What is the mechanism of action of vitamin C?

December 5, 2025 •

3 min read

Vitamin C, also known as ascorbic acid, is an essential nutrient that humans must obtain from their diet because they lack the ability to synthesize it internally. Its mechanism of action is surprisingly complex and goes far beyond its common reputation as an immune booster, encompassing multiple critical functions at a cellular level.

Quick Summary

Vitamin C works as a powerful antioxidant and a crucial enzymatic cofactor, driving essential processes like collagen formation, neurotransmitter synthesis, and epigenetic regulation.

Key Points

Antioxidant Action: Vitamin C neutralizes damaging free radicals (ROS, RNS) by donating electrons, thereby protecting cells and tissues from oxidative stress.
Enzymatic Cofactor: As a crucial cofactor, it maintains the reduced state of metal ions (iron, copper) required for the proper function of various enzymes.
Collagen Synthesis: Its role as a cofactor for hydroxylase enzymes is vital for forming the stable, triple-helical structure of mature collagen, essential for connective tissue.
Immune Cell Support: It accumulates in immune cells like neutrophils, protecting them from oxidative damage during infection fighting and enhancing their function.
Epigenetic Regulator: Newly identified mechanisms include acting as a cofactor for TET and JHDM enzymes, which control DNA and histone demethylation and influence gene expression.
Dual Role: Depending on its concentration and context, vitamin C can act as either an antioxidant at physiological levels or a pro-oxidant at very high pharmacological levels, with the latter being studied in cancer therapy.
Enhanced Iron Absorption: By reducing ferric iron ($Fe^{3+}$) to ferrous iron ($Fe^{2+}$), it significantly improves the body's absorption of non-heme iron from plant-based foods.

The Core Mechanisms of Vitamin C

Vitamin C's primary mechanisms of action stem from its ability to donate electrons, allowing it to function as both an antioxidant and a cofactor for various enzymes. It is absorbed in the small intestine, primarily as ascorbic acid (AA) via sodium-dependent transporters (SVCTs), and as dehydroascorbic acid (DHA) via glucose transporters (GLUTs), which is then reduced back to AA inside cells. This process ensures high concentrations in key tissues like the brain and adrenal glands.

Vitamin C as a Potent Antioxidant

Vitamin C acts as a water-soluble antioxidant, protecting cells from damage caused by reactive oxygen species (ROS) and reactive nitrogen species (RNS).

It directly neutralizes free radicals by donating electrons. The resulting semidehydroascorbic acid can be recycled back to its active form.
It regenerates other antioxidants, such as vitamin E, maintaining the body's antioxidant defenses.
It protects immune cells, like neutrophils, from oxidative damage they generate while fighting pathogens.

Vitamin C as an Enzymatic Cofactor

A critical function of vitamin C is its role as a cofactor for enzymes that utilize iron ($Fe^{2+}$) or copper ($Cu^{+}$), keeping these metal ions in their necessary reduced state.

Key vitamin C-dependent processes include:

Collagen Synthesis: Vitamin C is essential for the hydroxylation of proline and lysine residues in procollagen by serving as a cofactor for specific hydroxylases. This step is vital for the stability and structure of mature collagen. Deficiency leads to scurvy.
Neurotransmitter Production: It is a cofactor for dopamine β-hydroxylase, crucial for converting dopamine to norepinephrine.
Carnitine Synthesis: Required for the synthesis of carnitine, involved in fatty acid metabolism.
Epigenetic Regulation: Acts as a cofactor for TET and JHDM enzymes involved in DNA and histone demethylation, influencing gene expression and cell differentiation. This link is being explored in cancer research.

Transport and The Dual Nature

Specific transporters regulate vitamin C in the body. SVCTs actively transport ascorbic acid, while GLUTs transport dehydroascorbic acid, which is then reduced intracellularly. Vitamin C's role can shift depending on concentration; at high pharmacological doses, it can act as a pro-oxidant, unlike its antioxidant role at physiological levels.

Comparison of Vitamin C's Dual Roles


Feature	Antioxidant Role (Physiological Doses)	Pro-oxidant Role (High Pharmacological Doses)
Concentration	Physiological plasma ranges (40-80 μM).	Very high, supraphysiological (>1 mM).
Mechanism	Neutralizes free radicals and regenerates other antioxidants.	Generates reactive oxygen species, interacting with metal ions.
Cellular Impact	Protects from oxidative stress.	Can induce oxidative stress and cell death in certain cancer cells.
Application	Daily health, immune function, preventing scurvy.	Investigated as adjuvant cancer therapy.

Conclusion

Vitamin C's mechanism of action is multifaceted, extending beyond its antioxidant properties to include vital enzymatic cofactor roles in collagen synthesis, neurotransmitter production, carnitine synthesis, and epigenetic regulation. Its specific transport mechanisms ensure targeted delivery to tissues like the brain and immune cells. Understanding these roles highlights why consistent intake is crucial for health. For further information, consult resources like the Oregon State University Linus Pauling Institute.

Additional Mechanisms and Interactions

Vitamin C also influences immune system modulation, enhances non-heme iron absorption by reducing ferric iron to ferrous iron, participates in hormone and neuropeptide synthesis, and contributes to cardiovascular health through its antioxidant effects. Its role as an epigenetic regulator impacting DNA and histone demethylation is also significant for gene regulation and cell fate. These diverse actions underscore vitamin C's essential role in numerous bodily functions.

Frequently Asked Questions

Vitamin C protects the body by readily donating electrons to neutralize harmful molecules called free radicals and reactive oxygen species (ROS), preventing them from damaging lipids, proteins, and DNA within cells.

Vitamin C is an essential cofactor for enzymes called prolyl and lysyl hydroxylases. These enzymes hydroxylate proline and lysine residues, a step necessary for procollagen to form the stable triple-helix structure of mature collagen.

Scientific evidence suggests that regular, consistent vitamin C intake may slightly reduce the duration or severity of a cold, but taking a large dose after symptoms begin has limited benefits.

Severe vitamin C deficiency leads to scurvy, characterized by symptoms like bleeding gums, easy bruising, poor wound healing, and impaired immune function, primarily due to defective collagen synthesis.

Vitamin C (ascorbic acid) is transported into cells through active transport using sodium-dependent vitamin C transporters (SVCTs). Its oxidized form, DHA, can also enter via facilitated glucose transporters (GLUTs) and is then recycled.

At very high pharmacological concentrations, vitamin C can interact with transition metal ions like iron and copper to generate reactive oxygen species, acting as a pro-oxidant. This effect is being studied for its potential to selectively kill cancer cells.

Yes, vitamin C significantly enhances the absorption of non-heme iron (from plant-based foods) by reducing the iron to a more readily absorbable form in the intestine.