Vitamin C deficiency increases the risk of respiratory infections

- and supplementation has a therapeutic effect

Respiratory infections are a leading cause of illness and, in some cases, can become so complicated that they pose a life-threatening risk to vulnerable groups, which is why the body’s immune system and antioxidant defense are crucial. According to a population study published in Nutrients, with a special focus on children and adolescents, supplementation with vitamin C may play a therapeutic role in preventing and treating respiratory diseases.

Upper respiratory tract infections include the common cold, influenza, COVID-19, sinusitis, tonsillitis, and middle ear infections (otitis media). The two most common lower respiratory tract infections are bronchitis and pneumonia. Viral infections in the upper respiratory tract can also strain the immune system to the point where bacteria from the natural microflora of the throat spread and cause inflammation in the sinuses, middle ear, lungs, and elsewhere.
Influenza and COVID-19 can also spread to the lower respiratory tract, leaving older individuals particularly vulnerable. This is especially dangerous if the immune system overreacts with hyperinflammation, resulting in acute respiratory distress syndrome (ARDS) and related complications. The condition is also characterized by oxidative stress, in which potentially harmful free radicals outgun the protective antioxidants.
Lower respiratory tract infections are the most common cause of death among all infectious diseases and represent a major global health problem. Young children and teenagers are especially susceptible, particularly to bronchitis and pneumonia, as parts of their immune system are still developing. Older adults are also vulnerable due to the natural immune decline caused by ageing processes and chronic inflammation.
Globally, it is estimated that 3 million people die annually from pneumonia, including 740,000 children under the age of five.
Numerous studies have shown that a healthy diet plays a major role in preventing respiratory infections and that nutrient deficiencies increase the risk. It is also well established that vitamin C is important for the immune system and overall health. The new study aimed to investigate the relationship between blood levels of vitamin C and the risk of respiratory infections in children and adolescents.
The researchers obtained data from a large population study called NHANES (National Health and Nutrition Examination Survey), which included 1,344 participants aged 6–19 years. Participants’ vitamin C levels in the blood were analysed using laboratory tests. The incidence of respiratory infections was assessed using questionnaires. The relationship between vitamin C levels and respiratory infections was tested using various statistical models and methods, with adjustments made for potential confounding factors such as age, race, BMI, socioeconomic status, asthma, and smoking.
The study revealed that 238 participants (17.7%) had reported a respiratory infection within the last 30 days. It was also found that blood vitamin C levels were significantly and inversely associated with the risk of respiratory infection.
The researchers concluded that vitamin C deficiency increases the risk of respiratory infections. Supplementation with vitamin C may also be used therapeutically for prevention and as part of treatment.

What role does vitamin C play in the immune system?

Broadly, the immune system consists of the innate immune system and the acquired immune system, which develops after birth when it encounters pathogens.
The innate immune system, consisting mainly of proteins and phagocytes (specialized white blood cells), fights most pathogens without us noticing. It can also trigger inflammation and call for help from the acquired immune system, which consists of T-cells, B-cells, and antibodies.
When the immune system launches an attack, it uses free radicals as missile weapons. It is important to have sufficient antioxidants so these free radicals do not spiral out of control and damage healthy cells.
The immune system should ideally respond quickly and effectively and then stand down once pathogens have been dealt with. Here, vitamin C plays the following roles for the immune system and respiratory tract:

• Participates in the formation of connective tissue and protects epithelial cells in the airways.
• Stimulates the production of interferon, which prevents viruses from multiplying in infected cells.
• Stimulates phagocytes (neutrophils) in the innate immune system to attack pathogens rapidly.
• Regulates the production of cytokines, which white blood cells use for communication.
• Counteracts proinflammatory cytokines such as IL-6 and TNF-α, thereby preventing excessive inflammation.
• Reduces CRP (C-reactive protein), another inflammation marker.
• Acts as a powerful antioxidant, protecting cells from damage caused by free radicals and oxidative stress.

Overall, vitamin C can help prevent respiratory infections or keep them from becoming complicated. This is mainly because vitamin C strengthens the innate immune system, reduces inflammation in the airways, and improves respiratory function. Vitamin C can also shorten the duration of respiratory infections through its anti-inflammatory properties. As a strong antioxidant, it further helps prevent local tissue damage.

Why is vitamin C deficiency so widespread?

Good sources of vitamin C include citrus fruits, red peppers, berries, herbs, cabbage, and other fruits and vegetables. However, most people fail to follow the official dietary recommendations for fruit and vegetable intake. It is estimated that around 4 billion people, or more than 50% of the global population, are vitamin C deficient.
The problem is compounded by the fact that sugar from sweets, cakes, soft drinks, breakfast cereals, ketchup, and many other foods competes with vitamin C for uptake into cells. This means sugar can increase vitamin C requirements. Air pollution, other environmental toxins, and stress can also raise needs.
Of course, it is best to obtain adequate vitamin C from a healthy diet, which also provides fiber and other nutrients. The recommended intake (RI) for children and adults is 40 and 80 mg, respectively, but other studies suggest that the actual requirement may be higher.
Needs also increase during acute infections to support the immune system and antioxidant defenses. In such cases, it can be beneficial to start supplementation as soon as the first symptoms appear. Stronger, non-acidic supplements that are gentle on the stomach are available on the market, including child-friendly options.

References:

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

Simone Passarelli et al. Global estimation of dietary micronutrient inadequacies: a modeling analysis. The Lancet Global Health 2024

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