A meta-analysis to evaluate the effects of garlic supplementation on performance and blood lipids profile of broiler chickens

The present meta-analysis examined the effects of garlic supplementation on the average daily gain (ADG) and blood lipids profile of broiler chickens by aggregating 40 peer-reviewed articles published between 1991 and 2021. The studies assessing garlic intervention effects on broiler diets were selected from Scopus, Science Direct, Web of Science, PubMed Central, and DOAJ databases based on eligibility criteria developed referring to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Hedges' g effect size of dietary garlic treatment was calculated to estimate the standardized means difference (SMD) at 95% Confident Interval (95% CI) using random-effects models (REM) based on DerSimonian and Laird method. A subgroup meta -analysis was performed to clarify potentially significant effect sizes of covariates (study country, garlic form, rearing period, and strains of birds). A linear mixed-effect model was used to assess the inclusion levels effect along with the above-mentioned covariates. Results suggested that dietary garlic increased ADG (SMD = 3.239, 95% CI = 2.360 to 4.120, P < 0.001), and decreased feed intake (FI) (SMD =-2.230, 95% CI =-3.141 to-1.320, P < 0.001) and feed conversion ratio (FCR) (SMD = SMD =-0.059, 95% CI =-0.080 to-0.040, P < 0.001). The increasing effects on ADG were consistent among the type of garlic (garlic powder, extracted, and fermented), rearing periods (starter, grower, and finisher), most broiler strains, and in most countries where the study was carried out), with few exceptions such as studies from Pakistan, Poland, and Indonesia but the sample sizes were small. Highest increasing effect was observed during grower period (SMD = 3.828; P < 0.001). Subgroup meta -analysis also showed that decreasing effects on FI and FCR were found in grower and finisher periods (P < 0.01) but were not observed in the starter period (P > 0.05). Additionally, the relationship between levels and performance depended on the rearing periods as shown by the significant interaction effects between levels and rearing period (P < 0.001). In particular, increasing garlic levels linearly increased ADG and decreased FI and FCR in the finisher period (P < 0.05) but no relationship was observed for starter and grower periods. This study also provided strong evidence that garlic improved blood lipids profile in broiler chickens as shown by decreasing the concentration of cholesterol, triglycerides, and LDL while increasing HDL at the same time, regardless of dietary levels (P < 0.05). In conclusion, the present meta-analysis demonstrated that growth -promoting, hypolipidemic and hypocholesterolemic effects of garlic supplementation on broiler chickens were confirmed in most experimental conditions and rearing periods although the minimum effect was observed in the starter period in terms of ADG. It should be noted that high levels of garlic supplementation are typically un-favorable because it would decrease feed intake. Further study to evaluate low doses of specific bioactive compounds inclusion from garlic on broiler performance and health is warranted to minimize the deleterious effect on feed intake.