Vitamin C’s protective role in infections

Vitamin C plays a particularly important role in the innate immune system, which combats most pathogens without causing noticeable symptoms. The vitamin is also involved in regulating inflammatory processes, protecting cells against oxidative stress, and supporting wound healing, according to a new review article published in Nutrients.

Infectious diseases are caused by viruses, bacteria, fungi, parasites, or other pathogens invading the body. Various pathogens are typically spread through the air, or via direct contact, contaminated food or water, lesions, and insect bites. When resistance is reduced, potentially harmful bacteria from mucous membranes can also spread to the sinuses, bronchi, lungs, bladder, and other sites, where it causes so-called autoinfections.
Under normal circumstances, proteins, macrophages, and other white blood cells in the innate immune system handle most pathogens completely unnoticed. If the infection gains ground, the innate immune system triggers inflammatory processes and signals the adaptive immune system to produce more targeted T-lymphocytes, B-lymphocytes, and antibodies.
During inflammatory responses, white blood cells generate cascades of free radicals that attack pathogens, almost like missiles. Since free radicals are highly aggressive molecules, it is crucial that they are regulated by protective antioxidants.
If the inflammatory processes progress too slowly, there is a risk of chronic infections. If they become too intense, oxidative stress can occur, with free radicals attacking healthy cells and tissues and causing complications. Life-threatening cases of influenza, COVID-19, and sepsis are caused by hyperinflammation and other poorly regulated mechanisms that, in worst case, can result in organ failure.
In the prevention and treatment of infectious diseases, it is therefore vital to launch rapid and effective inflammatory responses while ensuring adequate antioxidant availability to prevent cellular damage from free radicals and oxidative stress. Vitamin C plays a key role here, and this
prompted the authors of the new review to examine its effects by analyzing numerous published studies.

Vitamin C stimulates immunity and protects against oxidative stress

Vitamin C exists in two forms: ascorbic acid (AA), the primary active form functioning as an antioxidant, and dehydroascorbic acid (DHA), which is formed when ascorbic acid donates electrons and also acts as an important metabolite. DHA is converted back to ascorbic acid inside cells, and the interplay between these forms is essential for vitamin C’s overall effects.
Vitamin C is involved in the synthesis of connective tissue, thereby protecting epithelial cells in the respiratory tract and elsewhere. White blood cells, such as phagocytes (granulocytes and macrophages), which play a crucial role in the innate immune system’s frontline defense and communication, contain significant amounts of vitamin C that are necessary for rapid activation.
In the case of viral infection, vitamin C also stimulates the production of interferon, which helps prevent viral replication in infected cells. Additionally, vitamin C inhibits the formation of pro-inflammatory markers (CRP, IL-6, and TNF-α), preventing excessive inflammation. Finally, vitamin C acts as a powerful antioxidant by countering cellular damage caused by free radicals and oxidative stress.
For the prevention of various infections, maintaining adequate vitamin C levels is essential. The review cites several studies in which vitamin C supplementation was used in cases of influenza, pneumonia, COVID-19, staphylococcal infections, other infections, and sepsis. In vitro and animal studies also suggest a potential beneficial effect in the treatment of AIDS, malaria, and tuberculosis.

Sources, needs, and relevant supplementation

Citrus fruits, berries, rose hips, herbs, broccoli, and other fruits and vegetables are good sources of vitamin C. Still, it is estimated that around four billion people worldwide are deficient due to poorly balanced or highly refined diets.
Healthy individuals can usually achieve a satisfactory blood concentration of 70 μmol/L with a daily intake of 200 mg of vitamin C. This is significantly higher than the official recommended intake of 80 mg. Smokers and sick individuals often have much lower blood levels of vitamin C and therefore require higher intake.
According to the review, healthy individuals should not exceed 1,000 mg of vitamin C per day, as excessive intake may increase the risk of kidney stones, although there is some disagreement on this issue. Regardless, it is important to maintain adequate vitamin C intake daily for infection prevention. During the onset of infections and as part of treatment, many individuals may also benefit from higher, pH-neutral doses taken multiple times throughout the day.

Vitamin C and the immune system

• Stimulates interferon production, helping prevent viral replication in infected cells.
• Promotes production and maturation of various white blood cells, including phagocytes (granulocytes and macrophages) and lymphocytes (T- and B-lymphocytes).
• Enhances the activity of white blood cells, including phagocytosis and respiratory burst (free radical attack).
• Regulates excessive inflammatory processes.
• Acts as a powerful antioxidant, protecting healthy cells from damage caused by free radicals and oxidative stress.

References:

Runze Li et al. The Roles of vitamin C in infectious diseases: A comprehensive review. Nutrition 2025

Adina Alberts et al. Vitamin C: A Comprehensive Review of Its Role in Health, Disease Prevention, and Therapeutic Potential. Molecules 2025

Ci Li. The association between serum vitamin C levels and respiratory infections in children and adolescents. Frontiers in Nutrition. 2025