Vitamin C (ascorbic acid) is a water-soluble vitamin that humans cannot synthesize due to a mutation in the L-gulonolactone oxidase gene — a trait shared with only a handful of other species including guinea pigs and some primates [2]. This means every milligram must come from diet or supplementation.
Its roles in the body are extensive:
- Collagen synthesis: Vitamin C is a required cofactor for prolyl and lysyl hydroxylase, enzymes that stabilize the collagen triple helix. Without it, collagen literally falls apart — this is what happens in scurvy [2].
- Iron absorption: Ascorbic acid reduces ferric iron (Fe3+) to ferrous iron (Fe2+) in the gut, dramatically increasing absorption of non-heme iron from plant sources. Taking vitamin C with iron-rich meals can boost absorption by 2-3 fold [2].
- Antioxidant defense: As an electron donor, vitamin C scavenges reactive oxygen species and regenerates other antioxidants including vitamin E [1].
- Immune function: Vitamin C accumulates in neutrophils and lymphocytes at concentrations 10-100 times higher than plasma levels. It enhances chemotaxis, phagocytosis, and microbial killing while also supporting lymphocyte proliferation [1].
The scurvy story is worth knowing. Sailors in the 18th century died by the thousands from this disease — bleeding gums, loose teeth, reopening of old wounds, and eventually death from hemorrhage or infection. In 1747, James Lind conducted one of the first controlled clinical trials, demonstrating that citrus fruits cured scurvy. It still took the British Navy another 42 years to mandate lemon juice on ships.
Regarding supplementation, plasma concentrations of vitamin C plateau at doses around 200-400 mg per day when taken orally [2]. The RDA is set at a modest 90 mg for men and 75 mg for women, but many researchers argue that 500-1000 mg per day is closer to optimal for antioxidant and immune support [1].
Liposomal vitamin C formulations claim higher bioavailability by encapsulating ascorbic acid in phospholipid spheres. While some small studies suggest modestly improved absorption compared to standard supplements, the difference is not as dramatic as marketing materials imply. Standard ascorbic acid remains well absorbed at doses under 500 mg [2].
The Linus Pauling controversy looms large in vitamin C history. Pauling, a two-time Nobel laureate, advocated megadoses (10-18 grams daily) for preventing colds and treating cancer. While his enthusiasm outpaced the evidence at the time, he was not entirely wrong that higher intakes could be beneficial — he was just wrong about the magnitude of the effect.
The 2013 Cochrane meta-analysis by Hemila and Chalker examined 29 trials with over 11,000 participants and found that regular vitamin C supplementation (200 mg or more daily) did not reduce cold incidence in the general population, but did reduce cold duration by 8% in adults and 14% in children [3]. Notably, in six trials involving participants under heavy acute physical stress (marathon runners, skiers, soldiers in subarctic conditions), regular supplementation halved the risk of catching a cold. The takeaway: vitamin C is not a magic cold shield for most people, but it does appear to help the immune system recover faster and offers meaningful protection under conditions of physical stress.
Carr and Maggini's 2017 review consolidated the mechanistic evidence for vitamin C's role in immune function, demonstrating that it supports both the innate and adaptive immune systems [1]. Vitamin C enhances epithelial barrier function, supports the oxidant-scavenging activity of the skin, and accumulates in phagocytic cells where it enhances chemotaxis, phagocytosis, generation of reactive oxygen species, and ultimately microbial killing. Deficiency impairs immunity and increases susceptibility to infections, creating a vicious cycle since infections further deplete vitamin C stores.
The NIH fact sheet notes that smokers require an additional 35 mg per day due to increased oxidative stress and turnover. Certain populations — including people with limited food variety, those with malabsorption conditions, and individuals with chronic diseases — are at higher risk of inadequate intake [2]. While frank scurvy is rare in developed nations, subclinical deficiency (plasma levels below 23 micromol/L) is more common than generally appreciated, affecting an estimated 7% of the US population.