Diet and the gut microbiota: how the gut: brain axis impacts on autism

Autism spectrum disorders (ASDs) are a group of neurodevelopmental conditions characterized by qualitative impairments in social interaction and communication, and by the presence of restricted interests and of repetitive and stereotyped behaviors. Although environmental factors are recognized to play an important role in ASD, there is little conclusive evidence linking diet with disease onset and progression. However, recent works examining how diet shapes the gut:brain axis may offer new insights into environmental contributions to disease mechanisms and raise the possibility of at least improving certain ASD symptoms through diet. Gut microbiota is a key player in development and regulation of the gut:brain axis and might thus play a critical role in ASD pathophysiology. Indeed, gut microbiota is altered in ASD. The recent appreciation of the core contribution of the human microbiome to host metabolic processes and in processing of dietary compounds has raised interest in how microbial activities might impact on ASD. This chapter provides a detailed overview of the current knowledge on the effect of gut microbiota on the metabolism of molecules (amino acids, γ-aminobutyric acid, unsaturated fatty acids, short-chain fatty acids, cholesterol, butyrate, acetate, N-acetylaspartate, polyphenols, etc.) and enzymes (disaccharidases, hexose transporters, and monocarboxylic acid transporters, among others) associated with ADS onset and progression. The role of dietary patterns, probiotics, and successional development of gut microbiota in brain function is also reviewed. Evidence points to the essential role of gut microbiota composition and metabolism in brain development and disease risk throughout life. However, there is a clear lack of human data describing the contribution of gut microbiota and its modulation through diet to hormonal, immunological, or metabolic communication along the information highways linking gut and brain, especially in early childhood.