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Zinc not for colds: heterogeneity, subgroup, toxicology and more

Carl Heneghan
Last edited 21st February 2011

There's no cure for the common cold, but according to a Cochrane Reviews, using zinc supplements may shorten the illness by a few days.

I can see all the hypochondriacs out there, start throwing away their vitamin C, the vitamin for staving off the cold, thanks to Linus Pauling, who said he’d never had a cold in his life. Well, you’ve been wasting your time, because now there is zinc. Wonderful, but is it any good.

Should we just take as fact that a Cochrane review is gospel and get on with sucking the pastels? Just because a reputable journal publishes a paper it doesn’t mean the findings will stand up to scrutiny. Reading the news won't help because there isn’t one critical analysis out there.

What the Cochrane review of Zinc for the common cold found from 13 trials was that the intake of zinc is associated with a significant reduction in the duration and severity of common cold symptoms. What concerns me about the result is the amount of heterogeneity for the main outcome, I squared of 93%.

At this amount of heterogeneity, which is the studies' results differ so much, it is implausible to combine them. Generally anymore than 50% heterogeneity you shouldn’t combine outcomes unless you have a good reason why it makes sense. They don't. The studies differed in terms of variable formulations (zinc gluconate or acetate lozenges, zinc sulphate syrup) and dose range (30 to 160 mg/day) as well as mean duration of symptoms prior to administration of zinc (varying from 24 to 48 hours).

The worst of it is, in the authors' limitations they state ‘the results from all the trials may not be comparable’. Couldn't agree more.

This review also has the added problem of multiple comparisons. Reporting day 3 and 5 results are not significant, and hey presto at day 7 there was a significant difference between the zinc and control group at a p value of 0.05. Doesn’t wash. For those of you who have no idea what I’m on about visit the bonferonni correction.

Oh and by the way, eleven of the studies were funded by pharmaceutical companies. And I didn’t even get the chance to say most trials relied on community-acquired infections in which the infecting agent was not identified. This is what the authors say ‘so it is unclear whether zinc helps those with rhinoviral cold or even cold due to other viruses.’ And one last moan, ‘given its toxicological profile, the potential for zinc to induce adverse effects at the doses participants are required to take also needs to be determined’.

Want my advice, keep taking the vitamin C, it tastes better.

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