In a recent study published in the journal Nutrients, a group of researchers in China conducted a systematic review and meta-analysis to investigate the effects of garlic on blood lipid and glucose levels in humans.

Systematic Review: Effects of Garlic on Glucose Parameters and Lipid Profile: A Systematic Review and Meta-Analysis on Randomized Controlled Trials. Image Credit: Igor Normann / Shutterstock

Background

Chronic non-communicable diseases, including cardiovascular diseases, chronic respiratory diseases, cancers, and diabetes, cause 41 million deaths annually. Glucose and lipids are crucial for energy, and their dysregulation can lead to atherosclerosis, diabetes, and fatty liver disease. Dyslipidemia, with high total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and low high-density lipoprotein cholesterol (HDL-C), is a major cardiovascular risk factor. Current treatments for metabolic diseases focus on symptom relief and have side effects. Garlic, rich in compounds like allicin, shows potential in regulating glucose and lipids. Further research is needed to understand its mechanisms, optimal dosage, and long-term effects.

About the study

In the present study, four databases- Embase, PubMed, Cochrane Library, and Web of Science were searched up to February 2024 using terms related to garlic, glucose, and lipid metabolism. Additional eligible trials were identified through manual searches, and the study adhered to the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020. Inclusion criteria were randomized clinical trials over two weeks, reporting outcomes like Hemoglobin A1c (HbA1c), fasting blood glucose (FBG), TC, HDL-C, LDL-C, and TG, involving adults aged 18 or older, with a placebo control group. Exclusions included non-garlic interventions, combined supplements, pregnant participants, non-clinical studies, and incomplete data.

Two researchers independently extracted data, including study details, sample size, demographics, and mean and standard deviation values for glucose and lipid indicators. Study quality was assessed using Cochrane Collaboration tools, evaluating bias risk factors.

Data analysis involved converting glycated hemoglobin units and standardizing blood glucose and lipid levels. Mean outcome changes were calculated from baseline and endpoint data, assessing heterogeneity with chi-square tests and the I² index. Significant heterogeneity led to using a random-effects model with a significance threshold of 0.05. Subgroup analyses and sensitivity analyses were conducted to explore heterogeneity sources and the impact of individual studies. Publication bias was evaluated using funnel plots and Egger's test, with trim and fill analysis for stability assessment.

Study results

A comprehensive search of four databases yielded 2,553 relevant papers. 550 duplicate publications were identified and excluded. Titles and abstracts were carefully evaluated, leading to the exclusion of 1,830 papers deemed irrelevant. The remaining 173 papers underwent a thorough qualification review, excluding 151 papers due to incomplete data, unavailability of full text, use of cinnamon in combination with other drugs, or the absence of a placebo in the control group. Ultimately, the analysis included 22 pieces of literature, comprising 29 trials.

The studies included a total of 1,567 participants from various countries, including Canada, the United States, Korea, Iran, Pakistan, India, Israel, Russia, Poland, Brazil, and Denmark. Participants' ages ranged from 18 to 80 years, and the intervention period for garlic spanned from 3 weeks to 1 year. Participants had various health conditions such as hyperlipidemia, type 2 diabetes, coronary artery disease, non-alcoholic fatty liver disease, myocardial infarction, obesity, hypertension, polycystic ovary syndrome, and some were healthy adults. Most subjects received no medication during the study, while some continued their daily medication. Garlic preparations included powder, raw garlic, oil, aged extract, and enteric-coated supplements, with varying daily doses.

The meta-analysis examined the impact of garlic on glucose metabolism indicators. Among studies focusing on FBG, eight studies with 12 effect sizes showed a significant reduction in FBG levels due to garlic intervention. Similarly, three trials with seven effect sizes indicated a noteworthy impact on HbA1c levels. The analysis of 17 studies with 19 effect sizes revealed a significant reduction in TC levels, while 19 studies with 22 effect sizes showed a positive influence on HDL-C. For LDL-C, data from 18 randomized controlled trials with 21 effect sizes indicated a substantial reduction due to garlic intake. However, 16 studies with 18 effect sizes did not show a significant impact on TG levels.

Subgroup analyses were conducted for indicators with heterogeneity exceeding 50%, identifying sources of heterogeneity such as garlic type, participant type, and study duration. Funnel plots and Egger's test assessed publication bias, revealing no bias except for TC and LDL. Trim and fill analysis confirmed the robustness of results despite publication bias. Sensitivity analyses indicated that excluding any single study did not significantly affect overall effect sizes, confirming the stability and reliability of the findings.

Conclusions 

To summarize, garlic significantly improved FBG, HbA1c, TC, LDL, and HDL levels but did not affect TG. The study used a random effects model due to high heterogeneity, with intervention periods ranging from 3 weeks to 1 year. Various forms of garlic, such as raw garlic, aged garlic extract, and garlic powder tablets, were effective. Despite some publication bias and variations in interventions, garlic's benefits on blood glucose and lipid profiles were evident.