Zinc could knock 2.25 days off a common cold

The common cold is one of the most frequent illnesses going, and it carries a genuine economic and social burden worldwide through lost work and general misery. Vitamin C has been investigated extensively as a way of preventing colds in healthy adults, but the evidence for other micronutrients has received far less attention. That leaves an obvious and practical question: do supplements such as zinc, vitamin D, vitamin A or vitamin E, taken singly by mouth, actually prevent colds, shorten them or make them milder? This review set out to pull together the trial evidence and answer it.

The authors searched four electronic databases and screened 660 identified studies, ultimately extracting data from 20 randomised controlled trials that met their criteria. Ten of those trials tested zinc, eight tested vitamin D and two covered vitamins A and E. Each trial provided a micronutrient singly through oral means to healthy adults, and the outcomes of interest were cold incidence, cold duration and symptom severity. The team also formally assessed the quality of the included trials and graded how much certainty the pooled outcomes deserved.

The headline result belongs to zinc: when it was provided on its own, cold duration was an estimated 2.25 days shorter, with the plausible range running from 1.12 to 3.39 days. The rest of the picture was largely null. Micronutrient supplementation, with the exception of vitamin C, did not appear to prevent colds from happening in the first place, and it did not appear to reduce how severe symptoms felt. In other words, zinc's benefit showed up in how long a cold lasted, not in whether people caught one or how rough it felt while it ran its course.

This review extends a cold-prevention literature long dominated by vitamin C, which has been studied extensively in healthy adults while other micronutrients were largely overlooked. It adds weight to the idea that zinc's value lies in shortening colds rather than stopping them, and it pours cold water on vitamin D, vitamin A and vitamin E as cold remedies for healthy people. The estimate for zinc comes with a wide plausible range, so the true size of the effect is still uncertain. Questions about the best form, dose and timing of zinc were not settled here either, and sharper trials would tighten the answer.

If you are a healthy adult, this is a reason to think zinc taken on its own might trim a couple of days off a cold, rather than a reason to expect it to stop one arriving. It is equally a reason for scepticism about vitamin D, vitamin A or vitamin E doing anything noticeable for colds. The evidence covers healthy adults taking a single micronutrient by mouth, so it says nothing about combinations or about people with other conditions. And as with all pooled evidence, the 2.25-day figure is an average estimate, not a promise for any individual cold.