Vitamin C supplements: what the evidence says and how much is too much

Ascorbic acid takes part in building collagen, the protein scaffold of skin, tendons, blood vessel walls and gums. It also assists wound repair, helps the gut absorb iron from plant foods and acts as an antioxidant that limits damage from free radicals.

Because it is needed for collagen, the earliest signs of too little vitamin C tend to appear where tissue is under mechanical stress or has fragile small vessels: gums, skin and joints.

The NHS lists citrus fruit, peppers, strawberries, blackcurrants, broccoli, Brussels sprouts and potatoes as good sources. MedlinePlus adds kiwi, berries and cantaloupe melon, and points out that raw produce keeps the most, because cooking and light exposure break the vitamin down.

Boiling vegetables in a large volume of water leaches a water-soluble vitamin into the pan. Steaming, microwaving with little water, or eating some produce raw all preserve more of it.

• Eat fruit or vegetables at most meals; one portion of a vitamin C-rich food a day is usually enough for the UK reference amount.

• Keep cut fruit covered and refrigerated, and avoid leaving juice in strong light for long.

• Frozen fruit and vegetables are often frozen soon after picking and can retain a good share of the vitamin.

Scurvy develops after a prolonged shortfall. MedlinePlus lists bleeding gums, easy bruising, slow wound healing and weakened immunity as features of low intake, and says severe deficiency leading to scurvy mainly affects malnourished older adults.

In practice, risk concentrates in people with very limited diets: those living alone with little fresh food, people with eating disorders or severe food restriction, people with alcohol dependence, and children or adults with highly selective eating. Anyone with bleeding gums, unexplained bruising or poor healing alongside a restricted diet should see their GP, who can check the diet history and arrange blood tests if needed.

Treatment is replacing the vitamin, usually with supplements for a time plus dietary change, under clinical direction.

This is the most common reason people buy vitamin C. The MedlinePlus entry states that supplements do not reduce the risk of getting a cold for most people, though people who take them regularly may find colds slightly milder. Taking a dose only after symptoms start has not been shown to be a reliable fix.

The picture is different for people exposed to unusual physical stress, but that does not describe most office workers, parents or commuters, so a daily tablet is not a sensible expectation of protection.

The NHS says that more than 1,000mg a day may cause stomach pain, diarrhoea and flatulence, usually settling when the supplement is stopped. The US upper level quoted by MedlinePlus is 2,000mg a day, with stomach upset, diarrhoea and a possibility of kidney stones above it. The two bodies set different thresholds, so the lower UK figure is the safer working limit.

People who have had kidney stones, who have a condition causing iron overload such as haemochromatosis, or who are on chemotherapy or other specialist treatment should ask their clinician before using high-dose products, because the vitamin enhances iron absorption and some treatments are affected by antioxidants.

MedlinePlus also advises against large supplemental doses in pregnancy. Vitamin C also appears as one ingredient in the AREDS-type eye formula, which is a separate, specific use described on a related page.

MedlinePlus quotes higher US reference amounts for breastfeeding women and an additional 35mg a day for smokers, because smoking increases turnover of the vitamin. The UK figure is the same for all adults in the 19 to 64 band, so smokers may reasonably be a little more attentive to diet, though the stronger health message for them is stopping smoking.