The effect and underlying mechanisms of garlic extract against cognitive impairment and Alzheimer's disease: A systematic review and meta-analysis of experimental animal studies

Alzheimer's disease (AD) is the main cause of dementia, and according to traditional Chinese medicine (TCM), it is leaded by the deficiency of essence, qi, and blood. Allii sativi bulbus, acrid and warm, is traditionally used as the important adjuvant and conductant drug to distribute essence-qi throughout the body, fortify the spleen and harmonize the stomach. Garlic (Allium sativum L., Alliaceae) has also been reported to display potential anti-AD effect both in vitro and in vivo studies, while no systematic review of these studies has been conducted. This review aims to provide a comprehensive evaluation of the effect and underlying mechanism of garlic extract against cognitive impairment and AD neuropathology through meta-analysis and sensitivity analysis. Eligible studies were searched from PubMed, Web of Science and EMBASE from February to March in 2020, and 13 studies describing the effect of garlic extract in AD animal models (551 mice and 88 rats) were identified. Analysis of these studies showed that garlic extract could reduce cerebral Aβ levels [Aβ40: SMD -8.62(-11.75, -5.49), p < 0.00001 and Aβ42: SMD -11.70(-18.01, -5.39), p=0.0003], and increase the number of right crossings in MWM [SMD 2.87(1.48, 4.26), p < 0.0001] in AD animals. However, moderate risk of bias (quality score ranged from 40% to 60%) is revealed by SYRCLE's checklist, mainly because of the lacks of sample size calculation, random allocation and blind assessment. This review shows that garlic extract may be effective in alleviating cognitive impairment and neuropathology in AD animal models. High quality AD animal studies with enough sample size and more comprehensive evaluation of outcomes are needed to further confirm the results.