Reviewed: Ried K, Toben C, Fakler P. Effect of garlic on serum lipids: an updated meta-analysis. Nutr Rev. May 2013;71(5):282-299.

Having high cholesterol or other lipid concentrations is a risk factor for cardiovascular disease (CVD). Statins are a common pharmaceutical intervention to decrease cholesterol or triglyceride (TG) concentrations; however, adverse side effects such as an increased risk of diabetes and problems with cognition and the muscles have been reported with statin use. Thus, alternative therapies are important. Garlic (Allium sativum, Amaryllidaceae) has been shown to decrease cholesterol in previous studies and is used to mitigate various CVD risk factors.1 This meta-analysis focused on impacts of garlic on total cholesterol (TC), low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and TG concentrations.

The authors searched MEDLINE®, Cochrane, and Google Scholar databases using the terms garlic, allium sativum, allicin, cholesterol, hyperlipidemia, and lipid. Included studies were randomized, placebo-controlled clinical trials published from 1955 to December 2011 in either English or German that addressed garlic’s effects on cholesterol. Inclusion criteria also consisted of treatment equal to or greater than two weeks, enrollment of adult participants, and use of garlic alone. Exclusion criteria were missing data, inclusion of pregnant participants or those who were taking cholesterol-lowering drugs, use of a combination treatment, inclusion of children or patients with kidney transplants, low compliance or high loss to follow-up, or inclusion of subjects from another study. Study quality also was measured with emphasis on randomization, blinding, follow-up loss, funding, and compliance. Studies were scored on these criteria to yield a total potential score of five. Studies with scores equal to or less than two or those with more than 20% attrition were excluded from the analysis.

Of the 63 trials found, 39 were included in this review with data on TC, LDL, HDL, and/or TG concentrations. The included trials comprised 2,298 subjects with an average age of 49.5 years. A parallel design was used in 32 trials, and a crossover design was used in five. The trials used garlic powder (600-5,600 mg/day), garlic oil (9-18 mg/day), aged garlic extract (1,000-7,200 mg/day; note: Aged Garlic Extract® or AGE is a trademark of the Japanese company Wakunaga; Osaka, Japan), or raw garlic (4-10 g/day). The authors of the meta-analysis reported that the various dosages and preparations are of limited comparability due to compound variation. Quality assessment of trials showed that most included descriptions of randomization, blinding, and placebo. Only two trials had more than 20% follow-up loss; compliance was high for most of the other trials (n=15). Financial support from industry was reported for 13 trials.

The analysis of trials that reported data for TC concentrations showed that garlic significantly lowered TC as compared to placebo (mean difference = -15.25 mg/dL, 95% confidence intervals [CIs], -20.72, -9.78 mg/dL, P<0.0001). The most common preparation was garlic powder (n=24), followed by AGE (n=5), and raw garlic (n=2). Significant effects with garlic preparations were observed with garlic powder (P<0.0001), AGE (P<0.001), and raw garlic (P<0.0001). A significant effect also was detected with enteric-coated garlic powder tablets (P<0.0003).

Of the total, 26 trials included data for LDL concentrations. A significant decrease of LDL was seen in those taking garlic as compared to the placebo group (mean difference = -6.41 mg/dL, 95% CI, -11.77, -1.05 mg/dL, P=0.02). The strength of this decrease was more pronounced in trials with a longer treatment period (P=0.01), elevated mean TC concentrations at baseline (P=0.0004), and trials without industry support (P=0.04). A significant effect of garlic was reported in 19 trials using garlic powder (P=0.04). The use of enteric-coated garlic powder tablets, different trial arms, low study quality, or the use of subjects taking other medication had no effect on the results.

Thirty trials assessed HDL concentrations and reported a significant effect (mean difference = 1.49 mg/dL, 95% CI, 0.19, 2.69 mg/dL, P=0.02). Of the preparation types, garlic oil provided the most significant impact (P=0.007, n=6). There were no significant effects of garlic on TG concentrations (n=32). Of those in treatment groups, 7% reported instances of adverse side effects, but the number of these patients was consistent with those in the placebo groups. Gastrointestinal problems such as bloating and belching were observed, but not correlated to garlic preparation. Also, no publication bias was found.

In conclusion, garlic supplementation was well tolerated and effective in decreasing TC and LDL, and in significantly elevating HDL. In particular, the decrease in TC is considered clinically relevant in those with elevated cholesterol at baseline; however, the bioavailability of garlic compounds is variable with preparation and form. Stated limitations of this meta-analysis include heterogeneity of treatment duration and varying levels of certain parameters at baseline. Also, the active compounds in many of the garlic preparations were unknown (Wakunaga has done extensive chemical, pharmacological, and clinical research on its proprietary AGE, elaborating the s-allylcysteine as a primary active compound). Standardization of garlic preparations, as well as dosage based on active compounds, should be included in future clinical trials.

—Amy C. Keller, PhD

Reference

1. Blumenthal M, Goldberg A, Brinckmann J, eds. Herbal Medicine: Expanded Commission E Monographs. Austin, TX: American Botanical Council; Newton, MA: Integrative Medicine Communications; 2000.