Narrative Review: The Effect of Vitamin A Deficiency on Gut Microbiota and Their Link with Autism Spectrum Disorder.

Autism spectrum disorder (ASD) is a group of neurodevelopmental disorders defined by a lack of social behaviors, repetitive behaviors and nonverbal interactions, such as limited eye contact, facial expression, and body gesturing. It is not a single condition, but rather a multi-factorial disorder caused by hereditary and non-genetic risk factors, as well as their interaction. According to several studies, the gut microbiota may have a role in the pathophysiology of autism spectrum disorder. Various studies have found differences in the composition of the gastrointestinal (GI) microbiota in children with ASD compared to unaffected siblings and/or healthy unrelated controls. The processes that relate the gut microbiota to brain dysfunctions (the gut-brain axis) in ASD are yet to be fully understood. However, the differences in the gastrointestinal composition might be due to vitamin A deficiency because vitamin A (VA) plays a role in the regulation of the intestinal microbiota. This narrative review discusses the impact of vitamin A deficiency on the gut microbiota composition and tries to understand how this may contribute for the development and severity of ASD.