Vitamin C

Vitamin C supports collagen production, wound healing, antioxidant defence and iron absorption. Most people obtain enough from fruit and vegetables. The popular reason for taking it — preventing colds — is the use the evidence supports least well, although regular intake may shorten a cold slightly.

Vitamin C is water-soluble and easily destroyed by heat, light and prolonged storage. Ascorbic acid, sodium ascorbate, calcium ascorbate and liposomal forms are sold; for ordinary intakes, absorption differences between them are modest.

Vitamin C is a cofactor for enzymes that build collagen and certain neurotransmitters, and it acts as an antioxidant and regenerates other antioxidants.

Correcting deficiency reliably resolves scurvy. Cochrane-level reviews of regular supplementation show a small reduction in cold duration but no reduction in how often adults catch colds, with more notable effects in people under extreme physical stress such as marathon runners. Trials for cancer, cardiovascular and cognitive prevention have not shown benefit.

Laboratory studies describe antioxidant chemistry, collagen hydroxylation, immune-cell functions and, at high concentrations, hydrogen peroxide generation in tumour models.

Evidence gap: Non-human evidence. High-concentration tumour findings depend on intravenous levels not achievable orally and do not support oral cancer claims.

Vitamin C increases iron absorption, which is useful for iron deficiency but unhelpful for people with iron overload. High doses may reduce the effect of some medicines and interfere with certain home glucose meters.

High doses commonly cause diarrhoea, nausea and abdominal cramps because absorption is saturable. People with a history of calcium oxalate kidney stones or kidney disease should avoid high-dose supplements, since vitamin C is metabolised to oxalate. People with haemochromatosis or other iron-loading conditions should be cautious because of increased iron absorption, and those with G6PD deficiency should avoid very high or intravenous doses. A tolerable upper intake level exists for adults. Anyone on cancer treatment should ask their oncology team before taking antioxidant supplements.

Tablets, powders, chewables and effervescent forms are common; chewable and effervescent products can affect dental enamel and add sodium respectively.

Fruit and vegetables cover requirements for most people and bring other nutrients with them. Supplementation is most defensible for deficiency risk, alongside iron therapy for absorption, and possibly for shortening cold duration with regular use. It does not prevent colds, and it does not substitute for treatment of any illness.

Educational example: a person with recurrent kidney stones takes high-dose vitamin C powder daily for immunity and has another stone episode. Decision factors are stone history, dose, and whether the expected benefit exists at all. Questions to ask: is a high dose appropriate for me, does it affect my iron or stone risk, and does it interact with my cancer treatment? Marketing that promises cold prevention overstates a well-characterised and modest effect on duration.