Zinc supplements may help stave off the symptoms of respiratory tract infections such as coughing, congestion and sore throat, and cut illness duration, according to a review of studies.
However, the study published in the journal BMJ Open cautions that the quality of the evidence on which these findings are based is variable, and it is not clear what an optimal formulation or dose of this nutrient might be.
Respiratory tract infections include colds, flu, sinusitis, pneumonia, and COVID-19.
The researchers, including those from Western Sydney University in Australia, noted that zinc has a key role in immunity, inflammation, tissue injury, blood pressure, and in tissue responses to lack of oxygen.
It has generated considerable interest during the current pandemic for the possible prevention and treatment of COVID-19 infection, they said.
“The marginal benefits, strain specificity, drug resistance and potential risks of other over-the-counter and prescription medications makes zinc a viable ‘natural’ alternative for the self-management of non-specific (respiratory tract infections),” the authors of the study noted.
“(Zinc) also provides clinicians with a management option for patients who are desperate for faster recovery times and might be seeking an unnecessary antibiotic prescription,” they added.
The review included 28 clinical trials involving 5,446 adults, published in 17 English and Chinese research databases up to August 2020.
None of the trials specifically looked at the use of zinc for the prevention or treatment of COVID-19.
The most common zinc formulations used were lozenges followed by nasal sprays and gels containing either zinc acetate or gluconate salts.
Doses varied substantially, depending on the formulation and whether zinc was used for prevention or treatment.
Pooled analysis of the results of 25 trials showed that compared with placebo, zinc lozenges or nasal spray prevented five respiratory tract infections in 100 people a month.
In three studies, these effects were strongest for curbing the risk of developing more severe symptoms, such as fever and influenza-like illnesses.
On average, symptoms cleared up two days earlier with the use of either a zinc spray or liquid formulation taken under the tongue than when a placebo was used.
During the first week of illness, participants who used sublingual or nasal spray zinc were nearly twice as likely to recover as those who used placebo.
As many as 19 more adults out of 100 were likely to still have symptoms a week later if they didn’t use zinc supplements, the researchers said.
While zinc wasn’t associated with an easing in average daily symptom severity, it was linked with a clinically significant reduction in symptom severity on day 3, they said.
Side effects, including nausea and mouth or nose irritation, were around 40 per cent more likely among those using zinc, but no serious side effects were reported in the 25 trials that monitored them.
However, compared with placebo, sublingual zinc didn’t reduce the risk of developing an infection or cold symptoms after inoculation with human rhinovirus, according to the researchers.
There was also no differences in illness duration between those who used zinc supplements and those who didn’t, they said.
The researchers cautioned that the comparative effectiveness of different zinc formulations and doses was also not clear.
They also noted that the quality, size, and design of the included studies varied considerably.
The team also included researchers National University of Natural Medicine, US, Southern Cross University and University of Sydney in Australia, and McMaster University in Canada.