Vitamin C for Frequent Infections

Expect roughly 8% shorter colds in adults on daily 200 mg+, and up to 50% fewer colds in marathon runners, soldiers and skiers.

Overview

Expect roughly 8% shorter colds in adults on daily 200 mg+, and up to 50% fewer colds in marathon runners, soldiers and skiers.

Verdict

Mixed evidence

Regular vitamin C does not stop you catching colds, but it shortens them modestly — and it clearly helps people under heavy physical stress.

How It Works

Vitamin C concentrates in neutrophils and lymphocytes at 50-100 times plasma levels, supporting chemotaxis, phagocytosis and oxidative-burst microbial killing while protecting the cells from their own oxidants. Levels fall sharply during infection, which is why requirements rise when you are unwell.

Dosing & Protocol

Typical dose

Recommended dose: 200-500 mg daily preventively; 1-2 g daily in divided doses during an infection.
Expected timeframe: Preventive effect requires continuous use before exposure; duration benefit is seen within the illness episode.

Protocol

dose: 200-500 mg daily; 1-2 g split during illness
form: Ascorbic acid, or buffered sodium/magnesium ascorbate if acidic forms upset your stomach
note: Absorption efficiency falls sharply above 400 mg per single dose
timing: With food; split doses above 500 mg
duration: Ongoing

Evidence

What the studies say

The Cochrane review of over 11,000 participants found regular supplementation did not reduce cold incidence in the general population but reduced duration by about 8% in adults and 14% in children. In five trials of people under extreme physical stress, incidence halved. Starting vitamin C after symptoms begin shows inconsistent benefit, although higher therapeutic doses at onset may help.

Vitamin C for preventing and treating the common cold

Score: 9/10

2013

systematic_review

n=11306

Hemila H, Chalker E

Regular vitamin C did not reduce cold incidence in the general population but shortened duration modestly

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Vitamin C supplementation reduces the odds of developing a common cold in Republic of Korea Army recruits: randomised controlled trial

Score: 7/10

2022

rct

n=1444

Kim TK, Lim HR, Byun JS

The vitamin C group had a 0.80-fold lower risk of getting a common cold than did the placebo group.

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Vitamin C and immune function

Score: 8/10

2017

systematic_review

Carr AC, Maggini S

Vitamin C contributes to immune defense by supporting various cellular functions of both the innate and adaptive immune system.

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Safety

Caveats

Above 2 g/day expect osmotic diarrhoea. High doses increase oxalate excretion and may raise kidney-stone risk in susceptible men. Vitamin C interferes with some glucometer readings.

Less likely to help if

People with recurrent bacterial infections, and anyone with unexplained frequent infection who has not been assessed for immunodeficiency, diabetes or iron deficiency.

Medical Disclaimer

The information provided on this website is for educational and informational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or before starting any supplement regimen.

Individual results may vary. The statements on this website have not been evaluated by the Food and Drug Administration. Products and information are not intended to diagnose, treat, cure, or prevent any disease.