Probiotics for dementia: a systematic review and meta-analysis of randomized controlled trials.

CONTEXT: Dementia is the fifth leading cause of death in the world. Animal studies indicate that in addition to the aging process, intestinal microbiota may play an important role in the neurodegeneration process through the modulation of the gut-brain axis. OBJECTIVE: A systematic review and meta-analysis was conducted to determine the effectiveness of probiotic and synbiotic supplementation on the cognitive function of individuals with dementia. DATA SOURCES: MEDLINE, BVS, SciELO, CENTRAL, Embase, and grey literature were searched from their inception to January 2019. STUDY SELECTION: We included data from randomized clinical trials (RCTs) that addressed dementias and assessed the following outcomes: cognitive function; inflammatory, oxidative stress, and metabolic markers; nutritional status; and intestinal microbiota composition. DATA EXTRACTION: Data searches, article selection, data extraction, and risk-of-bias assessments were performed according to the Cochrane guidelines. Data were pooled by inverse-variance random-effects meta-analyses. GRADE (Grading of Recommendations Assessment, Development, and Evaluations) was used to assess the quality of evidence. RESULTS: Data from 3 RCTs involving 161 individuals with Alzheimer's disease receiving Lactobacillus and Bifidobacterium strains showed no beneficial effect of probiotic supplementation on cognitive function (standardized mean difference, 0.56; 95%CI: -0.06 to 1.18), with very low certainty of evidence. However, probiotic supplementation improved plasma triglycerides, very-low-density lipoprotein cholesterol, insulin resistance, and plasma malondialdehyde. No RCTs included synbiotic supplementation or assessed microbiota composition. CONCLUSION: Current evidence regarding the use of probiotics and synbiotics for individuals with dementia is insufficient to support their clinical application. SYSTEMATIC REVIEW REGISTRATION: PROSPERO registration no: CRD42018116148.