Vitamin C supplements have long been promoted as a way to ward off colds, but two professors say the belief that high doses can stop people from getting sick is not supported by evidence.

Speaking on the ZOE Science & Nutrition Podcast, Professor Daniel Davis described vitamin C's supposed cold-prevention benefits as a '50-year-old myth', explaining that taking large amounts does not stop someone from catching a cold.

However, the evidence is more nuanced than simply dismissing the supplement. Regular vitamin C supplementation may modestly shorten the duration of a cold, although taking it only after symptoms appear has not been shown to provide the same benefit. Research into vitamin D and zinc has produced mixed results.

Vitamin C does not reliably prevent colds

Davis said high doses of vitamin C would not prevent someone from catching a cold. The distinction is supported by research reviewed by the US National Institutes of Health (NIH), which found that taking vitamin C regularly does not significantly reduce cold risk in the general population.

However, the same evidence suggests regular supplementation can modestly shorten the duration of a cold. Adults who take vitamin C consistently may experience colds that are around 8% shorter on average, although the benefit is relatively small.

The findings do not mean taking vitamin C after symptoms appear will have the same effect. Research has generally not found that starting supplementation once a cold has begun significantly reduces its duration or severity.

There may be exceptions among people exposed to extreme physical exercise or very cold environments, for whom regular vitamin C intake has been associated with a lower risk of catching a cold.

Vitamin D may offer limited protection

Professor Tim Spector discussed the evidence surrounding vitamin D, saying research suggests people with higher levels may have a slightly lower risk of respiratory infections.

He described the evidence as weak, however, and suggested that supplementation may be more relevant to people who are deficient in vitamin D.

Research has not established that vitamin D supplements reliably prevent respiratory infections in everyone. A 2025 systematic review and meta-analysis published in The Lancet Diabetes & Endocrinology, which examined 40 studies involving more than 61,000 participants, found no statistically significant reduction in overall respiratory infection risk.

Vitamin D remains important for other reasons, including maintaining healthy bones and muscles. In the UK, the NHS advises adults and children over four to consider taking 10 micrograms daily during autumn and winter because sunlight exposure may be insufficient for the body to produce enough.

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Zinc may help shorten a cold, not stop one

Davis also questioned whether zinc clearly prevents colds. Evidence from clinical trials does not establish that taking the mineral regularly reduces the likelihood of becoming ill.

However, zinc may have a role after symptoms begin. The NIH says zinc lozenges or syrup taken early in a cold may shorten its duration, although findings are mixed and the evidence does not consistently show reduced symptom severity.

A 2024 Cochrane review found that zinc may have little or no effect on preventing colds, but could shorten their duration when used as a treatment. Researchers cautioned that the evidence was generally of low or very low certainty.

This does not mean taking high doses is advisable. Excessive zinc intake can cause nausea and other adverse effects, and prolonged high intake can interfere with copper absorption.

A balanced diet matters more than supplement claims

Davis said maintaining a healthy gut and a balanced diet could support normal immune function. He also highlighted mucus as part of the body's natural defence system, containing molecules that help protect against viruses.

The broader message from the professors is that supplements should not be treated as guaranteed protection against seasonal illnesses.

Vitamin and mineral deficiencies can affect normal bodily functions, and correcting them is important. But taking additional supplements when nutrient levels are already adequate does not necessarily provide extra protection against infection.

People considering supplements should follow recommended doses and seek medical advice if they have an underlying condition, take medication or are unsure whether they have a deficiency. A supplement cannot replace established measures such as vaccination, hand hygiene and avoiding close contact with people who are ill.

Frequently Asked Questions

  • Does vitamin C prevent colds?
    No, high doses of vitamin C do not prevent colds, but regular intake may shorten their duration.
  • Can vitamin D prevent respiratory infections?
    Research suggests vitamin D may offer limited protection, but it is not a reliable preventive measure for everyone.
  • Does zinc prevent colds?
    Zinc does not prevent colds, but it may help shorten their duration if taken early.
  • What is the best way to support immune function?
    Maintaining a balanced diet and healthy lifestyle is more effective than relying solely on supplements.
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