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)
[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.
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.
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:
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: