. Plain ascorbic acid does the job. Liposomal, buffered and "ester" forms cost three to five times more for a difference that doesn't show up in outcomes; the only real reason to pick a buffered form is a sensitive stomach.

Side effects and limits

Over 1,000 mg in one hit often causes diarrhoea, cramps and nausea — that's an osmotic effect, not an allergy.
In men with a history of oxalate kidney stones, doses of 1,000 mg a day and up raise the risk.
With haemochromatosis and similar conditions, the boost to iron absorption is unwanted.
High doses can throw off readings on a home glucose meter.
The upper safe limit for adults is 2,000 mg a day from all sources combined.

When to see a doctor

Bleeding gums with no obvious cause, bruising easily, wounds that don't close for weeks, joint pain alongside a very limited diet — get that checked rather than self-treating.
Before you go above 500 mg a day if you've ever had kidney stones.
Fatigue, pallor and breathlessness on exertion — ask for a blood count and ferritin, not vitamin C.
If you're on cancer treatment, high-dose antioxidants are a conversation with your oncologist.

The short version

You need 75-90 mg a day; plasma saturates around 200 mg and the excess goes out in urine.
It won't stop you catching a cold; it shortens one by about half a day, unless you're under extreme physical stress.
The clearest benefit is plant iron absorption: 50-100 mg with the meal, two to three times better uptake.
1,000 mg daily can blunt training adaptations — stay at 200-500 mg and keep it away from your session.
Peppers, kiwi, broccoli and strawberries get you there cheaper than any capsule.