What Is Vitamin C Good For? UK Dose, Deficiency Signs and Colds

Vitamin C is good for making collagen, the protein that holds skin, blood vessels, bones, cartilage, gums and teeth together; for the normal working of the immune system; for absorbing iron from plant foods; and for protecting cells from oxidative stress. Those are its authorised UK roles, along with normal energy-yielding metabolism, normal functioning of the nervous system, normal psychological function, the reduction of tiredness and fatigue and the regeneration of vitamin E.¹ The UK reference intake is 40 mg a day, a little less than one orange, and a diet with regular fruit and vegetables usually covers it. Supplements are sold at 500 to 1,000 mg, within the UK guidance level of 1,000 mg a day from supplements, and they suit people whose intake is low or whose needs run higher: smokers, restricted diets, small appetites and gut conditions among them.² This page sets out what the evidence supports, including the cold question and the blood sugar claim.

• Roles: collagen formation, immune function, iron absorption, antioxidant protection, tiredness and fatigue
• Dose: 40 mg a day reference; upper guidance 1,000 mg from supplements
• Colds: shortens them slightly; prevents them only under extreme physical stress
• Low vitamin C: tiredness, bleeding gums, bruising, slow healing, rough skin; rare but real in restrictive diets and smokers
• Too much: loose stools above 1,000 mg; kidney stone risk in those prone

What vitamin C does

Humans are among the few animals that cannot make vitamin C, so it has to come from food. Its central job is as a cofactor for the enzymes that build collagen; without it, collagen is weak, and the result is the bleeding gums, loose teeth, bruising and wounds that will not heal of scurvy. It is also an antioxidant that protects cells and recycles vitamin E, it is concentrated in white blood cells where it supports normal immune function, and it converts plant iron into the form the gut can absorb, which is why a glass of orange juice with a lentil curry does more for iron than the lentils alone.^{1, 3} The authorised wording reflects all of this. What it does not include is anything about heart disease, arthritis, eye disease or diabetes, because the evidence does not support claims there. How vitamin C relates to glutathione, a molecule cells make for themselves, is set out in what does glutathione do in the body.

How much vitamin C per day in the UK?

40 mg a day for adults, which is lower than the American 75 to 90 mg and far lower than the doses in most supplements. Smokers need about 35 mg more because smoking depletes it. One orange has about 50 mg, a kiwi 60 mg, half a pepper 80 mg, a portion of broccoli 40 mg, a handful of strawberries 50 mg, and a jacket potato 10 mg; it is destroyed by long cooking, so lightly cooked or raw vegetables carry more.¹ The UK guidance level for supplements is 1,000 mg a day, above which loose stools and stomach upset are common and absorption falls sharply; at 1,000 mg the gut takes up perhaps half.² There is no benefit in going higher.

Does vitamin C help with colds?

Less than its reputation. The Cochrane review of dozens of trials found that taking vitamin C every day does not reduce how many colds ordinary people get, shortens the ones they do get by about 8% in adults and 14% in children, and makes symptoms a little milder. The exception is people under extreme physical stress, marathon runners, skiers and soldiers in cold conditions, in whom regular vitamin C halved the number of colds. Starting it once a cold has begun has no consistent effect.⁴ So the sensible use is a regular daily intake from food or a supplement if your diet is short; the benefit in the trials came from daily intake, not from a large dose once a cold had started. Vitamin C and zinc together covers the pairing and the zinc evidence.

Signs of low vitamin C

Tiredness and irritability come first, then rough, dry skin with small bumps around hair follicles, easy bruising, bleeding and swollen gums, slow wound healing, aching joints and, in long-standing deficiency, corkscrew body hairs and anaemia. Full scurvy is rare in the UK but not extinct: it turns up in people on very restrictive diets, in heavy drinkers and smokers, in older people eating little fresh food, in people with eating disorders or severe food aversions, and in those recovering from bariatric surgery.³ Weeks without fruit or vegetables are enough to start it. The fix is food or a modest supplement, and symptoms improve within days.

