Immune system and other benefits: the essential role of vitamin C

IN BRIEF

• Vitamin C (ascorbic acid) is a water-soluble nutrient involved in many biological reactions: antioxidation, immune system support, and as a cofactor in collagen synthesis.
• Systematic reviews show that regular supplementation modestly reduces the duration of the common cold but does not prevent its onset in the general population; however, it may have a protective effect under conditions of extreme physical stress.
• Vitamin C increases the absorption of non-heme iron consumed with food and promotes tissue repair processes; however, clinical evidence on important outcomes (e.g., survival in oncology) remains inconclusive.
• High intravenous doses are being studied in oncology and critical care, but solid clinical evidence is limited and requires larger controlled studies.
• Safety is good for dietary and supplemental intake in moderate doses; pro-oxidant effects are possible in particular contexts (e.g., presence of transition metals or renal insufficiency) and warrant attention.

Abstract: what does science say?

Vitamin C (ascorbic acid) is an essential micronutrient for humans, necessary as an enzymatic cofactor (e.g., collagen hydroxylase), antioxidant, and modulator of immune and energy responses. Experimental and clinical evidence indicates that: 1) in subjects with adequate nutritional status, ordinary supplementation does not reduce the incidence of the common cold in the general population, but can shorten its duration; 2) vitamin C increases the absorption of non-heme iron and supports collagen synthesis during healing; 3) at pharmacological doses administered intravenously, antitumor effects are observed in preclinical studies and preliminary clinical results, but robust confirmations on survival or tumor control are lacking. The effect depends on dose, route of administration, baseline status (deficiency versus adequacy), and clinical context; interpretations require caution and consideration of the differences between observational data and causal evidence.

Biological roles and mechanisms of action

Vitamin C is involved in diverse and well-characterized processes: it acts as an electron donor (antioxidant), as a cofactor for enzymes that require iron in a reduced state, and participates in collagen synthesis and maintenance. Recent literature synthesizes these roles, highlighting how ascorbate supports epithelial function and innate immunity, participates in neurotransmitter synthesis, and facilitates biosynthetic reactions that require hydroxylation of amino acids in structural proteins [1].

From a cellular perspective, vitamin C enters cells via specific transporters (SVCT) and protects cellular components from oxidative damage; under certain conditions at high concentrations, it can also exert pro-oxidant effects that are being studied as a possible anti-cancer lever through the production of hydrogen peroxide in tumor microenvironments [7]. These mechanisms make a multifactorial role for vitamin C plausible: defensive support in the initial phases of infection, support for tissue repair, and possible pharmacological actions at concentrations much higher than those achievable orally [1][7].

Clinical evidence on respiratory infections and the common cold

The most authoritative systematic reviews show that regular vitamin C supplementation does not reduce the incidence of the common cold in the general population, but tends to reduce its duration and severity to a modestly consistent extent [3]. This effect is more pronounced in children and in people exposed to acute physical stress (marathon runners, hikers in extreme conditions) where a reduction in risk has been observed in some controlled trials [3].

For therapeutic use initiated after the onset of symptoms, the evidence is less solid: some studies with high doses have reported benefits, but the results are not uniform and the studies are heterogeneous in terms of dose and duration [3]. In practical terms, the literature supports the biological plausibility of a modulating effect, but not proof of absolute prevention for the entire population; therefore, recommendations must be based on context, baseline nutritional status, and individual factors [3].

Use and limitations of vitamin C in oncology (H3)

There is growing interest in high-dose intravenous vitamin C as a complementary therapy in oncology. Systematic reviews and preclinical reviews show potential mechanisms of action (production of reactive species in tumor microenvironments, modulation of epigenetic enzymes) and some preliminary clinical studies indicate improved quality of life or biological effects, but there is currently no robust evidence of benefit in terms of survival consolidated by large RCTs [6][5].

Iron absorption and role in collagen synthesis

Vitamin C increases the intestinal absorption of non-heme iron by converting ferric iron (Fe3+) into the more absorbable ferrous form (Fe2+) and reducing the inhibitory effect of phytates and other dietary components. Meta-analyses of absorption studies indicate a significant increase in iron absorption when ascorbic acid is consumed with a meal, with dose-dependent effects that are particularly relevant in populations at risk of iron deficiency [4].

In parallel, vitamin C is essential as a cofactor for proline and lysine hydroxylases, enzymes necessary for the hydroxylation of procollagen and the stabilization of the collagen triple helix. Reviews and preclinical studies indicate that adequate vitamin C availability facilitates tissue repair and collagen synthesis, with limited but consistent clinical evidence in contexts of musculoskeletal injuries and wound healing [5].

Safety, pharmacokinetics, and possible adverse effects

The pharmacokinetics of ascorbic acid show that moderate oral doses saturate plasma concentration and that further increases in oral dose lead to limited increases in blood concentration due to mechanisms of renal elimination and regulated cellular transport [2]. This explains why many pharmacological actions observed in vitro require plasma concentrations that are only achieved with intravenous administration.

