Vitamin C (Ascorbic Acid)

Vitamin C (ascorbic acid) is a water-soluble antioxidant with multiple critical roles. It is the rate-limiting cofactor for prolyl and lysyl hydroxylase enzymes, which hydroxylate proline and lysine residues in collagen — without adequate vitamin C, collagen cannot cross-link and is structurally defective, leading to poor wound healing and bleeding gums. It supports immune function by enhancing neutrophil phagocytosis, lymphocyte proliferation, and interferon production. Vitamin C reduces ferric iron (Fe³⁺) to ferrous iron (Fe²⁺), dramatically enhancing non-heme iron absorption — a single 100 mg serving can triple plant iron uptake. It is also essential for neurotransmitter synthesis (dopamine, norepinephrine, serotonin), serving as a cofactor for dopamine-β-hydroxylase. Finally, it functions as a powerful antioxidant, scavenging free radicals and protecting against lipid peroxidation and oxidative damage to DNA and proteins.

Scurvy develops after approximately 4-12 weeks of near-zero vitamin C intake. Early signs include vague fatigue, irritability, joint pain, and rough bumpy skin with bleeding around hair follicles (perifollicular hemorrhages). As deficiency advances, gums become red, swollen, and bleed; teeth loosen and fall out; hair becomes coiled or corkscrew-shaped; and bruising and poor wound healing become pronounced. Advanced scurvy includes anemia, increased infection susceptibility, and potential death from infection or bleeding. Historically called the "pirate disease," scurvy still occurs today in vulnerable populations: older adults with restricted diets, people with severe alcoholism, individuals with malabsorption disorders (Crohn's disease, cystic fibrosis), and smokers with extremely low intake. Subclinical deficiency (serum vitamin C <30 μmol/L) affects approximately 6% of US adults and manifests as mild fatigue, slow wound healing, and increased infection susceptibility.

The Recommended Dietary Allowance (RDA) is 90 mg/day for adult men and 75 mg/day for adult women. The FDA Daily Value on supplement labels is 90 mg. The Tolerable Upper Intake Level (UL) is 2,000 mg/day. Notably, smokers need an additional 35 mg/day (125 mg/day for men; 110 mg/day for women) because smoking depletes vitamin C through increased oxidative stress. The body saturates vitamin C at approximately 400 mg/day — beyond this, plasma levels plateau and excess is excreted unchanged in urine, providing no additional benefit. For immune support during acute illness, 200 mg/day from food or supplements is effective; megadoses (>1,000 mg/day) do not provide additional immune benefit and increase urinary oxalate and kidney stone risk.

Per 100 g serving (3.5 oz): guava is the richest at 229 mg, followed by raw red bell pepper (190 mg), kiwi (92 mg), raw broccoli (89 mg), papaya (62 mg), strawberries (59 mg), and orange (59 mg). One medium orange (~150 g) provides approximately 70-100 mg vitamin C. It is critical to eat these foods raw or lightly cooked: cooking, boiling, and prolonged storage destroy vitamin C. Raw vegetables and fresh fruit retain maximum vitamin C; frozen berries and vegetables (frozen at peak ripeness) often retain more vitamin C than fresh produce stored at room temperature for several days. Canned citrus juice retains some vitamin C (approximately 50-70%), whilst fresh-squeezed juice consumed immediately contains nearly all the original vitamin C.

The evidence is nuanced: regular vitamin C supplementation (200+ mg/day year-round) does NOT prevent colds in the general population, contrary to popular belief. However, it does reduce cold duration by approximately 8% in adults (e.g., reducing a 10-day cold to 9.2 days) and by approximately 14% in children — a modest but measurable benefit. People under extreme physical stress (marathon runners, soldiers in cold climates) see a 50% reduction in cold incidence with adequate supplementation, suggesting vitamin C is most protective in conditions of severe immune challenge. During acute illness, high-dose vitamin C (500-1,000 mg/day) may reduce symptom severity slightly, though the evidence is weaker than for prevention. The landmark Cochrane review (Hemilä & Chalker, 2013) concludes that regular supplementation warrants consideration only for those with heavy physical stress exposure.

Yes, strongly supported by mechanistic and clinical evidence. Vitamin C is the rate-limiting cofactor for collagen hydroxylation; without it, collagen cannot cross-link properly and is structurally defective. Adequate dietary vitamin C saturation (approximately 400 mg/day) ensures optimal collagen synthesis. Topical vitamin C (10-20% L-ascorbic acid formulations) penetrates the skin barrier, neutralises free radicals, and stimulates collagen synthesis, reducing visible wrinkles and improving skin elasticity. Dietary vitamin C (combined with topical vitamin C) protects skin from UV-induced oxidative damage and reduces photoaging. Studies show that combining adequate dietary vitamin C (200-400 mg/day) with topical vitamin C serums provides superior anti-ageing effects compared to either alone. This is why vitamin C is a cornerstone of evidence-based anti-ageing skincare, and why collagen supplements require concurrent vitamin C intake to be effectively utilised by the body.

Yes, high-dose vitamin C supplementation (>1,000 mg/day) increases urinary oxalate excretion and is associated with higher kidney stone risk, particularly in men and individuals with a history of stones. The landmark Ferraro et al. (2016) study published in JAMA Internal Medicine found that men taking 1,000+ mg/day vitamin C supplements had approximately 2-fold increased risk of kidney stones compared to those without supplementation. The mechanism is simple: excess vitamin C is metabolised to oxalate and excreted in urine; high urinary oxalate is a major risk factor for calcium oxalate stone formation. For most people without stone history, intakes up to 500 mg/day from food and supplements are safe. Those with personal or family history of kidney stones should limit supplemental vitamin C to <500 mg/day and avoid megadoses entirely.

One of the most evidence-based nutrient interactions: vitamin C dramatically enhances non-heme iron absorption through a simple reduction reaction. Plant-based (non-heme) iron exists primarily in the ferric form (Fe³⁺), which is poorly absorbed. Vitamin C (ascorbic acid) reduces ferric iron to ferrous iron (Fe²⁺), the form that intestinal iron transporters (DMT1) preferentially absorb. Just 100 mg vitamin C (e.g., half a bell pepper, one kiwi, or 240 mL / 8 fl oz orange juice) consumed with a plant iron source can triple non-heme iron bioavailability. Practical examples: lentil soup with lemon juice, fortified breakfast cereal with strawberries, spinach salad with orange segments, tofu stir-fry with bell peppers. This is the single most effective dietary strategy for vegetarians and vegans to correct or prevent iron deficiency without requiring supplementation. Animal-based (heme) iron absorption is not enhanced by vitamin C, as heme iron is absorbed via a separate mechanism.