Vitamin C is essential for many different aspects of health and particularly for immune function. Most animals can produce their own vitamin C, yet somewhere along the evolutionary line, humans and other higher primates, guinea pigs, most bats, and some species of birds and fish lost their ability to synthesise this important nutrient. As well as not being able to make it ourselves, we only have a very limited capacity to store it, so regular and adequate intake through dietary sources is essential. Historically, sailors learnt this the hard way when scurvy (a devastating connective tissue disease caused by vitamin C deficiency) was a prevalent feature of life at sea due to lack of regular access to fresh vitamin C-rich foods for months on end. Simply adding back into the diet, vitamin C-rich foods such as oranges and lemons was discovered to be the best treatment for the disease and so today we understand just how important regular dietary intake of vitamin C is. Cooking vitamin C-rich foods, storing them for a long period of time or exposure to light can all reduce the vitamin C content, hence why the best sources of vitamin C are raw or gently steamed fruits and vegetables – eaten as fresh as possible. Foods rich in vitamin C include citrus fruits, kiwi fruit, berries and bell peppers.
Why is vitamin C so important for immune function?
Vitamin C has many important roles to play in the immune system, and similar to vitamin D, is involved in both innate and adaptive immune function. Innate immune function is your first line of defence against harmful invaders; it is fast and non-specific, whereas adaptive immune function is more specific and long lived but takes a few days to appear. A robust and resilient immune response requires both parts to work together and vitamin C helps to enable this. Vitamin C is also a potent antioxidant which means it helps to protect the body against harmful oxidative stress. In short, vitamin C is necessary for the immune system to be able to mount and sustain an adequate response against harmful invaders, and yet also helps to avoid excessive damage to the host when this immune response is in full swing.
Vitamin C is also essential for wound healing as it is a vital ingredient for collagen and elastin synthesis – two vital structural proteins in the skin (hence why severe vitamin C deficiency causes the connective tissue disease, scurvy). Collagen is the most abundant protein in the human body, found in bones, muscles, skin, blood vessels, cartilage, tendons and other connective tissues; it is the ‘tough glue’ that holds the body together, whereas elastin gives skin its ‘elasticity’. Together they support the protective barrier effects of the skin and other connective tissues which is another crucial part of our defence against harmful pathogens.
When do bodily needs for vitamin C increase?
We know that regular intake of vitamin C is important for many aspects of health including immune function. Unfortunately, epidemiological studies indicate that low levels of vitamin C are still relatively common in Western populations.1 This is likely due to reduced intake of vitamin C-rich foods combined with our limited ability to store the nutrient. In addition, bodily requirements for vitamin C increase due to factors such as pollution and smoking, during times of stress and illness, fighting infections, and chronic diseases with oxidative and inflammatory components such as type 2 diabetes and metabolic syndrome.1
Vitamin C & Immune health - Research highlights:
• Vitamin C has been shown to shorten the duration of the common cold, and even prevent it in some conditions, for example when people are exposed to brief periods of intense physical exercise.2
• In a 2017 study, intravenous vitamin C was successfully added to a protocol to significantly reduce mortality in patients with severe sepsis and septic shock.3
• Evidence suggests that vitamin C levels decline dramatically in critically ill patients. In a 2019 meta-analysis published in Nutrients involving 18 controlled trials and 2004 patients, researchers evaluated the effects of vitamin C on practical outcomes in an intensive care unit (length of stay and duration of mechanical ventilation). They found that vitamin C shortened the length of stay in the intensive care unit and shortened the duration of mechanical ventilation.4 Another meta-analysis found that vitamin C shortened the duration of mechanical ventilation in patients who required the longest ventilation.5
• Research is now underway to study the clinical efficacy and safety of vitamin C for the clinical management of severe acute respiratory infection (SARI) (the severe pneumonia which has been caused by the new coronavirus strain) through randomised controlled trials during this current outbreak.6
Regular dietary intake of vitamin C is essential for your immune health
From an immune health perspective, it is crucial to remember that regular dietary intake of vitamin C is essential. And particularly important to note that vitamin C is used up rapidly during illness or infection and thus necessary to replenish regularly to help the body to meet these increased needs. For general health and wellness, and prevention of infection it is important to achieve dietary intakes that provide at least 100 – 200mg per day.7 In contrast however, when vitamin C needs increase such as during illness or infection significantly higher (gram) doses are needed to compensate for the increased metabolic demand.8-11
1. Carr AC, Maggini S. Vitamin C & Immune function. Nutrients 2017 Nov; 9 (11): 1211
2. Hemila H, Chalker E. Vitamin C for preventing the common cold. Cochrane Database Systematic Review. 2013 Jan 31 (1)
3. Marik PE, Khangoora V, et al. Hydrocortisone, Vitamin C and thiamine for the treatment of severe sepsis and septic shock. Chest. June 2017 Volume 151, Issue 6, pages 1229-1238
4. Hemila H & Chalker E. Vitamin C can shorten the length of stay in the ICU: A meta-analysis. Nutrients 2019 11 (4)
5. Hemilä H, Chalker E. Vitamin C may reduce the duration of mechanical ventilation in critically ill patients: a meta-regression analysis. J Intensive Care. 2020; 8: 15.
6. ZhiYong Peng. Vitamin C infusion for the treatment of severe 2019-nCoV Infected Pneumonia. Clinical Trials.gov February 11, 2020
7. Levine M., Conry-Cantilena C., et al. Vitamin C pharmacokinetics in healthy volunteers: Evidence for a recommended dietary allowance. Proc. Natl. Acad. Sci. USA. 1996; 93: 3704–3709.
8. Long C.L., Maull K.I., et al. Ascorbic acid dynamics in the seriously ill and injured. J. Surg. Res. 2003; 109: 144–148.
9. Rümelin A., Jaehde U., et al. Early postoperative substitution procedure of the antioxidant ascorbic acid. J. Nutr. Biochem. 2005; 16: 104–108.
10. Rümelin A., Humbert T., et al. Metabolic clearance of the antioxidant ascorbic acid in surgical patients. J. Surg. Res. 2005; 129: 46–51.
11. De Grooth H.J., Manubulu-Choo W.P., et al. Vitamin C pharmacokinetics in critically ill patients: A randomized trial of four iv regimens. Chest. 2018; 153: 1368–1377.
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