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Evidence summaries

Garlic and Cardiovascular Risk Factors

Garlic may reduce blood pressure and have small short-term benefits on lipid-lowering and antiplatelet factors compared with placebo, but there is insufficient evidence on mortality and cardiovascular morbidity. Level of evidence: "C"

A Cochrane review[Abstract] 1 included 2 studies with a total of 87 subjects. One study included 47 hypertensive patients and showed that garlic (200 mg garlic powder 3 times daily) significantly reduces mean supine systolic blood pressure by 12 mmHg (95% CI 0.56 to 23.44 mmHg) and mean supine diastolic blood pressure by 9 mmHg (95% CI 2.49 to 15.51 mmHg) versus placebo. The second trial (n=40) could not be meta-analysed as they did not report the number of people randomized to each treatment group. They did report that 200 mg of garlic powder given 3 times daily, in addition to hydrochlorothiazide-triamterene baseline therapy, produced a mean reduction of systolic blood pressure by 10-11 mmHg and of diastolic blood pressure by 6-8 mmHg versus placebo. Neither trial reported clinical outcomes and insufficient data were provided on adverse events.

A systematic review 2 including 45 RCTs with a total of 2 987 subjects was abstracted in DARE. All garlic preparations combined, compared with placebo: the total cholesterol was reduced by 0.19 mmol/L (95% CI: 0.03 to 0.34) after 4 to 6 weeks of therapy (14 studies), by 0.44 mmol/L (95% CI: 0.32 to 0.57) after 8 to 12 weeks (24 studies), and by 0.03 mmol/L (95% CI: -0.21, +0.28) after 20 to 24 weeks (6 studies). The authors concluded that garlic has potentially small short-term benefits on lipid-lowering and antiplatelet factors, whereas it has insignificant effects on blood-pressure and no effect on glucose levels. The conclusions regarding clinical significance were limited by the marginal quality and short duration of many trials, and by the unpredictable release and inadequate definition of the active constituents in the preparations studied.

A technology assessment report 3 abstracted in the Health Technology Assessment Database 4 confirms that garlic preparations may have small, positive, short-term effects on lipids; whether effects are sustainable beyond 3 months is unclear. There are insufficient data to draw conclusions regarding garlic's effects on clinical cardiovascular outcomes such as claudication and myocardial infarction. Consistent reductions in blood pressure with garlic were not found, and no effects on glucose or insulin sensitivity were found. Some promising effects on antithrombotic activity were reported, but few data are available for definitive conclusion.

Multipleadverse effects, including breath and body odour, dermatitis, bleeding, abdominal symptoms, and flatulence have been reported but the causality of the adverse effects was not clear, except for breath and body odour. Whether adverse effects occur more commonly with certain preparations than others was not established.

Comment:The quality of evidence is downgraded by study limitations (unclear allocation concealment) and by imprecise results (few patients).

    References

    • Stabler SN, Tejani AM, Huynh F et al. Garlic for the prevention of cardiovascular morbidity and mortality in hypertensive patients. Cochrane Database Syst Rev 2012;(8):CD007653.[PubMed]
    • Ackermann RT, Mulrow CD, Ramirez G, Gardner CD, Morbidoni L, Lawrence VA. Garlic shows promise for improving some cardiovascular risk factors. Arch Intern Med 2001 Mar 26;161(6):813-24. [PubMed] [DARE]
    • Agency for Healthcare Research, Quality (AHRQ). Garlic: effects on cardiovascular risks and disease, protective effects against cancer, and clinical adverse effects. Rockville, MD: Agency for Healthcare Research and Quality (AHRQ). 2000. 140. Agency for Healthcare Research and Quality (AHRQ).
    • Health Technology Assessment Database: HTA-20010948. The Cochrane Library, Issue 1, 2004. Chichester, UK: John Wiley & Sons, Ltd.

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