Vitamin C and the Immune System: What is the Suggested Intake?

Vitamin C, also known as ascorbic acid, is a water-soluble micronutrient which can be found in some fruits such as oranges, kiwis, lemons, grapes and many vegetables like tomatoes, broccoli, and peppers, among others. Vitamin C exists in the form of an L-ascorbic acid and in the oxidized form, L-dehydroascorbic acid [1]. It is an essential micronutrient due to the fact that we cannot synthesize it so it is important to ensure an adequate daily intake. Vitamin C has many important functions in the body, interacting with many metabolic processes [2] and has an important role in improving human health.

Recent studies [1, 3, 4] have shown the role of vitamin C as a regulatory nutrient of the immune system [1]. This can be through its antibacterial activity, stimulation of the natural killer cells, by interfering in the synthesis of pro-inflammatory cytokines or by preventing tissue damage by inhibiting an excessive activation of the immune system [5]. Vitamin C is also a very efficient antioxidant; it can protect immune cells from the toxic effects of reactive oxygen species (ROS) [1, 6].

Neutrophils are immunological cells involved in the first line of defense against invading pathogens [3]. They are the most abundant type of leukocytes (40-70 percent) and participate in the innate immune response [1]. Neutrophils are also known to accumulate intracellular concentrations of vitamin C [6, 7] that do not only protect them from their oxidative effect, but also increase their bactericidal activity [8] and improve their chemotaxis and chemokinesis [1], which refers to speed and direction of cell movements.

A new review [1] analyzes recent studies in order to link neutrophil motility to vitamin C requirements and suggests that a daily intake of 200 mg might be advisable for the general adult population [1].

NEW DEVELOPMENTS ON DIETARY INTAKE

Several agencies have studied recommendations for daily intake of vitamin C based on different factors such as: age, sex or special periods with incremental needs like pregnancy or lactation. The European Food Safety Authority (EFSA) has guided the recommended intakes in a way to avoid deficiencies and to ensure an adequate functioning of the processes requiring vitamin C, while avoiding high-doses that could cause adverse effects [1, 9]. The EFSA research panel supports the recommended intakes by considering the latest research results, determining the average requirement (AR) in healthy adults from the quantity of vitamin C that balances metabolic vitamin C losses and maintains fasting plasma ascorbate concentrations at about 50 μmol/L [9]. Therefore, the panel considered a daily population reference intake (PRI) of 110 mg/day of vitamin C derived for adult men (≥19 years) and a PRI of 95 mg/day derived for adult women (≥19 years) [9]. Another agency, the Institute of Medicine (IoM) in the U.S., recommends vitamin C daily Allowances (RDA) of 90 mg/day for adult men and 75 mg/day for adult woman [1, 10].

The effect of vitamin C on neutrophils motility tends to be a good marker to readjust daily reference intakes. In a recent in vitro study [4], the impact of vitamin C on the speed and direction (CT and CK)  of immune cells has been investigated and  it was shown that it  improves  the process of migration of neutrophils from the blood into the concerned tissues  [1]. Another human study [3], confirms the effect of vitamin C supplementation on neutrophils function. A group of healthy young men with suboptimal plasma vitamin C status (<50 μmol/L) were supplemented with vitamin C rich kiwi fruits (260 mg/day vitamin C) for four weeks [1, 3]. The outcome of the study showed that participants with daily vitamin C intake through kiwi fruit presented an important improvement in neutrophil vitamin C status, as well as in their motility and oxidant production [3].

Vitamin C plasma level of ≥ 70 μmol/L can ensure an optimal function of neutrophils [1]. Results from another study showed that a daily intake of approximately 110mg of vitamin C resulted in saturated levels of vitamin C on neutrophils, but not in plasma. Increasing the doses to approximately 210 mg was associated with saturated vitamin C levels on plasma improving leukocyte functions, which supports the recommendation of a daily intake of 200 mg. [1, 11].

It is necessary to increase the research related to neutrophils motility as a possible functional biomarker to redefine daily recommendations for vitamin C. Research suggests the current recommendations are set too low and that an ingestion of ≥ 200 mg/day of vitamin C might imply a better functioning of the immune system [1]. Fruits and vegetables are the ideal way to improve the intake of vitamin C, although, there is evidence suggesting that nowadays a high percentage of the general population is not achieving the actual recommendations. As changing people’s food habits is notoriously difficult, supplements and fortified foods could be used as a short-term solution [1].

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REFERENCES

1. Elste, Volker, et al. "Emerging Evidence on Neutrophil Motility Supporting Its Usefulness to Define Vitamin C Intake Requirements." Nutrients 9.5 (2017): 503.
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3. Bozonet, Stephanie M., et al. "Enhanced human neutrophil vitamin C status, chemotaxis and oxidant generation following dietary supplementation with vitamin C-rich SunGold kiwifruit." Nutrients 7.4 (2015): 2574-2588. PMC. Web. 3 July 2017.
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9. EFSA NDA Panel. Scientific opinion on dietary reference values for Vitamin C. EFSA J. 2013. EFSA Journal 2015;13(3):4028
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