"Cold-eeze — Clinically proven to cut colds by nearly one half." So reads an advertisement on the Internet for zinc lozenges. The claim is said to be supported by a randomized, controlled, double-blind study published in 1996.
A recent analysis from Stanford medical center, however, which looked at 105 reports published over the past 40 years on the effects of zinc on the common cold, casts doubt on the Cold-eeze claim.
Only 14 of the 105 studies were of the best design — a randomized, controlled trial like the Cold-eeze study mentioned above. Seven of these 14 concluded that zinc had a beneficial effect, whereas the other seven found no such effect.
The Stanford researchers, however, found that 10 of these 14 randomized trials had enough major flaws in their design to possibly invalidate their results. And only one of the four well-designed trials reported a beneficial effect for zinc — and that trial used a zinc nasal gel rather than a zinc lozenge.
The authors of this systematic review concluded that zinc lozenges are not useful for either the prevention or treatment of the common cold. They left open the possibility that a zinc nasal gel may be beneficial.
This analysis from Stanford points out the need to be wary about the utility of diets, drugs, or other products that are touted as having been proven affective by randomized, controlled trials. Many such trials turn out to be so poorly designed that the results can't be trusted. And, unfortunately, population studies and small, uncontrolled trials often are even less reliable.
So caveat emptor! I was asked by a lay publication just this week to review a write-up of a research paper that purported to prove that elderly people could benefit from frequent snacking. When I read the original paper, I found that the snacking habits it advocated would lead to a weight gain of 26 pounds over the period of one year! Though published in a reputable medical journal, this report had serious flaws.
And if you still believe the promotion for zinc lozenges cited above, keep in mind that this type of lozenge can cause sore mouth, upset stomach, and temporary loss of smell.
Vitamin C
Though the benefits of vitamin C supplements have been controversial for about 60 years, this vitamin is still widely used for both prevention and treatment of the common cold.
A systematic analysis completed in 2007 examined a large number of published papers describing the effects of vitamin C on the prevention of colds, as well as on the severity and duration of cold symptoms.
The reviewers concluded that vitamin C supplementation did not reduce the incidence of colds in the normal population. The evidence did show, however, that vitamin C could be protective against colds in a person exposed to brief periods of severe physical exercise and/or cold weather.
No benefit was evident from taking large amounts of vitamin C after the onset of a cold. In adults and children who developed a cold, however, despite taking large doses of vitamin C for prevention, the severity and duration of symptoms were both reduced by about 10 percent.
