To be published on Wednesday in The Cochrane Library, the review updates an earlier Cochrane Systematic Review from 1999 that found there was no strong evidence that zinc lozenges reduced the duration of the common cold and highlighted numerous side effects, including mouth irritation, unpleasant taste, nausea and diarrhoea.
In this latest review, lead researcher Meenu Singh and colleague Rashmi R. Das of the Postgraduate Institute of Medical Education and Research in Chandigarh, India, pooled and re-analyzed data from 15 trials covering a total of 1,360 participants as if they had taken part in one large trial.
They found that:
- Taking zinc in syrup, lozenge or tablet form within a day of onset of cold symptoms reduced the severity and duration of illness.
- At seven days after onset, a higher proportion of participants who took zinc had cleared their symptoms compared to those who took placebo.
- Children who took zinc in syrup or lozenge form for five months or more contracted fewer colds and had less time off school.
The suggestion that zinc might be effective for the common cold came from a 1984 study that indicated zinc lozenges could shorten the duration of illness.
Since then, trials to test the effect of zinc on the common cold have shown conflicting results and while various plausible explanations for how it might work have been put forward, for instance that it suppressess inflammation and directly stops the virus binding and reproducing in the tissues lining the airways, none has been confirmed.
Singh told the press that this latest review strengthens the evidence for zinc as an effective treatment for the common cold, but cautioned that it was too early to make any recommendations because we don't know enough to say what the optimum dose, formulation or length of treatment should be.
Singh also said more research was now needed to look at the benefits of zinc on treating the common cold in particular groups.
"Our review only looked at zinc supplementation in healthy people," she said.
"But it would be interesting to find out whether zinc supplementation could help asthmatics, whose asthma symptoms tend to get worse when they catch a cold," she suggested.
The authors also called for further studies based in low-income countries where zinc deficiency is common.
Zinc is an essential mineral that is present in some foods, including oysters, which contain more zinc per serving than any other food.
Most Americans get their zinc from red meat and poultry. Other good sources include beans, certain seafoods like lobster and crab, fortified cereals, dairy food, whole grains and nuts.
Plant-based zinc, such as from legumes and grains, is more difficult for the body to absorb because these foods also contain phytates that bind to the mineral and reduce its bioavailability compared to animal-based foods.
Zinc is essential for normal growth and development during pre-birth, childhood and adolescence, and for our sense of smell and taste.
It plays a key role in cell metabolism. For instance it helps synthesize DNA, proteins and enzymes, and it is also important for the immune system, wound healing, and cell division.
We need to consume zinc every day because the body has no specialized storage mechanism for zinc.
The common cold accounts for about 40% of sickness absence from work and millions of school days missed every year.
-- The Cochrane Library
-- Info on dietary zinc from the Office of Dietary Supplements (NIH, US)
Additional source: Wiley-Blackwell.