Garlic has received a lot of attention in the last several years, much of it focused on its effect on serum cholesterol. Scores of scientists have explored this issue, but an article in the most recent issue of the Annals of Internal Medicine suggests that faulty study designs may have led some of them to overestimate its ability to lower blood cholesterol levels.

Scientists from the University of Exeter in England pooled the results of a group of studies that they felt were particularly well done into one body of evidence. They were highly selective in the type of study included in their analysis. They included only those that were randomized and placebo-controlled (equal numbers of participants are randomly assigned to either the treatment group or the placebo, or inactive treatment group); and double-blind (neither the participants nor researchers know who in the study received which kind of treatment). This type of study is considered the research "gold standard," and the English scientists hoped that, by including only very well-designed studies in their analysis, they would get a fairly realistic assessment of how garlic fits into cholesterol control.

Careful analysis of 13 such studies showed that garlic does, indeed, have a beneficial effect on serum cholesterol levels, but its impact may be less impressive than previously reported. The researchers estimated that in these studies, garlic supplements reduced serum cholesterol levels by an average 0.41 mmol/L (15.7 mg/dL in the measurement commonly used in the US). The scientists emphasize that these are averaged results, since the studies included in this analysis tested varying forms and doses of garlic extracts.

The authors of this analysis say that, while garlic does appear to have some ability to lower cholesterol, they question whether the effect is great enough for it to be considered a practical treatment component. They do point out, though, that the majority of the studies they included were fairly short in duration; the longest followed participants for ten months. It is possible that a longer period of time is needed to fully appreciate the cholesterol-lowering potential of garlic, and that a well-designed, long term study of the effect of garlic on serum cholesterol levels could help clarify the issue.