Maybe. Maybe not. Some studies find that zinc supplements may help prevent or treat colds, but many others find no benefit. On balance, the case for boosting zinc to fight colds is weak, unless you are zinc deficient.
Zinc is a metal that we all need in tiny amounts for optimal health. Zinc helps your immune system fight off invading bacteria and viruses, and is also involved in growth, development, wound healing, sensing taste, and more. In lab experiments, high doses of zinc have been shown to have antiviral properties.
Image Source: European Union, Food Facts for Healthy Choices
Is more zinc better when it comes to colds? In people who are lacking in zinc, the answer is a clear: yes! However, in people who are *not* deficient in zinc, the answer is unclear.
To understand why the answer isn’t black and white, let’s dive into a recent meta-analysis (a study that synthesizes results across many studies) based on 26 randomized controlled trials (RCTs) of zinc supplementation. The studies varied in a number of ways, including the outcome (prevention vs treatment), formulation (lozenge vs spray), dose, and study population. Six of the studies were “inoculation studies” in which healthy people signed up to be infected with a cold virus (human rhinovirus) and then received either zinc or a placebo for prevention or treatment. The other twenty studies involved “community-acquired” illnesses, in which people picked up a respiratory virus in the real world, rather than in the lab. This set of studies was more of a mixed bag because it included people with colds as well as other respiratory infections, and in many cases, there was no viral lab test.
Nerd Note: A meta-analysis typically provides both an answer and a confidence level for every question they ask. In other words, a study may conclude “yes, zinc prevents colds – and we are really confident in this answer” or “we think that zinc may prevent colds – but we are not very sure of this answer”. These confidence levels are based on the quality of the data (i.e. sample size, risk that the results are biased, and consistency across trials) and are just as important as the overall “yes or no” answer!
Here are the key takeaways:
➡️ Across studies of lab-based cold infections (“inoculation” studies), there were no clear benefits of zinc supplementation. Zinc supplements did not help prevent or treat colds in people who volunteered to get infected with a cold virus (human rhinovirus). This conclusion was rated as moderate confidence.
➡️ Across studies of “community-acquired” respiratory illnesses (colds and then some), results were mixed. Some studies found benefits related to prevention, severity or duration of symptoms, and others did not. There was a trend towards a shorter duration of symptoms, but the authors rated this conclusion as low-certainty/quality. On average, zinc supplements did not impact symptom severity but there was a hint of lower symptom severity on day three (a finding that was also rated as “low certainty/quality”). The authors concluded (with moderate confidence) that zinc may offer a small benefit in terms of prevention – they estimate that zinc could help prevent one cold by treating 20 people (this metric is called “number needed to treat, or NNT”).
➡️ Mild side effects from zinc were common, especially when taking high doses (above 40 mg). People taking zinc were more likely to report side effects, including nausea, bad taste and mouth/nasal irritation, compared to study subjects that took a placebo pill (i.e. a pill containing no treatment).
➡️ Zinc supplements were found to be generally safe. No serious adverse reactions were reported across 25 trials that monitored them, including roughly 5000 people, although not every study included adequate monitoring or reporting. We can’t rule out rare serious reactions until we have even larger studies with meticulous monitoring.
➡️ It’s unclear whether a certain dose or formulation of zinc is better than others. While it’s possible that variations in dose and formulation explain some of the mixed results, there was no clear “winner” identified.
Are you getting enough daily zinc?
Most people can get enough zinc by consuming a healthy, varied diet. Dietary sources of zinc include shellfish (especially oysters), meats, dairy, beans, nuts, and seeds. Fortified grains (like breads and cereals) are also rich in zinc. Vegetarians may benefit from a bit of extra zinc because zinc absorption is generally lower from plant-based sources than animal sources. Other populations that should be mindful of zinc include people with inflammatory bowel syndrome, people who are pregnant or lactating, people with alcohol use disorder, and older babies who are exclusively breastfed.
The official daily recommendation for zinc intake varies by age and sex, and pregnancy/lactation status, and may vary across countries. For example, the US National Institutes of Health (US NIH) recommends 11 mg daily for adult men and 8 mg for adult women, while the United Kingdom National Health Services (UK NHS) recommends 9.5 mg and 7 mg respectively in these populations.
How much zinc is too much?
As with all vitamins and minerals, more is not always better! The safe daily limit for zinc varies by age, and is lower in kids and babies compared to adults. For adults, the US NIH recommends staying below 40 mg daily for adults while the UK NHS recommends a daily limit of 25 mg in adults. It seems safe to go beyond this level for a week or so, as shown in zinc supplement trials, but it can be very dangerous to really crank up your zinc in the long term. High, sustained doses of zinc can lead to copper deficiency and permanent neurological damage.
The Bottom Line
It remains unclear whether or not zinc supplementation helps prevent or treat colds. If you opt to reach for zinc, don’t expect miracles in terms of prevention or recovery from a cold or other respiratory virus. While zinc supplements are generally safe, you may experience uncomfortable side effects with higher doses. When taking zinc, be mindful of the dose and talk to your clinician about potential interference with your medications.
Personally, this Nerdy girl is skeptical of the cold-fighting power of zinc and suggests doubling down on the fundamentals of immune health: a healthy diet, good sleep, exercise, and stress management.
If you like gory details, your best bets to learn more about zinc are the US National Institutes of Health and the Linus Pauling Institute. For a briefer take, check out the Mayo Clinic or Harvard Health. You may find that each website has a slightly different take on the utility of zinc because the body of evidence is so mixed. An optimist may embrace the positives while a skeptic may remain unconvinced!