Vitamin C is one of the most popular supplements in the UK — but most people take it wrong. Here's what the evidence says about immune function, collagen, and the optimal dose.
Vitamin C (ascorbic acid) is the bestselling supplement in the UK. Most people associate it with immune support and reach for it when they feel a cold coming on. The reality is more nuanced — vitamin C is a critical nutrient with well-established roles in immune function, collagen synthesis, antioxidant protection, and iron absorption. But the dose matters, and timing matters.
Vitamin C supports immune function through several mechanisms:
The evidence for colds specifically: a 2013 Cochrane review (29 RCTs, 11,000+ participants) found regular vitamin C supplementation doesn't prevent colds in the general population but significantly reduces cold duration (8% shorter in adults) and severity. The preventive effect IS significant in people under extreme physical stress (marathon runners, soldiers in subarctic conditions) — 50% reduction in cold incidence.
Bottom line: take it daily for duration and severity benefit, not as a last-minute intervention when symptoms start.
This is one of vitamin C's most important and underappreciated roles. Vitamin C is an essential cofactor for the enzymes that form the collagen triple helix — prolyl hydroxylase and lysyl hydroxylase. Without vitamin C, collagen cannot be properly synthesised. This is the mechanism behind scurvy (severe vitamin C deficiency), which causes collagen breakdown throughout the body.
For anyone taking collagen supplements for skin or joint health, ensuring adequate vitamin C intake is critical — it's the cofactor that allows the collagen peptides to be converted into new tissue. Many quality collagen supplements now include vitamin C specifically for this reason.
Vitamin C is the primary water-soluble antioxidant in human plasma. It neutralises free radicals in aqueous environments and regenerates fat-soluble vitamin E (converting the oxidised form back to active vitamin E). Chronic oxidative stress contributes to cardiovascular disease, ageing, neurodegeneration, and cancer. Dietary antioxidants, including vitamin C, help buffer this — though translating antioxidant capacity to clinical outcomes in healthy individuals is complex.
The adrenal glands have one of the highest vitamin C concentrations in the body, as it's involved in cortisol synthesis and regulation. High-dose vitamin C (1–3g) has been shown in some studies to blunt cortisol response to acute stress. A 2001 trial found 1g vitamin C significantly reduced cortisol and blood pressure response to a public speaking test. This makes it a useful addition to anti-stress supplement protocols.
Vitamin C significantly enhances non-haem iron absorption (the form found in plant foods) — converting Fe³⁺ to the more absorbable Fe²⁺ form. Taking vitamin C (even from food) alongside iron-rich plant foods or iron supplements can increase absorption by 2–3x. Critical for vegetarians, vegans, and anyone with iron deficiency.
The UK RNI (Reference Nutrient Intake) is 40mg/day — easily achievable from diet for most people eating fruit and vegetables. However, optimal for supplemental benefits differs:
Vitamin C is water-soluble — excess is excreted. Doses above 1–2g can cause diarrhoea in some people ("bowel tolerance"). This is safe but uncomfortable. Spread doses through the day for better absorption.
Rich dietary sources include: kiwi (93mg each), red bell pepper (190mg/100g), broccoli (89mg/100g), orange juice (50mg/glass), and strawberries (59mg/100g). Many people consuming a varied diet rich in fruit and vegetables already achieve 200–300mg/day. Supplementation is most beneficial for those with restricted diets, high stress loads, smokers (who deplete vitamin C faster), and during illness.
Vitamin C's role in immune function, collagen synthesis, and antioxidant protection is well-established. For cold prevention, daily supplementation doesn't prevent colds in healthy people but reduces duration and severity. The collagen cofactor role is critical and underappreciated. 500mg–1g daily is a reasonable general supplementation dose; higher doses during illness or stress are supported by evidence. Always take alongside collagen supplements for maximal benefit.
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