Effect of garlic on blood pressure: a systematic review and meta-analysis

Abstract

Background: Non-pharmacological treatment options for hypertension have the potential to reduce the risk of cardiovascular disease at a population level. Animal studies have suggested that garlic reduces blood pressure, but primary studies in humans and non-systematic reviews have reported mixed results. With interest in complementary medicine for hypertension increasing, it is timely to update a systematic review and meta-analysis from 1994 of studies investigating the effect of garlic preparations on blood pressure.

Methods: We searched the Medline and Embase databases for studies published between 1955 and October 2007. Randomised controlled trials with true placebo groups, using garlic-only preparations, and reporting mean systolic and/or diastolic blood pressure (SBP/DBP) and standard deviations were included in the meta-analysis. We also conducted subgroup meta-analysis by baseline blood pressure (hypertensive/normotensive), for the first time. Meta-regression analysis was performed to test the associations between blood pressure outcomes and duration of treatment, dosage, and blood pressure at start of treatment.

Results: Eleven of 25 studies included in the systematic review were suitable for meta-analysis. Meta-analysis of all studies showed a mean decrease of 4.6 +/- 2.8 mm Hg for SBP in the garlic group compared to placebo (n = 10; p = 0.001), while the mean decrease in the hypertensive subgroup was 8.4 +/- 2.8 mm Hg for SBP (n = 4; p < 0.001), and 7.3 +/- 1.5 mm Hg for DBP (n = 3; p < 0.001). 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 = -0.315; p = 0.02).

Conclusion: Our meta-analysis suggests that garlic preparations are superior to placebo in reducing blood pressure in individuals with hypertension.

Figure 1

Flow diagram of study selection for systematic review and meta-analysis. Abbreviations: BP, blood pressure; SBP, systolic blood pressure; DBP, diastolic blood pressure; SD, standard deviation; RCT, randomised controlled trial

Figure 2

Meta-analysis graphs on the effect of garlic on systolic blood pressure (A) or diastolic blood pressure (B). Abbreviations: N, number of participants; SD, standard deviation; WMD, weighted mean difference; CI, confidence interval; SBP, systolic blood pressure; DBP, diastolic blood pressure; s1, study 1 [ref 11].

Figure 3

Subgroup meta-analysis on the effect of garlic on systolic blood pressure of hypertensive subjects (≥140 mm Hg at start of intervention) (A) or 'normotensive' subjects (<140 mm Hg at start of intervention) (B); on diastolic blood pressure of hypertensive subjects (≥90 mm Hg) (C) or normotensive subjects (<90 mm Hg) (D). For abbreviations see Fig 2.

Figure 4

Funnel plots of studies included in meta-analysis on the effect of garlic on systolic blood pressure (A) and diastolic blood pressure (B). The vertical line of Begg's funnel plot represents the pooled mean effect size, the dotted lines the 95% confidence interval, p-values are derived from Egger's test. Abbreviations: SB, systolic blood pressure; DBP, diastolic blood pressure; SE, standard error; mm Hg, millimetre mercury.

Figure 5

Mean BP against time for garlic-only intervention arm(s) of studies using subjects with SBP≥130 mm Hg at start of intervention (A), SBP < 130 mm Hg (B), DBP≥85 mm Hg (C), DBP < 85 mm Hg (D). The plot incorporates garlic-only intervention arms of studies included in the systematic review: study arms in meta-analysis are in red/orange and marked with *, others are in black/grey. Diamonds illustrate trials using garlic powder and circles illustrate other garlic preparations. Studies in legend boxes are sorted by baseline blood pressure. Abbreviations: K88_2 = Kandziora 1988 (Study 2) [25], garlic vs drug; K88_1*= Kandziora 1988 (Study 1) [11], garlic+drug vs placebo+drug; A90* = Auer et al. 1990 [12]; DJ95 = De A Santos & Johns 1995 [28]; D04hyp = Durak et al. 2004 [32], hypertensive study arm; DJ04hyp = Dhawan & Jain 2004 [37], hypertensive study arm; VS90* = Vorberg & Schneider 1990 [13]; H92*= Holzgartner et al. 1992 [15]; DG93 = De A Santos & Grünwald 1993 [27]; J05swall = Jabbari et al. 2005 [35], swallowing garlic study arm; J05chew = Jabbari et al. 2005 [35], chewing garlic study arm; H88 = Harenberg et al. 1988 [24]; S96para = Steiner et al. 1996 [19], parallel study arm ; DJ04norm = Dhawan & Jain 2004 [34], normotensive study arm; S96cross = Steiner et al. 1996 [19], crossover study arm; J93 = Jain et al. 1993 [16]; S95 = Simons et al.1995 [18]; S94 = Saradeth et al. 1994 [17]; D04norm = Durak et al. 2004 [32], normotensive study arm ; Z00 = Zhang et al. 2000 [21]; Ki91 = Kiesewetter 1991 [26]; T04 = Turner et al. 2004 [33], median BP.