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Vitamin C, also known as ascorbic acid, is essential for at least 300 metabolic reactions in the body. Vitamin C:
As vitamin C cannot be stored in the body, a regular intake is essential. We mainly obtain vitamin C from fruit and vegetables such as berries, citrus, green leaves and capsicum peppers – an excellent reason to aim for your 5-a-day.
However, vitamin C is one of the most unstable vitamins: up to two thirds is lost during food processing, cooking and storage. Fruit juices rapidly lose their vitamin C content when exposed to air, even if chilled, but this is somewhat mitigated by the high levels present in juice in the first place.
Interestingly, most animals make their own vitamin C, but for some reason humans lack the enzyme (L-glionolactone oxidase) needed for its synthesis - possibly because caveman obtained good amounts of vitamin C from berries and green leaves, so there was no advantage from keeping this metabolic trick.
Possibly the best-known consequence of vitamin C deficiency is scurvy, where blood vessels break down due to lack of collagen: vitamin C is vital for collagen production.
However, vitamin C also plays an important role in immunity, keeping skin, bones and joints healthy, maintaining energy levels and helping the body absorb iron.
Vitamin C is important for normal immunity, especially during and after intense physical exercise. It helps to suppress the activation of viral genes to protect against colds, and mops up inflammatory chemicals to improve symptoms.1
Studies involving schoolchildren and students2 found that taking vitamin C reduced the risk of catching a cold by as much as 30%. The results from 29 trials involving 11,300 people showed a consistent reduction in the duration of a cold by 8% for adults and 14% for children. The severity of cold symptoms was also reduced.3
Men doing heavy physical exercise are particularly prone to respiratory infections, and vitamin C supplements were found to halve the risk of cold symptoms for military troops under training, as well as for participants in a 90km running race.4
In people with asthma, cold viruses are a well-known trigger for attacks. At least two studies showed that vitamin C supplements decreased the occurrence of respiratory infection-induced asthma attacks by as much as 78% – partly through the antiviral action and by reducing airway sensitivity.5
Vitamin C is needed to make collagen, an important structural protein in the body. This makes vitamin C vital for maintaining healthy blood vessels, bones, cartilage, gums, skin and teeth.
Vitamin C also stimulates bone-building cells (osteoblasts), enhances vitamin D activity and boosts calcium absorption to improve bone mineral density and reduce the risk of osteoporosis.
By stimulating collagen production, vitamin C may also help to reduce cartilage loss and disease progression in osteoarthritis, while its anti-inflammatory action reduces joint inflammation.
In the skin, vitamin C helps to protect against UV exposure to reduce sun damage and the effects of long-term sun exposure, which causes skin to become increasingly thickened, yellow, scaly, mottled and wrinkled with a coarse, leathery texture.
Because of this, vitamin C is now added to many cosmetic creams designed to slow the visible signs of skin ageing.
Vitamin C is important for a normal energy-yielding metabolism and helps to reduce tiredness and fatigue.
A study that assessed the effects of performing aerobic exercise to exhaustion, before and after taking vitamin C supplements for 30 days, found that men with a low vitamin C concentration had a significantly decreased physical performance.6
As an antioxidant, vitamin C is also popular among athletes to help reduce muscle soreness after exercise.
Dietary iron that is found in meats (haem iron) is readily absorbed from the gut into the circulation. However, the iron found in non-meat sources is not bound to protein, and exists in two oxidation states: ferrous (Fe2+) and ferric (Fe3+), which are absorbed in different ways.
Most plant-derived iron is in the form of ferric iron, which is less well absorbed due to low solubility at the pH found in the small intestines. Vitamin C increases iron absorption by converting ferric iron to more easily absorbed ferrous iron.
The EU nutrient reference value (NRV) for vitamin C is 80mg.
Vitamin C supplements are available in the form of:
Vitamin C is also added to many anti-ageing skin creams.
Doses of above 1g of vitamin C per day may cause indigestion and have a laxative effect. This is largely due to the acidity of vitamin C rather than a specific sign of toxicity. Some people are more sensitive to the acidity of vitamin C than others; 'gentle' vitamin C supplements are available to help.
Some urine test kits used to monitor diabetes are affected by vitamin C – check with your doctor.
People with iron-storage disease (haemochromatosis) should only take vitamin C supplements under medical advice.
People who are recurrent kidney stone formers and who have a defect in ascorbic acid or oxalate metabolism, and people with renal failure should seek medical advice before taking vitamin C.
Dr Sarah Brewer is Healthspan's Medical Director and holds degrees in Natural Sciences, Surgery and Medicine from the University of Cambridge. Having worked as a GP and hospital doctor, Dr Sarah now holds an MSc in Nutritional Medicine from the University of Surrey and specialises in nutrition. She is also an award-winning writer and author.
Nothing beats a healthy, balanced diet to provide all the nutrients we need. But when this isn't possible, supplements can help. This article isn't intended to replace medical advice. Please consult your healthcare professional before trying supplements or herbal medicines.
1Paliing, L (1971). Vitamin C and the common cold, Canadian Medical Association Journal 105(5), p.448
2Hemilä, H. (2013). Vitamin C and common cold-induced asthma: a systematic review and statistical analysis, Allergy, Asthma & Clinical Immunology 9(1), p.46.
3Hemilä, H. and Chalker, E. (2013). Vitamin C for preventing and treating the common cold, Cochrane Database of Systematic Reviews (1)
4Hemilä, H. (1996). Vitamin C and common cold incidence: a review of studies with subjects under heavy physical stress, International journal of sports medicine 17(05), pp.379-383
5Hemilä, H. (2013). Vitamin C and common cold-induced asthma: a systematic review and statistical analysis, Allergy, Asthma & Clinical Immunology 9(1), p.46.
6Paschalis, V., Theodorou, A.A., Kyparos, A., Dipla, K., Zafeiridis, A., Panayiotou, G., Vrabas, I.S. and Nikolaidis, M.G. (2016). Low vitamin C values are linked with decreased physical performance and increased oxidative stress: reversal by vitamin C supplementation, European journal of nutrition 55(1), pp.45-53.