Vitamin C: New NIH Guidance on Immune Function

Why Is Vitamin C Essential for Human Health?

According to the NIH Office of Dietary Supplements, vitamin C (ascorbic acid) is an essential micronutrient that humans must obtain from food because we lack the enzyme L-gulonolactone oxidase needed to synthesize it. The vitamin serves as a cofactor for at least eight enzymes, most notably those involved in collagen biosynthesis, which is critical for maintaining skin, blood vessels, tendons, and wound healing.

Beyond its structural role, vitamin C functions as a potent antioxidant, scavenging free radicals that contribute to oxidative stress and chronic disease. It also regenerates other antioxidants such as vitamin E, supports immune cell function including phagocytosis and chemotaxis, and significantly enhances the absorption of non-heme iron from plant-based foods. The NIH notes that plasma vitamin C concentrations reach a maximum at intakes of around 200-400 mg per day, with excess amounts excreted in urine.

Who Is at Risk of Vitamin C Deficiency?

The NIH identifies several groups at elevated risk of inadequate vitamin C status. Smokers experience increased oxidative stress and metabolic turnover of vitamin C, requiring an additional 35 mg per day beyond standard recommendations. People with limited food variety — including those with food insecurity, alcohol use disorder, or restrictive diets — frequently consume insufficient fruits and vegetables to meet daily needs. Individuals with malabsorption disorders, end-stage renal disease on hemodialysis, and certain cancers may also develop deficiency.

Severe vitamin C deficiency causes scurvy, characterized by fatigue, gum disease, joint pain, poor wound healing, and corkscrew hair follicles. While scurvy is rare in industrialized countries, sporadic cases continue to appear, particularly among children with autism spectrum disorders who have highly selective eating patterns and elderly individuals with social isolation. Marginal deficiency, defined as plasma levels below 23 micromoles per liter, may affect up to one in ten adults in some populations and can impair immune function before clinical scurvy develops.

Does Vitamin C Supplementation Prevent Colds and Chronic Disease?

The most rigorous evidence on vitamin C and the common cold comes from Cochrane systematic reviews showing that regular supplementation does not reduce cold incidence in the general population. However, supplementation may shorten cold duration by approximately 8% in adults and 14% in children, and people under heavy physical stress — such as marathon runners and soldiers in subarctic conditions — may experience reduced cold incidence with regular intake.

For chronic disease prevention, the NIH summarizes evidence as inconclusive. Observational studies have linked higher vitamin C intake from foods with lower risk of cardiovascular disease and certain cancers, but randomized controlled trials of supplementation have generally not confirmed these benefits. Intravenous high-dose vitamin C remains under investigation as an adjunct cancer therapy, though it should never replace standard oncology treatment. The NIH emphasizes that obtaining vitamin C from a varied diet rich in fruits and vegetables provides additional phytochemicals and fiber that supplements cannot replicate.