CRD summary

The authors concluded that garlic produced modest reductions in total cholesterol, driven mainly by modest reductions in triacylglycerol levels, without any appreciable lowering in low-density lipoprotein levels or increase in high-density lipoprotein levels. The review has some methodological weaknesses, but the conclusions reflected the evidence presented and appear to be reliable.

Authors' objectives

To determine the effects of garlic on lipid parameters.

Searching

MEDLINE, CINAHL, and the Cochrane Library were searched from inception to November 2007. Search terms were reported. References of retrieved reviews, meta-analyses and trials were handsearched. Only trials published in English or German were eligible for inclusion in the review.

Study selection

Randomised placebo-controlled double-blind trials (RCTs) that evaluated the effects of garlic on total cholesterol, low-density lipoprotein, high-density lipoprotein or triacylglycerol were eligible for inclusion. Trials where garlic was combined with other products or nutraceuticals with lipid-altering effects were excluded. Both parallel and cross-over trials were eligible for inclusion. Cross-over trials which did not include a washout period were considered if the follow-up after the intervention and control was each at least three weeks.

The majority of trials assessed Kwai brand garlic powder. The types of garlic preparations were varied including garlic powder, garlic oil, aged garlic extract and spray-dried garlic. Baseline total cholesterol levels ranged from 4.4 to 7.6mmol/L. Baseline triacylglycerol levels ranged between 0.62 to 4.47mmol/L. Diet modification (counselling) was implemented in half of the included trials. The washout periods in cross-over trials ranged from 0 to 4 weeks. The trial duration ranged from two to 26 weeks.

Two authors independently assessed studies for inclusion. Disagreements were resolved through discussion.

Assessment of study quality

Randomisation, allocation concealment, blinding and follow-up were assessed according to the Jadad criteria. Trials could score up to a maximum of 5 points for quality; trials that scored less than 3 points were considered to be of poor quality.

The authors did not state how many reviewers assessed study quality.

Data extraction

The mean differences and variances were extracted. Triacylglycerol levels reported in mg/dL were multiplied by 0.0113 to derive the mmol/L value.

Two reviewers independently extracted data using a standardised data abstraction tool.

Methods of synthesis

The pooled weighted mean differences (WMDs) with 95% confidence intervals (CIs) were calculated using a random-effects meta-analysis. Sensitivity analyses were performed based on Jadad quality scores, study designs, trial duration, brands of garlic, baseline total cholesterol levels and diet counselling. Statistical heterogeneity was assessed using the I² statistic. Publication bias was assessed using funnel plots and by the Egger’s test.

Results of the review

Twenty-nine RCTs were included (n=1,794 participants): 22 parallel-group designs (n=1,296 participants) and seven crossover designs (n=498 participants). Twenty-five studies had a Jadad score of 3 points or more and were considered high quality. Twenty-seven studies had proper allocation concealment. Trial duration ranged from two to 26 weeks.

Garlic in comparison with placebo was associated with a reduction in levels of total cholesterol (WMD -0.19 mmol/L 95% CI -0.33 to - 0.06; I²=64.3%; 28 RCTs), triacylglycerol (WMD -0.11mmol/L, 95% CI - 0.19 to -0.03; I²=26.7%; 28 RCTs), low-density lipoprotein (WMD - 0.059mmol/L, 95% CI -0.151 to 0.032; I²=24%), and high-density lipoprotein (WMD 0.025mmol/L, 95% CI -0.001 to 0.051; I²=9.6%). No evidence of publication bias was found.

Sub-group analyses suggested that there were no statistically significant differences in the effects of garlic on total cholesterol, low-density lipoprotein, high-density lipoprotein and triacylglycerol with respect to trial quality, trial duration, type/brand of garlic, baseline total cholesterol, baseline triacylglycerol and diet control.

Authors' conclusions

Garlic produced modest reductions in total cholesterol levels, driven mainly by modest reductions in triacylglycerol levels, without any appreciable lowering in low-density lipoprotein levels or increase in high-density lipoprotein levels.

CRD commentary

The review question was clear with regard to the eligible study designs, interventions and outcome measures, but criteria relating to patients were not explicitly stated. Relevant databases were searched for studies published in English or German, so there was a risk of language bias. No attempts were made to search for unpublished studies and the review appeared to only include published studies, raising the risk of publication bias. However, the potential for publication bias was assessed and no evidence found. Appropriate steps were taken to minimise the risk of reviewer error and bias during study selection and data extraction, but not explicitly during validity assessment. Study quality was assessed using appropriate criteria. The decision to combine data was supported by absence of evidence for heterogeneity.

The review had some weaknesses, but the conclusions reflected the evidence presented and appear to be reliable.

Implications of the review for practice and research

Practice: The authors did not state any implications for practice.

Research: The authors stated that future studies should assess the effect of adjunctive garlic therapy with fibrates or statins on triacylglycerol concentrations.

Funding

Not stated.

Bibliographic details

Reinhart KM, Talati R, White CM, Coleman CI. The impact of garlic on lipid parameters: a systematic review and meta-analysis. Nutrition Research Reviews 2009; 22(1): 39-48. [PubMed: 19555517]

Original Paper URL

http://journals.cambridge.org/action/displayAbstract?aid=5877640

Indexing Status

Subject indexing assigned by NLM

MeSH

Cholesterol /blood; Garlic; Humans; Plant Preparations /pharmacology; Randomized Controlled Trials as Topic; Triglycerides /blood

AccessionNumber

12009107666

Database entry date

26/05/2010

Record Status

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.