Allicin release under simulated gastrointestinal conditions from garlic powder tablets employed in clinical trials on serum cholesterol.

The failure of five recent clinical trials to show significant reduction in elevated serum cholesterol by a single brand of allicin-standardized garlic powder tablets is in contrast to many prior positive studies with the same brand. The hypocholesterolemic activity of garlic is mainly due to allicin, a compound that is produced by the acid-sensitive garlic enzyme, alliinase, only after tablet consumption. Therefore, the allicin-releasing ability of ten lots of these tablets--manufactured over the same years that the positive and negative clinical trials were conducted (1989-1997)--was determined under simulated gastrointestinal dissolution conditions, as defined by U.S. Pharmacopeia Method 724A. It was found that the older lots were more resistant to acid-disintegration (2.5 h vs. 1.3 h, P < 0.001) and that they released three times as much allicin (44% vs. 15 % of their potential, P < 0.001) as the newer lots. A second brand of tablets employed in a recent negative trial released no detectable amount of allicin, while a third set of tablets with high allicin release was used in a trial that gave positive effects. Hence, the persons involved in the recent negative clinical trials probably received considerably less allicin than did those in the older positive studies, possibly accounting for much of the discrepancy in the outcomes. In conclusion, clinical trials using garlic powder tablets to assess any effect of garlic that might be related to allicin, as most are, cannot be considered valid for garlic when the trial shows no effect, unless the expected allicin release from the tablets has at least been determined under standardized drug release conditions (USP 724A).