Vitamin C

Vitamin C serves as an electron donor for at least 15 mammalian enzymes. It is an essential cofactor for prolyl and lysyl hydroxylases, which stabilize collagen's triple-helix structure — without it, collagen cannot form properly, leading to scurvy. As a cofactor for dopamine beta-hydroxylase, it is required for norepinephrine synthesis. Vitamin C regenerates alpha-tocopherol (vitamin E) from its oxidized form, maintaining the lipid-soluble antioxidant network. In immune cells, ascorbate accumulates at concentrations 10–100 times higher than in plasma. It enhances neutrophil chemotaxis, phagocytosis, and reactive oxygen species generation while protecting host tissues from oxidative damage. Vitamin C also supports the differentiation and proliferation of T-cells and B-cells. In the gut, it reduces ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), dramatically increasing non-heme iron absorption. It participates in carnitine biosynthesis, peptide hormone amidation, and epigenetic regulation through TET dioxygenases that demethylate DNA.

A Cochrane review (Hemilä & Chalker, 2013) of 29 RCTs with 11,306 participants found that regular vitamin C supplementation (≥200 mg/day) did not reduce cold incidence in the general population but reduced cold duration by 8% in adults and 14% in children. In individuals under physical stress (marathon runners, soldiers), regular supplementation halved cold incidence. A meta-analysis by Carr & Maggini (Nutrients, 2017) found that vitamin C at 1–2 g/day consistently shortened ICU stays and duration of mechanical ventilation in critically ill patients. Linus Pauling Institute research has shown that plasma vitamin C levels of 60–80 µmol/L are associated with reduced risk of cardiovascular disease, stroke, and certain cancers. High-dose IV vitamin C (not oral) is being studied as adjunctive cancer therapy through pro-oxidant mechanisms in tumor cells. For skin health, topical and oral vitamin C increase dermal collagen density, with clinical trials showing improved photoaging markers.

L-ascorbic acid, dehydroascorbic acid

Oral bioavailability is dose-dependent: nearly 100% at doses under 200 mg, dropping to ~50% at 1 g and ~33% at doses above 1.25 g. Divided doses and liposomal delivery significantly improve absorption at higher intakes.

• Red bell peppers
• Citrus fruits (oranges, lemons, grapefruit)
• Kiwifruit
• Strawberries
• Broccoli and Brussels sprouts
• Guava
• Acerola cherries