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Vitamin C: Antioxidant & Immune Support

Vitamin C, also known as ascorbic acid, is a readily available water-soluble vitamin found in many fruits and vegetables and is commonly known for its antioxidant and immune supportive functions. Vitamin C is an essential cofactor for about 15 enzyme systems within the body. This nutrient has a wide range of activity across multiple body systems making it a crucial vitamin for human health.

Vitamin C as an Antioxidant

Sources of oxidative exposure are ubiquitous and sometimes unavoidable as free radicals are generated from normal metabolic activity, exposure to toxins and pollutants, or even medications like chemotherapy. Vitamin C functions as a reducing agent by donating electrons to other molecules. This can essentially recharge other antioxidants, like vitamin E, from their oxidized forms, making both antioxidants more impactful against oxidative stress in the body.1 Damage from free radicals or reactive oxygen species can have a wide-ranging impact on the body, yet vitamin C, even in small amounts, can be effective for mitigating the impact of oxidative stress and damage.

The reducing potential of vitamin C also makes dietary iron more bioavailable. Vitamin C reduces ferric iron (Fe3+) to ferrous iron (Fe2+), forming an absorbable acetic acid iron-containing complex.2 This complex renders the iron more absorbable in the intestine. This is especially useful for plant-based non-heme sources of iron which can be inhibited by other nutrients present at the time of ingestion. This vitamin C iron combination is essential for optimal iron absorption in plant-based diets and can be a useful strategy for treating iron deficiency anemia.

Vitamin C for Immune Support

The production and function of leukocytes (white blood cells) has been shown to be stimulated by vitamin C in lab studies. Neutrophils and lymphocytes, two white blood cell types, protect themselves from oxidative damage by demonstrating the ability to the accumulate high concentrations of vitamin C intracellularly.3 This protection from oxidative damage is useful during times of immune activation as reactive oxygen species are released to kill pathogens as part of the normal immune response. While it is commonly thought that vitamin C prevents the common cold and boosts immune function, research to date has produced conflicting results likely due to the complex pharmacokinetics of vitamin C.3

Impaired immunity and increased susceptibility for infection is associated with vitamin C deficiency; however, human studies supplementing vitamin C have proven inconsistent results in the general population. In one meta-analysis, a daily dose of 200 mg of vitamin C reduced the duration of the common cold in some groups exposed to physical stressors, like running a marathon, by eight percent. The severity of the symptoms reported was also reduced in these groups. These results were not seen in the general population.4 In one older study of pneumonia patients, providing vitamin C supplementation at a dose of 0.25 to 0.8 g per day reduced hospital stays by 19 percent. In the same study, patients given 0.5 to 1.6 g per day found a reduction of 36 percent.5

Vitamin C Deficiency

Unlike many other mammals who can synthesize vitamin C in their tissues, humans lack the needed enzyme to complete the synthesis and therefore must obtain this vitamin from the diet.6  It is estimated that over half of American adults are not meeting vitamin C intake recommendations.7 The NIH Office of Dietary Supplements recommends the adult males intake 90 milligrams (mg) daily, while the target for adult females is 75 mg daily. These targets are set for healthy individuals. Requirements increase during pregnancy and lactation and for other high-risk groups.

While vitamin C deficiency is relatively rare in the U.S., it still can occur in people with a limited diet or with certain behaviors like smoking.5 Research looking at vitamin C levels in people who smoke have shown decreased vitamin C levels in plasma and leukocytes largely due to the oxidative stress of this behavior. The Institute of Medicine recommends an additional 35 mg daily for smokers.

One important function connected to vitamin C is in the production and maintenance of collagen, the most common protein found in the body, comprising blood vessels, connective tissue, and bone. Vitamin C deficiency can lead to scurvy, where a person develops a constellation of problems related to collagen biosynthesis including:

  • Impaired wound healing
  • Bruising
  • Gingivitis
  • Hair loss
  • Tooth loss

Sources of Vitamin C

Vitamin C is readily available in many common fresh fruits and vegetables. Excellent sources include citrus, bell peppers, berries, and kiwis. Acerola berries are an extremely rich source of vitamin C. One berry provides the daily recommendation for females and near the recommendation for males, with an estimated 80 mg per berry according to the USDA Department of Agriculture. Camu camu is another vitamin C powerhouse with approximately 2.5 g per 100 g of pulp.8

Some studies suggest that the bioavailability and impact of natural sources may be stronger than their synthetic counterparts due to the other phytochemicals present.9 The synergy of the combination of polyphenols and other phytochemicals may act to potentiate the efficacy of the vitamin C. More research is needed to fully understand the complex relationships between plant derived versus synthetic sources of vitamin C and human health. Epidemiological research suggests positive associations from increased consumption of plant sources of vitamin C and disease prevention.9


Emerging research points to the potential benefit of vitamin C for cardiovascular health, age related macular degeneration, and cataracts. Higher intakes of fresh fruits and vegetables have been associated with certain types of cancer prevention, which is likely related to the vitamin C content of these foods.3 Although more research is needed to understand the full potential of vitamin C, its antioxidant capacity, immunomodulation, and role in structural support are just a few of the reasons that this crucial nutrient is beneficial for promoting health and wellbeing.


  1. Lykkesfeldt, J., Michels, A. J., & Frei, B. (2014). Vitamin C. Advances in Nutrition (Bethesda, Md.), 5(1), 16–18.
  2. Gaby, A. (2017). Nutritional medicine.
  3. Vitamin C. (2014, April 22). Linus Pauling Institute.
  4. Hemilä, H., & Chalker, E. (2013). Vitamin C for preventing and treating the common cold. The Cochrane Database of Systematic Reviews1, CD000980.
  5. Carr, A. C., & Maggini, S. (2017). Vitamin c and immune function. Nutrients9(11).
  6. Drouin, G., Godin, J.-R., & Pagé, B. (2011). The Genetics of Vitamin C Loss in Vertebrates. Current Genomics, 12(5), 371–378.
  7. Schleicher, R. L., Carroll, M. D., Ford, E. S., & Lacher, D. A. (2009). Serum vitamin C and the prevalence of vitamin C deficiency in the United States: 2003–2004 National Health and Nutrition Examination Survey (NHANES). The American Journal of Clinical Nutrition, 90(5), 1252–1263.
  8. Justi, K. C., Visentainer, J. V., Evelázio de Souza, N., & Matsushita, M. (2000). Nutritional composition and vitamin C stability in stored camu-camu (Myrciaria dubia) pulp. Archivos Latinoamericanos De Nutricion50(4), 405–408.
  9. Carr, A. C., & Vissers, M. C. M. (2013). Synthetic or food-derived vitamin C–are they equally bioavailable? Nutrients5(11), 4284–4304.

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