Vitamin C is generally well tolerated; however, significant doses can increase oxalate excretion and, in the presence of renal insufficiency or predisposition to stone formation, increase the risk of nephrolithiasis. Furthermore, in the presence of free transition metals (iron or copper), ascorbate can participate in Fenton reactions that promote the formation of free radicals: in this regard, chemical models and experimental studies describe both antioxidant and pro-oxidant roles depending on the chemical context [8].

What this means in practice

For most people, a varied diet rich in fruits and vegetables provides the necessary vitamin C: citrus fruits, kiwi, strawberries, bell peppers, and brassicas are effective sources. If nutritional status is sufficient, the additional benefits of daily supplementation to prevent the common cold are limited for the general population, but it may be useful for reducing the duration of symptoms or for individuals exposed to severe physical stress [3].

In specific clinical contexts (documented deficiency, iron deficiency anemia with low dietary intake, complex wounds), supplementation may be useful as part of an overall strategy; however, the decision should take into account concomitant conditions (renal insufficiency, oncological therapies) and be based on medical advice. Intravenous therapies at pharmacological doses are experimental: protocols are variable and the evidence does not allow for their routine use outside of controlled clinical trials [5][6][2].

KEY POINTS TO REMEMBER

• Vitamin C is essential for health: an enzymatic cofactor, antioxidant, and immune modulator.
• Regular supplementation slightly reduces the duration of the common cold but not its incidence in the general population; greater benefits are observed under conditions of extreme physical stress [3].
• It promotes the absorption of non-heme iron and supports collagen synthesis during tissue repair [4][5].
• Oral dosages have pharmacokinetic limitations; very high plasma concentrations are only achieved intravenously, and its clinical use remains an object of research [2][7].
• Safety is good at dietary and moderate doses; caution is advised in renal insufficiency and in the presence of a risk of kidney stones or iron overload [8].

Limitations of the evidence

It is crucial to distinguish between observed associations (observational studies), biological plausibility (experimental mechanisms), and causal evidence obtained through randomized controlled trials. Many positive effects of vitamin C are supported by mechanistic data and preclinical studies, but robust clinical evidence varies by area: for the common cold, meta-analyses and reviews report modest effects; for oncological use, definitive clinical evidence on important outcomes is still insufficient and often based on small or non-randomized studies [3][5][6].

Some recurring methodological limitations: heterogeneity of doses and routes of administration, poor representativeness of certain populations in trials, heterogeneous clinical outcomes, and short follow-up. Furthermore, observational studies can be confounded by baseline nutritional status, eating behavior, and socioeconomic factors; therefore, recommendations must be based on systematic reviews and updated guidelines [3].

Editorial conclusion

Vitamin C remains one of the most studied nutrients with a biological profile that warrants clinical and nutritional attention. Evidence confirms its central roles in collagen synthesis, iron absorption, and antioxidant and immune processes. However, the magnitude and clinical relevance of the effects vary: many therapeutic applications, particularly the use of high-dose intravenous doses, require further controlled studies to establish efficacy and safety. For now, the most prudent and evidence-based strategy remains an adequate diet and targeted supplementation in specific clinical contexts under medical supervision.

Editorial note

This article was originally published in the past and updated according to scientific and popular criteria. The content is for informational purposes only and does not replace professional medical advice. For individual clinical decisions, consult a qualified healthcare professional.

SCIENTIFIC RESEARCH

1. Carr AC, Maggini S. Vitamin C and Immune Function. Nutrients. 2017;9(11):1211. https://doi.org/10.3390/nu9111211
2. Levine M et al. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. Proc Natl Acad Sci USA. 1996;93(8):3704-3709. https://doi.org/10.1073/pnas.93.8.3704
3. Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;Art. No.: CD000980. https://doi.org/10.1002/14651858.CD000980.pub4
4. Heffernan A et al. The regulation of dietary iron bioavailability by vitamin C: a systematic review and meta-analysis. Proc Nutr Soc. 2017. https://doi.org/10.1017/S0029665117003445
5. DePhillipo NN et al. Efficacy of vitamin C supplementation on collagen synthesis and oxidative stress after musculoskeletal injuries: a systematic review. Orthop J Sports Med. 2018; DOI: https://doi.org/10.1177/2325967118804544
6. Fritz H et al. Intravenous Vitamin C and Cancer: A Systematic Review. Integr Cancer Ther. 2014;13(2):85-101. https://doi.org/10.1177/1534735414534463
7. Giansanti M et al. High-Dose Vitamin C: Preclinical Evidence for Tailoring Treatment in Cancer Patients. Cancers. 2021;13(6):1428. https://doi.org/10.3390/cancers13061428
8. Shen J et al. Ascorbate oxidation by iron, copper and reactive oxygen species: review, model development, and derivation of key rate constants. Sci Rep. 2021;11:7417. https://doi.org/10.1038/s41598-021-86477-8