Vitamin C and everyday immunity

Why Vitamin C Still Earns Its Place in Your Medicine Cabinet

Vitamin C (ascorbic acid) is perhaps the most recognisable supplement in Britain. It lines pharmacy shelves in every format imaginable — effervescent tablets, chewable gummies, time-release capsules — and sits in the corner of every cold-weather conversation. Yet beyond the cultural familiarity lies a genuinely impressive body of evidence covering immune modulation, collagen synthesis, and antioxidant defence.

The Biochemistry: More Than an Antioxidant

Ascorbate is a water-soluble reducing agent. Its primary biological role is to donate electrons, quenching free radicals before they oxidise lipids, proteins, and DNA. But calling it "just an antioxidant" undersells its function considerably.

Vitamin C is an essential cofactor for prolyl and lysyl hydroxylases — the enzymes that hydroxylate proline and lysine residues in procollagen chains. Without adequate ascorbate, collagen triple helices fail to form properly, producing the fragile capillaries and impaired wound healing that characterise scurvy. This collagen role matters not just in historical cases of maritime deficiency but in everyday tissue maintenance, skin integrity, and recovery from injury or surgery.

Immune Modulation: What the Evidence Actually Shows

Vitamin C accumulates in immune cells at concentrations 50 to 100 times higher than in plasma. Neutrophils, macrophages, and lymphocytes all actively transport and concentrate ascorbate, using it to support respiratory burst activity (the oxidative killing of pathogens) and to regulate cytokine signalling. When the immune system mounts a response, cellular vitamin C is rapidly consumed, which is why requirements increase during infection.

The most rigorous population-level analysis comes from the Cochrane systematic review by Hemilä and Chalker, updated multiple times since its original publication. The headline findings are more nuanced than both the enthusiasts and the sceptics tend to acknowledge.

Regular prophylactic supplementation in the general population does not reduce the incidence of the common cold. This was the hypothesis that originally drove mass supplementation in the 1970s following Linus Pauling's advocacy, and it has been fairly definitively refuted in healthy adults who are not under extreme physical stress.

However, regular supplementation consistently reduces cold duration. Across over 30 placebo-controlled trials involving more than 11,000 participants, regular vitamin C supplementation reduced the duration of colds by approximately 8% in adults and around 14% in children. For an average seven-day cold, that represents roughly half a day of illness shortened — modest in absolute terms, but statistically robust and potentially meaningful at population scale.

In sub-groups under heavy physical stress — marathon runners, skiers, soldiers on Arctic exercise — prophylactic vitamin C supplementation reduced cold incidence by approximately 50%. This finding is consistent with the idea that extreme exertion depletes tissue ascorbate and temporarily compromises immune function.

Therapeutic high-dose vitamin C (taken after a cold begins) showed weaker and less consistent effects in the Cochrane analysis.

Vitamin C and Serious Illness

Intravenous vitamin C at pharmacological doses (10–75g per infusion) is an area of active clinical research in critical care and oncology, but this is a separate question from oral supplementation and should not be conflated with standard supplement use. The evidence base for IV vitamin C in sepsis and cancer is preliminary and contested.

Collagen Synthesis, Skin, and Wound Healing

Beyond immunity, the collagen-cofactor role has practical implications for anyone interested in skin health or musculoskeletal recovery. Ascorbate is required at two steps in collagen biosynthesis, and several randomised trials have examined oral vitamin C supplementation and skin outcomes.

A 2015 study published in Nutrients found that higher dietary vitamin C intake was associated with reduced wrinkle appearance and skin dryness in a cross-sectional analysis of 4,025 women. While correlation is not causation, the mechanistic plausibility is strong. More interventional work has examined wound healing, particularly in surgical and pressure-ulcer contexts, where ascorbate adequacy consistently correlates with faster healing.

Dosing and Safety

The UK Reference Nutrient Intake for vitamin C is 40mg per day — sufficient to prevent deficiency but not necessarily optimal for immune support. Many practitioners recommend 500mg to 1g daily for general health purposes, with evidence suggesting that plasma concentrations plateau at around 200–400mg due to saturation of intestinal transporters.

At 1g daily, vitamin C is extremely well tolerated. Above 2–3g daily, some individuals experience gastrointestinal symptoms including loose stools — this is sometimes used as a practical ceiling (the "bowel tolerance" concept). The upper safe level in the UK is set at 1,000mg/day for adults.

People with a history of kidney oxalate stones should exercise caution, as ascorbate is metabolised in part to oxalate. Those with haemochromatosis should note that vitamin C enhances non-haem iron absorption.

Food Sources vs Supplements

Citrus fruit is the classic source, but red and yellow peppers are substantially richer — a single 100g portion of raw red pepper contains around 200mg. Kiwi fruit, strawberries, broccoli, and Brussels sprouts are all excellent sources. However, vitamin C is heat-labile and leaches into cooking water, so the vitamin C content of cooked vegetables is considerably lower than raw equivalents.

For those with inconsistent dietary intake — common during winter when fruit and vegetable consumption tends to fall — a daily supplement of 500mg to 1g fills the gap reliably and inexpensively.

Bottom Line

Vitamin C is not the panacea Pauling envisaged, but it is a genuinely important nutrient with well-documented roles in immunity, collagen synthesis, and antioxidant defence. The Cochrane evidence is clear: it will not stop you catching a cold, but regular supplementation meaningfully shortens illness duration. For most adults, 500mg to 1g daily is safe, evidence-supported, and represents a sensible baseline for year-round immune resilience.