Vitamin C for skin

From the inside, vitamin C contributes to normal collagen formation for the normal function of skin, which is a maintenance role, not a treatment: it does not reverse ageing or clear dark spots from a tablet. The topical vitamin C serums sold for pigmentation and brightness are cosmetics with their own evidence and are a different subject from this one. Do you need vitamin C to absorb collagen answers the pairing question, and what destroys collagen covers sun, smoking and sugar. Vitamin C is also the constant ingredient in plant "collagen" blends; is there a vegan collagen covers what those products contain.

Does vitamin C lower blood sugar?

A claim circulates that 500 mg of vitamin C twice a day lowers blood sugar by 36% and is "particularly important" for people with diabetes. It rests on a single small trial and is not a basis for advice: vitamin C has no authorised role in blood sugar, and people with diabetes should not adjust anything on the strength of it.

Who should consider a supplement?

People who eat little fruit and vegetables, smokers, heavy drinkers, older adults with small appetites, people with gut conditions affecting absorption, and anyone whose diet is restricted for medical or other reasons. A dose of 80 to 500 mg a day is ample. People prone to kidney stones should stay well below 1,000 mg, as should anyone with haemochromatosis, because vitamin C increases iron absorption. Vitamin C is often paired with zinc in a supplement; vitamin C contributes to the normal function of the immune system, normal collagen formation and the reduction of tiredness and fatigue; does zinc help with colds covers the lozenge trials.

What we don't know

• Whether intakes above 200 mg add anything for people who are not deficient. The trials of high doses measured chronic disease outcomes rather than day-to-day status.
• Why vitamin C prevents colds in extreme exertion but not in everyday life. Plausibly oxidative stress; unproven.⁴

Frequently asked questions

What is vitamin C good for?

Collagen formation for skin, blood vessels, bones, gums and teeth; normal immune function; iron absorption from plant foods; antioxidant protection; and reducing tiredness and fatigue when you were short of it.¹

How much vitamin C per day in the UK?

40 mg a day for adults, 75 mg for smokers. The upper guidance for supplements is 1,000 mg.^{1, 2}

Does vitamin C help with colds?

It shortens colds slightly when taken daily and prevents them only in people under extreme physical stress. Starting it after symptoms begin has no consistent effect.⁴

Can you take too much vitamin C?

Above about 1,000 mg a day, loose stools and stomach upset are common and kidney stone risk rises in people prone to them. Excess is otherwise excreted.²

Does vitamin C help bleeding gums?

Vitamin C contributes to normal collagen formation for the normal function of gums, and bleeding, swollen gums are a classic sign of running very short of it.^{1, 3} A 2021 meta-analysis of 15 trials in 1,140 mostly healthy people found that vitamin C supplementation reduced gum bleeding tendency where baseline plasma vitamin C was below 28 µmol/L, but not unequivocally where it was already above 48 µmol/L.⁵ In the UK, bleeding gums are more often gum disease from plaque than a vitamin shortfall: the NHS gum disease page says "it's important to get it checked by a dentist" and lists gums that bleed "when you brush your teeth or eat hard foods" among the reasons to see one.⁶ Where collagen itself fits in is on is collagen good for your gums.

Sources

1. NHS. Vitamin C. nhs.uk; Great Britain nutrition and health claims register.
2. Expert Group on Vitamins and Minerals. Safe Upper Levels for Vitamins and Minerals, 2003. UK Government Web Archive.
3. Pullar JM, Carr AC, Vissers MCM. The roles of vitamin C in skin health. Nutrients, 2017. PubMed 28805671.
4. Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews, 2013. PubMed 23440782.
5. Hujoel PP, Kato T, Hujoel IA. Bleeding tendency and ascorbic acid requirements: systematic review and meta-analysis of clinical trials. Nutrition Reviews, 2021. PubMed 33517432.
6. NHS. Gum disease. nhs.uk.

This article is general information, not medical advice. If you have symptoms of deficiency, kidney stones or haemochromatosis, speak to your GP before taking vitamin C supplements.