Vitamin C and immunity for better

Vitamin C is a potent antioxidant and cofactor for multiple immune functions. It supports the production and function of white blood cells that attack bacteria, the most abundant immune cells and first responders to bacterial infection. It enhances the skin's barrier function and supports the production of collagen in the gut lining and respiratory mucosa, the physical barriers that helps reduce the risk of threats from entering the body in the first place. It regenerates other antioxidants including vitamin E during the oxidative burst that white blood cells that attack bacteria use to destroy threats. And it supports the production and function of immune cells during the targeted immune response. These are genuine, well-established mechanisms. The question that the evidence is less settled on is whether extra vitamin C helps people who already get enough.

What is happening

The most rigorous and comprehensive analysis of vitamin C and the common cold is a independent medical review covering over 11,000 participants across multiple trials. Its conclusions are often misquoted in both directions. The review found no significant reduction in cold incidence in the general population from regular vitamin C supplementation, but found a significant reduction in cold duration, approximately 8 percent in adults, and a reduction in severity. Importantly, it found a significant protective effect on cold incidence specifically in populations under heavy physical or psychological stress, including marathon runners, skiers and soldiers in subarctic conditions. This population-specific finding is often overlooked. For people in high-demand, high-stress roles, the evidence for vitamin C supplementation is considerably stronger than for the general population.

Plasma vitamin C levels deplete rapidly during infection and illness, as the immune response consumes antioxidants at an accelerated rate during inflammation. Studies measuring plasma levels in hospitalised patients with severe infections consistently find significant depletion even in patients who had adequate levels beforehand. This depletion impairs the function of your bacteria-fighting white blood cells at precisely the time it is most needed. Supplementation during illness, rather than just before it, addresses this depletion and maintains the function of your bacteria-fighting white blood cells during the active immune response. The evidence for this application is stronger than the evidence for prophylactic supplementation in healthy people.

What to do next

Dietary sources of vitamin C are abundant and underutilised. A single red pepper provides around 170 milligrams. A kiwi fruit provides around 90 milligrams. A cup of broccoli provides around 100 milligrams. The recommended daily intake is 80 milligrams for adults, rising to 100 milligrams for smokers and under higher physiological stress. Most Europeans eating any meaningful quantity of vegetables meet the recommended intake without supplementation. Where supplementation is most defensible is during periods of high stress or illness, during winter when fresh vegetable intake tends to fall, and for people working high-demand schedules where dietary quality is inconsistent. A standard 500 to 1000 milligram supplement is adequate for these purposes. Very high doses above 2 grams per day produce gastrointestinal side effects without proportionally greater immune benefit.

Sources: WHO healthy diet guidance, EFSA water guidance, NCBI sleep stages, WHO stress guidance.

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