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Studies question garlic effectiveness

Two recently published clinical studies produced negative results on the effectiveness of freeze-dried odorless garlic (Allium sativum L., Liliaceae) in lowering blood fat and cholesterol levels. The first was a multicenter, randomized, placebo-controlled trial that tested the effectiveness of garlic powder tablets (Kwai®) in lowering cholesterol levels in 50 patients with elevated cholesterol [Isaacsohn et al., 1998]. As with earlier trials, the dose used was 900 mg per day of garlic powder standardized to yield 5.4 mg of allicin, approximately equivalent to one clove of fresh garlic per day. The 12-week study reported no significant changes in blood lipids or lipid proteins for either placebo- or garlic-treated groups, and no significant difference between the two groups. In addition, the study found that the garlic tablets had no statistically significant effects on blood pressure. Two previous trials using a similar study design and the same garlic preparation [Neil et al., 1996, and Simons et al., 1995] also reported no significant effects on cholesterol levels. However, these results are in marked contrast to those of previous studies, including a 1993 meta-analysis that showed an approximate 12 percent reduction of cholesterol levels among subjects taking various forms of garlic [Warshafsky et al., 1993].

All of the negative studies just discussed included a modified low cholesterol diet and evaluated dietary compliance throughout the trial period. When the results of clinical studies are expected to be modest, according to the authors, "dietary stabilization becomes a crucial factor in establishing the true lipid lowering capacity of the treatment under investigation." They also suggested that another reason for the difference between these negative results and earlier positive results could be "publication bias," implying that investigators and sponsors are reluctant to report negative outcomes. Another possibility, not addressed by the reviewers, is that garlic may be more effective for those who do not modify their diet, or who modify it less severely than did the subjects in this study.

Another new study, reported in the Journal of the American Medical Association (JAMA), tested the effects of a steam-distilled garlic oil preparation in 25 patients with moderate hypercholesterolemia [Berthold et al., 1998]. Patients took either placebo or 5 mg steam-distilled garlic oil twice a day. No special low cholesterol diet was used, but food intake was assessed at the end of the two treatment periods. The 12-week study found no significant differences between placebo and garlic groups, and "virtually no effect of garlic" on parameters of cholesterol metabolism. The authors of this study erroneously concluded, "Based on the results of the present study, however, there is no evidence to recommend garlic therapy for lowering serum lipid levels."

This study can be criticized for its small sample size of 25 patients, the type of garlic preparation used, and the low dosage used in the study. Because of this and other problems with the study, as detailed in the accompanying letter by Dr. Lawson (see sidebar), the JAMA study fails to make a credible case against the use of garlic therapy for lowering serum lipid levels. - Rob McCaleb (HRF)

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[Berthold HK, Sudhop T, von Bergmann K. Effect of a garlic oil preparation on serum lipoproteins and cholesterol metabolism: a randomized controlled trial. JAMA 1998; 279(23): 1900-1902.

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Isaacsohn JL, Moser M, Stein EA, Dudley K, Davey JA, Liskov E, Black HR. Garlic powder and plasma lipids and lipoproteins: a multicenter, randomized, placebo-controlled trial. Arch Intern Med 1998; 158: 1189-1194.

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Neil LA, Silagy CA, Lancaster T, et al. Garlic powder in the treatment of moderate hyperlipidemia: a controlled trial and meta-analysis. JR College Physicians Lond 1996; 30: 329-334.

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Simons LA, Balasubramaniam S, von Konigsmark M, Parlitt A, Simons J, Peters W. On the effect of garlic on plasma lipids and lipoproteins in mild hypercholesterolemia. Atherosclerosis 1995; 113: 219-225.]

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