Reviewed: Ried K, Frank OR, Stocks NP, Fakler P, Sullivan T. Effect of garlic on blood pressure: a systematic review and metaanalysis. BMC Cardiovasc Disord. 2008;8:13. doi:10.1186/1471-2261-8-13.

Hypertension (high blood pressure) affects about 1 billion persons worldwide. Clinically, hypertension is defined as systolic blood pressure (SBP) equal to or greater than (≥) 140 mm Hg (millimeters of mercury) and diastolic blood pressure (DBP) ≥ 90 mm Hg. Recent guidelines extend the management of blood pressure to include prehypertensive persons with SBP of 120-139 mm Hg and DBP of 80-89 mm Hg. In research studies, animal results have suggested that garlic (Allium sativum, Liliaceae) preparations produce moderate reductions in blood pressure while primary studies in humans and nonsystematic reviews have reported mixed results. Because of the increased use of alternative and complementary therapies for hypertension, these authors, from the Discipline of General Practice, The University of Adelaide, Adelaide, South Australia, conducted a systematic review and meta-analysis of trials investigating the effect of garlic preparations on blood pressure.

The authors searched the Medline, Embase, and Cochrane databases for studies published between 1955 and October 2007, and they checked reference lists of previously published systematic reviews and meta-analyses for more primary studies. For the systematic review, they included published intervention studies (including randomized controlled trials and non-placebocontrolled trials) in English and German that reported effects of garlic preparations on blood pressure. For the meta-analyses, they included only studies with placebo control groups, using garlic-only supplements, and reporting mean SBP and/or DBP and standard deviation (SD).

The number of subjects in intervention and control groups, mean SBP and DBP at start and end of intervention, and SD were collated from text, tables, or figures. Methodological quality was assessed independently by two of the investigators using guidelines of the Cochrane Collaboration.

The authors also conducted a subgroup meta-analysis by baseline blood pressure (hypertensive/normotensive) for the first time and a meta-regression analysis to test the associations between blood pressure outcomes and duration of treatment, dosage, and blood pressure at the start of treatment.

Eleven of 25 studies included in the systematic review and investigating the effect of garlic preparations on blood pressure met the inclusion criteria for meta-analysis. Fourteen studies were excluded from meta-analysis: 6 trials had no placebo control group, another 6 reported incomplete data for mean SBP, DBP, or SD, and 2 studies used garlic combination supplements containing other potentially hypotensive agents.

Nine studies compared garlic preparations to placebo, and 2 studies compared the effect of garlic on blood pressure in addition to a drug compared to drug plus placebo. Nine studies used garlic powder (the commercial standardized garlic product Kwai^®, Lichtwer Pharma AG, Berlin, Germany), one study used aged garlic extract (Kyolic®, Wakunaga, Japan), and another used distilled garlic oil. Dosage of garlic powder ranged between 600 and 900 mg per day (providing potentially 3.6 to 5.4 mg of allicin, an active compound produced in fresh garlic but not in the aged garlic extract), and duration of intervention ranged from 12 to 23 weeks. A total of 252 subjects allocated to a garlic intervention group and 251 subjects allocated to a control group were included in the meta-analysis on SBP from 10 studies, and 283 (garlic) versus 282 (control) on DBP. Mean blood pressure at start of intervention varied markedly, with 4 studies reporting mean SBP in the hypertensive range (≥140 mm Hg) and 3 studies reporting mean DBP in the hypertensive range (≥90 mm Hg) before treatment.

The authors report that their meta-analysis suggests that garlic supplements exert a hypotensive effect compared with placebo, particularly in subjects with high blood pressure. Meta-analysis of 10 studies of the effect of garlic on SBP showed a significant difference between garlic and control groups, with garlic having a greater effect in reducing SBP than placebo by 4.56 (95% CI, -7.36, -1.77) mm Hg compared with placebo (P<0.001). Subgroup analysis of studies with mean SBP in the hypertensive range at start of intervention revealed a greater SBP reduction in the garlic group than in the placebo group by 8.38 (95% CI, -11.13, -5.62) mm Hg (P<0.001). Subgroup analysis of the remaining studies with mean SBP in the normotensive range (<140 mm Hg) at start of intervention showed no significant difference between the garlic and placebo groups.

The meta-analysis of the effect of garlic on DBP did not show a significant difference between garlic and placebo groups (-2.44 [95% CI, -4.97, 0.09] mm Hg, P=0.06). However, subgroup analysis of studies with mean DBP in the hypertensive range at the start of treatment revealed a significant difference between garlic and control groups. The results indicate that garlic was more effective in reducing DBP than placebo in hypertensive subjects by 7.27 (95% CI, -8.77, -5.76) mm Hg (P<0.001). In contrast, subgroup meta-analysis of normotensive subjects was not significant.

Regression analysis revealed a significant association between blood pressure at the start of the intervention and the level of blood pressure reduction (SBP: R=0.057; P=0.03; DBP: R=0.351; P=0.02).

The authors report that their findings on the effects of garlic preparations on SBP/DBP are comparable to the hypotensive effects of commonly prescribed blood pressure drugs.

“This systematic review and meta-analysis suggest that garlic preparations are superior to placebo in reducing blood pressure in individuals with hypertension. Future large-scale long-term trials are needed to investigate whether standardized garlic preparations could provide a safe alternative or complementary treatment option for hypertension in clinical practice,” say the authors.

—Shari Henson