Systematic review of evidence to support the theory of psychobiotics.

CONTEXT: The theory that supplemented probiotic bacteria could affect psychological outcomes has recently been outlined in narrative reviews; to date, however, this area of research has not been systematically reviewed.

OBJECTIVE: The objective of this review was to compare the effects of probiotics with those of placebo on psychological outcomes and symptoms of psychiatric disorders.

DATA SOURCES: The Cochrane Central Register of Controlled Trials, MEDLINE, Embase, PubMed, PsycINFO, and PsycARTICLES databases were searched electronically for studies published up to July 17, 2014. Reference lists of relevant articles were searched manually.

STUDY SELECTION: Only double-blind, randomized, placebo-controlled human trials that used a standardized, validated scale to assess the effects of probiotic interventions compared with placebo on psychological outcomes or symptoms of psychiatric disorders were included.

DATA EXTRACTION: Two researchers independently assessed trials and evaluated them for methodological quality. Data were extracted from the included studies using a data extraction form.

DATA SYNTHESIS: Ten trials met the inclusion criteria. Overall, there is very limited evidence for the efficacy of probiotic interventions in psychological outcomes. The evidence base is incomplete and lacks applicability.

CONCLUSIONS: More trials are necessary before any inferences can be made about the efficacy of probiotics in mental health applications.