Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder marked by progressive loss of motor neurons and a median survival of 2 to 3 years after symptom onset. Despite advances in genetics, particularly the identification of mutations in C9ORF72, SOD1, and TDP 43, substantial variability in disease onset and progression remains unexplained. Mounting evidence points to the gut microbiome as a potential modifier of ALS biology. Microbial communities within the intestine influence systemic and central immune responses, energy metabolism, and the bioavailability of nutrients and therapeutic agents. Animal studies reveal that dysbiosis contributes to intestinal barrier dysfunction, immune activation, and altered metabolite production, while supplementation with beneficial metabolites such as butyrate or nicotinamide can delay disease progression and extend survival. Human studies, though inconsistent in their findings, consistently identify microbial imbalances and loss of diversity in subsets of patients. The gut-brain axis provides a plausible framework for these effects, as microbial products can signal through endocrine, neural, and immune pathways to influence central nervous system function. Beyond motor decline, microbiota alterations may also contribute to non-motor symptoms such as depression, anxiety, and gastrointestinal dysfunction, further shaping quality of life. While methodological variability complicates interpretation, integration of microbiome research with host genomics and metabolomics offers a path toward precision medicine. Targeting microbial composition and function may ultimately represent a novel therapeutic approach capable of modifying both disease biology and patient outcomes in ALS.
