Gut microbes may offer clues to a potentially devastating and confounding disorder named autism.
Whereas the causes of autism spectrum disorder (ASD) are unclear, genetic anomalies, nutritional deficiencies, toxins and infections are suspected. Now scientists are looking more closely at gut microbes and complex interactions along the gut-brain axis.
Quick review: Autism is a developmental disorder characterized by behaviors that include impaired language development, resistance to change, repetitive behaviors and social difficulties. The World Health Organization estimates ASD effects as many as 1 in 160 children across the globe.
Dysbiosis in ASD
Intestinal distress—constipation, abdominal pain, gaseousness, diarrhea, and flatulence— is common in ASD. Prevalence ranges from 9% to 90% in people with ASD, a much higher rate than in neurotypical individuals. Furthermore, irritable behaviors like social anxiety and withdrawal have been reported to be more frequent in those with intestinal distress.
An underlying dysbiosis may influence emotional behaviors and brain neurotransmitter systems via the gut-brain axis. Dysbiosis in ASD, reported in numerous studies, may lead to compromised gut permeability, production of toxins, impaired immune response, and metabolic abnormalities.
Microbiota in ASD
Patients with ASD do harbor different compositions of gut microbiota compared to controls. However, inconsistent results have muddled the picture. For example, one study reported a higher percentage of Bacteroides in the total detected microflora in children with ASD, whereas another showed a lower proportion. Further, are difference in microbiota the cause and/or effect of ASD?
A 2019 meta-analysis from China to assess the differences in microbial populations between patients with ASD and age-matched controls reported that participants with ASD had a lower abundance of Akkermansia, Bacteroides, Bifidobacterium, Escherichia coli, and Enterococcus, a higher abundance of Faecalibacterium and Lactobacillus, and a slightly increased abundance of Ruminococcus and Clostridium.
But another meta-analysis, from Spain, reported in 2020 found different results:
Compared to neurotypical children, those with ASD showed a greater abundance of Bacteroidetes (Bacteroides and Parabacteroides) and some Firmicutes genera (specifically Clostridium, Faecalibacterium and Phascolarctobacterium) along with a lower abundance of Coprococcus and Bifidobacteria.
A key limitation in both analyses was the impossibility of evaluating bacterial diversity at the species level as few studies included it. Also, people with ASD may harbor different gut microbes because they tend to have specific food preferences.
Still, an abundance of potentially harmful bacteria and the low presence of beneficial bacteria microbiota could trigger GI as well as neurological symptoms in ASD. As illustration, an abundance of Clostridium bacteria with their related inflammatory toxins and metabolites have been linked with repetitive behaviors and GI problems in ASD.
The gut–brain microbiome axis with its bi-directional communication employing hormones and neurotransmitters released by the gut endocrine system may explain some of this. Notably, dysbiosis is an association and the direction of causation is not clear.
Probiotic Treatment in ASD
The hope is that probiotics may modulate GI dysfunction and behavioral symptoms in ASD.
There are a number of animal studies whose results support the positive effect of probiotics. A downloadable PDF from a 2020 review paper provides a comparison of probiotic supplementation across different pre-clinical studies.
Studies have shown promise in human trials also. Clinical trials performed mainly on autistic children or children with ASD symptoms to study the effect of probiotics are summarized in the above paper in this PDF.
Caution: the range of methodologies (e.g. probiotic supplements were different in form, dose, bacterial strains, and combinations of bacterial strains) used in the different clinical trials makes comparisons difficult; this prevents precise conclusions and recommendations, but does reveal a clear beneficial potential.
The authors also noted that studies using multi-strain probiotic supplements showed a better effect in alleviating ASD symptoms.
By re-establishing a balanced microbial composition with subsequent balanced metabolites secretion, probiotics may present potential for the management of GIS and ASD symptoms. Obviously, standardized clinical studies should lead to more robust outcomes.
Abdellatif B et al.A. The Promising Role of Probiotics in Managing the Altered Gut in Autism Spectrum Disorders. International Journal of Molecular Sciences. 2020; 21(11):4159.
Iglesias-Vázquez, Lucía et al. “Composition of Gut Microbiota in Children with Autism Spectrum Disorder: A Systematic Review and Meta-Analysis.” Nutrients vol. 12,3 792. 17 Mar. 2020, doi:10.3390/nu12030792
Xu, Mingyu et al. “Association Between Gut Microbiota and Autism Spectrum Disorder: A Systematic Review and Meta-Analysis.” Frontiers in Psychiatry vol. 10 473. 17 Jul. 2019, doi:10.3389/fpsyt.2019.00473