Vitamin C (ascorbic acid) is a powerful antioxidant with many important functions in the body, including ensuring the proper functioning of the immune system. It must be provided with the diet, as humans cannot produce it and have a low capacity to store it.
Its richest sources are:
- rosehip
- blackcurrant
- citrus fruit
- strawberries
- parsley
- red pepper
- Brussels sprouts
- broccoli
Vitamin C is susceptible to destruction when exposed to heat (including during cooking), oxygen, high pH, metals and enzymes found in some plants (including ascorbicase, peroxidase).
The role of vitamin C in building immunity
Ascorbic acid is essential for mobilising the immune system to build and maintain an adequate response to pathogens, while preventing the excessive host damage that inflammation can generate. This can be evidenced by the fact that leukocytes (neutrophils and monocytes) actively accumulate this essential compound against a concentration gradient, reaching values up to 50-100 times higher than plasma concentrations. Vitamin C enhances the body’s immune defences by influencing the innate and acquired immune system. This ranges from improving epithelial barrier function against pathogens to stimulating various immune cells. This compound accumulates in neutrophils (phagocytic cells) and enhances various processes (chemotaxis, phagocytosis, generation of reactive oxygen species) to ultimately kill microorganisms. After the whole process, it participates in apoptosis and elimination of spent neutrophils from sites of infection by macrophages. This reduces the risk of potential damage and inhibits necrosis. In the case of lymphocytes, on the other hand, ascorbic acid enhances the differentiation and proliferation of B and T immune cells.
Furthermore, vitamin C is a powerful antioxidant that not only directly removes numerous free radicals, but also indirectly inhibits free radical-producing enzymes (NADPH, XO) and additionally regenerates other important cellular and membrane antioxidants (including glutathione and vitamin E) present in the body.
For effective infection prevention, an adequate dose of ascorbic acid is required to reach saturating plasma levels (i.e. 100-200 mg/day). However, treatment of already concomitant infections requires much higher (in gram quantities) doses of this compound to compensate for the increased metabolic demand. Probably the greatest benefit can be derived from this antioxidant in the treatment of the common cold if supplementation is started within 24 hours of the onset of symptoms, in doses of up to 8 g per day, continuing for another 5 days. A review of 29 comparisons concluded that regular administration of ascorbic acid (1-2 g/day) reduces the duration of infection in adults by 8% and in children by 14%.
Effects of vitamin C deficiency
People who are at risk of ascorbic acid deficiency are:
- those suffering from various diseases related to inflammation and oxidative stress
- those exposed to pollution and smoke, including cigarette smokers
- those with an unhealthy lifestyle (alcohol abuse, weight loss and/or unbalanced diet)
- those facing periods of excessive physical or mental stress
- those who have undergone surgery
- elderly people
Vitamin C deficiency primarily results in impaired immunity and greater susceptibility to infection. Supplementation with gram doses of ascorbic acid by healthy individuals has been shown to improve the chemotactic capacity of neutrophils. It has been noted that patients with acute respiratory tract infections have reduced plasma vitamin C concentrations compared to control subjects. However, administration of ascorbic acid to these patients restores its level in plasma to normal and alleviates the severity of respiratory symptoms. It has been confirmed that elderly patients hospitalised with acute respiratory infections are in significantly better condition when they take this powerful antioxidant than those who do not take.
What to consider when choosing vitamin C?
Vitamin C occurs naturally in fruit, where it is accompanied by other bioactive compounds, so when choosing a supplement it is worth checking the composition for that. Especially as some studies have shown greater availability of ascorbic acid contained in citrus extract than as a synthetic ingredient alone. An interesting compound is pepper extract, or piperine, which increases the absorption and bioavailability of nutrients by acting as a thermo-nutrient. Care should be taken to store ascorbic acid properly, as exposure to heat, light or air oxidises the ascorbate. Excess vitamin C is excreted in the urine, so a standard prophylactic dosage of 1g per day may be chosen, as only doses of 5-10g result in a temporary onset of diarrhoea, bloating and abdominal pain, depending on tolerance. A single dose of more than 1g is not recommended, it is then best to divide it into several portions.
Bibliography:
Carr AC, Maggini S. Vitamin C and Immune Function. Nutrients. 2017 Nov 3;9(11):1211.
Martí N, Mena P, Cánovas JA, Micol V, Saura D. Vitamin C and the role of citrus juices as functional food. Nat Prod Commun. 2009 May;4(5):677-700. PMID: 19445318.
Fernández-Lázaro D, Mielgo-Ayuso J, Córdova Martínez A, Seco-Calvo J. Iron and Physical Activity: Bioavailability Enhancers, Properties of Black Pepper (Bioperine®) and Potential Applications. Nutrients. 2020 Jun 24;12(6):1886.
Bucher A, White N. Vitamin C in the Prevention and Treatment of the Common Cold. Am J Lifestyle Med. 2016 Feb 9;10(3):181-183.
Ghalibaf MHE, Kianian F, Beigoli S, Behrouz S, Marefati N, Boskabady M, Boskabady MH. The effects of vitamin C on respiratory, allergic and immunological diseases: an experimental and clinical-based review. Inflammopharmacology. 2023 Apr;31(2):653-672.
Jarosz M., Rychlik E., Stoś K. i wsp. Normy żywienia dla populacji Polski i ich zastosowanie., PZH, 2020