Vitamin C: a small, real edge on colds and little beyond that

Taking vitamin C every day does not stop the average person catching colds, and taking it once a cold has started does nothing measurable — the only reliable effect is that regular daily use makes colds slightly shorter, by about 8% in adults [s1]. Beyond the common cold, the large claims made for vitamin C do not hold up: a major randomised trial found no cardiovascular benefit, and a well-fed adult reaches the recommended intake from ordinary food [s2][s3].

How much you need, and what deficiency looks like

The Recommended Dietary Allowance for vitamin C is 90 mg a day for adult men and 75 mg a day for adult women, with an extra 35 mg a day for smokers, who metabolise the vitamin faster [s3]. Those are small amounts, easily met from fruit and vegetables — median intakes among adults were reported at 102 mg a day in the United States and 72 mg a day in Canada — and the Tolerable Upper Intake Level for adults is set at 2 g a day, above which the main problem is osmotic diarrhoea [s3]. Genuine deficiency causes scurvy, a connective-tissue disease, but the amount needed to prevent its gum-disease sign is under 10 mg a day, far below the RDA [s3]. In other words, the gap between "enough to avoid deficiency" and "what supplements sell" is enormous.

The common cold, tested for 70 years

The cold question has the strongest evidence, and it is unusually clear. A Cochrane review pooled 29 trial comparisons in 11,306 people taking vitamin C regularly and found that, in the general community, it did not reduce the number of colds people caught: the pooled relative risk was 0.97 (95% confidence interval 0.94 to 1.00) [s1]. Regular use did shorten colds — by 8% in adults (95% confidence interval 3% to 12%) and by 14% in children (7% to 21%), rising to 18% in children given 1 to 2 g a day — a real but small effect on a self-limiting illness [s1]. And crucially, taking vitamin C only after symptoms appeared, the way most people actually use it, produced no consistent effect on the duration or severity of colds across seven comparisons [s1].

There is one striking exception. In five trials totalling 598 marathon runners, skiers and soldiers on subarctic exercise, regular vitamin C roughly halved the risk of catching a cold (relative risk 0.48; 95% confidence interval 0.35 to 0.64) [s1]. That points to a real benefit in people under heavy acute physical stress — not the sedentary majority the supplements are marketed to.

No shortcut for the heart

The claim that vitamin C protects the heart, which came out of observational data, was tested directly and failed. The Physicians' Health Study II randomised 14,641 male physicians aged 50 or older to 500 mg of vitamin C daily or placebo and followed them for a mean of eight years, recording 1,245 major cardiovascular events [s2]. Vitamin C had no effect on major cardiovascular events (hazard ratio 0.99; 95% confidence interval 0.89 to 1.11), nor on heart attack, stroke or cardiovascular death individually [s2]. A supplement that does nothing for cardiovascular events in a long, well-powered trial is doing nothing worth paying for on that count.

How to read this

Vitamin C is a nutrient the body genuinely needs and most people already get. Daily supplements shorten colds a little and, in people under extreme physical stress, help prevent them; used at the first sniffle they do nothing, and for the heart the trial evidence is null [s1][s2]. The pattern — an essential vitamin whose supplement claims outrun its proven effects — repeats across the aisle: see the null cancer results for beta-carotene and vitamin A supplements, the targeted case for vitamin B12, and, on colds specifically, the mixed evidence for zinc lozenges.

This article is informational and not dietary or medical advice.