The science of vitamin C

What was seen?

In a trial with 48 people, 1 g twice a day increased urinary oxalate and the stone risk index in about 40% of participants, with and without previous stones. There are also isolated case reports of kidney damage from oxalate crystals in people who took high doses for months, especially with kidney disease or bowel surgery, and one report of iron overload in a patient with hemochromatosis who took large doses; they show what can happen, not how often. People who have had kidney stones should talk to their doctor before taking 1 g or more a day, and so should people with kidney disease or hemochromatosis.

What changes with colds?

In 10 trials with oral vitamin C (1 g a day or more), colds were about 15% less severe; the effect was seen in severe symptoms, not mild ones. In the largest trial, with 2,349 volunteers, no vitamin C group showed a clear difference from placebo; in another trial in adults, those who took 1 g a day had about 30% fewer days of incapacity (at home or off work).

In people under short, intense physical stress (marathon runners, skiers, soldiers), vitamin C cut colds by about half, in 5 trials with 598 people. In a trial with 1,444 army recruits, there were fewer colds with vitamin C.

With regular use, colds lasted about 8% less in adults and 14% less in children. Taking vitamin C only once a cold starts had no consistent effect: in the largest trial, 10 g over 2.5 days did not relieve symptoms.

In the Cochrane review, with 29 comparisons and about 11,300 people, taking vitamin C every day did not reduce the number of colds in the general population (relative risk 0.97).

What changes in the blood vessels?

In 17 trials with 478 people, vitamin C alone (500 to 2,000 mg a day, for more than 2 weeks) slightly improved artery dilation; in trials with people over 56 the effect was larger, and up to age 56 no effect was seen. In a trial with 520 adults, 3 years of vitamin C alone did not slow thickening of the carotid artery.

In 29 short trials (about 8 weeks, 500 mg a day), vitamin C lowered systolic blood pressure by about 4 mmHg and diastolic by about 1.5 mmHg, slightly more in people with high blood pressure. In a 5-year Japanese trial, 500 mg a day did not lower blood pressure compared with 50 mg. In people with type 2 diabetes, two small trials saw decreases of 7 to 10 mmHg; a third saw no difference.

And heart attacks and strokes?

In the largest trial, PHS II (14,641 male doctors, 500 mg a day for about 8 years), vitamin C did not reduce heart attacks, strokes or cardiovascular deaths. In WACS, with 8,171 women at high cardiovascular risk, it did not either (relative risk 1.02).

Is one form better absorbed?

After a single dose, liposomal vitamin C gave a blood peak 27% higher than plain ascorbic acid, in 27 people. PureWay-C gave higher levels than calcium ascorbate from 1 to 6 hours, in a small trial. Ester-C gave plasma levels similar to ascorbic acid and caused fewer stomach complaints in people sensitive to acidity. Calcium ascorbate did not give more vitamin C than ascorbic acid, either in plasma or in white blood cells. Vitamin C from a tablet and from kiwi fruit are absorbed equally. There are no trials showing health differences between forms. INSYGMA products provide 1,000 mg of vitamin C a day with 2 capsules.

The only human trial with PureWay-C is small and used a single dose; a 2025 review describes it as randomized and double-blind, with 10 people per form.

What happens with larger doses?

Blood vitamin C rises with the dose: in 24 people, it rose step by step from 60 to 1,000 mg a day. A large dose is absorbed less: with 500 mg at once, less than 30% of the proportion absorbed with 60 mg was absorbed. Over 5 years, 500 mg a day raised blood vitamin C by 38.5%. INSYGMA products provide 1,000 mg of vitamin C a day with 2 capsules.