Taking zinc may cut the time adults suffer with a common cold, but consuming it will likely come with unpleasant side effects such as nausea, according to a review of past studies.
The researchers, whose findings appeared in the Canadian Medical Association Journal, found that the benefits of zinc also appear to be modest and don't extend to children. But they could add up given that there are about 62 million cases of the common cold in the United States every year.
Michelle Science, from the Hospital for Sick Children in Toronto, and her colleagues compiled data from 17 clinical trials covering 2,121 people, comparing those who took zinc orally to another group that either took a placebo or received no treatment.
“The results of our meta-analysis showed that oral zinc formulations may shorten the duration of symptoms of the common cold,” Science and her colleagues wrote.
“However, large high-quality trials are needed before definitive recommendations for clinical practice can be made.”
Overall, the researchers noted that in the trials, which included people from one to 65-years-old, those taking zinc had colds that were shortened by a little more than two and a half days compared to those taking the placebo.
Little difference was seen in cold duration in children, though - perhaps, said the authors, because adults tended to use a different form of zinc than children.
Peoples' cold symptoms also seemed to clear up faster if they took a higher dosage of zinc compared to those who took the least. The various studies used different dosages, said Science.
While adults who didn't take zinc tended to have colds lasting a week or more, there was no difference in the severity of cold symptoms on day three in any of the groups.
But side effects were more common in people taking zinc. They were 64 percent more likely to experience nausea and 65 percent more likely to detect an unpleasant taste in their mouths.
The authors could not say why zinc stops the rhinovirus, a frequent cause of the common cold, from reproducing, but some believe it acts as an astringent on important facial nerves where viruses tend to congregate.
Meenu Singh, a paediatrician at the Post Graduate Institute of Medical Education and Research in Chandigarh, India, told Reuters Health that people shouldn't be afraid to take zinc, but should consult their doctors first.
Singh was not involved in the current review but led another review that drew similar conclusions last year and was published by the Cochrane Collaboration, an international organisation that evaluates medical research.
“The findings are more or less the same. The side effects are the same,” Singh said.
Science and her colleagues warned that their results were based on studies that varied from one another in their methods and quality. Also, they did not look at zinc nasal sprays that have been linked to the loss of smell or zinc in collaboration with other vitamins.
“Until further evidence becomes available, there is only a weak rationale for physicians to recommend zinc for the treatment of the common cold,” they concluded.
“Otherwise, for healthy adults ... it's probably an individual decision and it's something they can talk to their physician about.” - Reuters