Zinc supplements might prevent symptoms of viral respiratory infections and reduce the length of illness, a pooled analysis of randomised controlled trials shows.

After searching 17 English and Chinese databases, researchers found 28 randomised controlled trials that looked at zinc to prevent or treat acute viral respiratory tract infections (RTIs) in adults.

They had initially hoped to find trials specific to SARS-CoV-2, but results were pending on all relevant studies and could not be included in the analysis.

When oral or intranasal zinc was used for prophylaxis, they found that on average five RTIs per 100 person-months were prevented compared with placebo.

'The prophylactic effects were greatest for reducing the [relative risk] of developing more severe symptoms, such as fever and influenza-like illness. However, only four studies were identified, and none used laboratory tests to confirm a viral infection,' the researchers reported in BMJ Open.

When used for treatment, symptoms resolved on average two days earlier with sublingual or intranasal zinc compared with placebo and 19 more adults per 100 were likely to remain symptomatic on day seven without zinc.

There were clinically significant reductions in day three symptom severity scores but not average daily symptom severity scores.

'In contrast to these promising findings, following human rhinovirus inoculations, compared with placebo, sublingual zinc did not reduce the risk of developing an infection or symptoms of a clinical cold, nor were there any significant effects on symptom severity or duration of illness when zinc was compared with an active control,' the study authors wrote.

There was an increased risk of non-serious adverse events with zinc that researchers cautioned may limit tolerability for some people, but the risk of serious adverse events was low.

The most common zinc formulations were lozenges followed by nasal sprays and gels containing either zinc acetate or gluconate salts.

The quality of the evidence included in the analysis was variable, the study authors cautioned, adding they were unable to answer questions about the comparative efficacy, effectiveness and acceptability of different zinc formulations and doses, and their mechanisms of action.

'The marginal benefits, strain specificity, drug resistance and potential risks of over-the-counter and prescription medications makes zinc a viable ‘natural’ alternative for the self-management of non-specific RTIs,' the researchers wrote.

'It also provides clinicians with a management option for patients who are desperate for faster recovery times and might be seeking an unnecessary antibiotic prescription.'

However, clinicians and consumers needed to be aware that considerable uncertainty remained regarding the clinical efficacy of different zinc formulations, doses and administration routes, and the extent to which efficacy might be influenced by the ever-changing epidemiology of the viruses that cause RTIs.

Further research was warranted to clarify the efficacy and mechanism of zinc in viral respiratory infections, including SARS-CoV-2 infections, they concluded.

It comes after a study found a high dietary intake of alpha linolenic acid (ALA), a type of omega 3 fatty acid, is associated with a lower risk of death from all causes and from cardiovascular disease and coronary heart disease.