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With the recall of the cold remedy Zicam nasal spray for possibly causing some people to lose their sense of smell and the prior failures of vitamin C and echinacea to prove effective in trials, viruses seem to be winning the war on colds.
Adding to the long line of mixed or null results, a new review of the research on garlic pills was suggestive of benefit, but inconclusive. Of course, unlike Zicam victims, friends and family of those who use this remedy might actually wish to smell less, given the well-known major side effect of this pungent vegetable.
Only one garlic study had strong enough data to be included in the review — but that study did find a large effect. It included 146 patients randomly assigned to take garlic pills or a placebo for 12 weeks. The number of days they were sick if they caught a cold decreased from five to less than two — reduced by more than half. There was also a dramatic reduction in the number of colds.
“The one relevant trial that we found did report a significant benefit: of those people taking garlic supplements, only 24 reported coming down with a cold, compared to 65 of the people taking the placebo tablet,” said lead review author Elizabeth Lissiman, a medical student at the University of Western Australia.
“Unfortunately,” she added, “that trial was small and reported an unusually high number of people getting colds within the study period, so it cannot be considered conclusive.” Side effects, however, were mild: the expected bad breath, body odor and in some cases, a skin rash.
So how might garlic work—if it does? “Some laboratory investigations have suggested that some components of garlic have antimicrobial properties,” Lissiman said. “Theoretically, these compounds in garlic could kill the viruses that cause the common cold.”
The review appears in the latest issue of The Cochrane Library, which is a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.
Christopher Gardner, an associate professor of medicine at Stanford University, has studied the use of garlic to lower cholesterol. He is skeptical of the results on colds, saying that the findings from the included study “could be a fluke or an outlier.” He also notes that reviews cannot answer questions about collections of data if they only include one study.
Gardner knows from personal experience, however, how difficult it is to study garlic. His study compared raw garlic to two different types of garlic supplements, finding no effect on cholesterol. Nevertheless, that does not necessarily deem it ineffective.
There are more than 100 different types of garlic and each type contains many different compounds. “It’s incredibly complicated,” Gardner said. “There are 14 sulfur-containing compounds and two non-sulfur compounds.”
To make matters even trickier, one of the compounds believed to be most active — known as allicin — is not present in raw garlic. It literally forms in your mouth as you eat the herb. When you chew garlic, an enzyme known as allinase turns a substance called allinin into allicin. This happens within six seconds.
If you take a garlic pill, however, the acidic environment of the stomach can prevent the necessary conversion from taking place. Many garlic supplements now use an “enteric” coating, so that they do not dissolve until they get to the small intestine, where this conversion is might be able to occur.
“It’s not as simple as just freeze-dry the powder and stick it in a pill,” Gardner said. “There are issues there; you might ruin some of molecules in real garlic. The biochemistry of garlic is really quite complex and it’s not even clear what the active agent might be.”
Lissiman noted that more trials exist which the Cochrane authors were unable to include in the review. Most excluded trials were not randomized or had other basic flaws that reduced their quality.
“Cochrane reviews are first planned carefully without knowledge of the evidence available; in this way, they remain objective,” she said. “It is still helpful to know that there is no conclusive evidence of the efficacy of garlic for the common cold. Garlic supplements are a multimillion- dollar industry and claims of effectiveness of garlic for the common cold are based on poor-quality evidence.”