Microbiome

The microbiome is the collective genome of microbial communities inhabiting human tissues, with the gut microbiome the largest and most studied. It contains approximately 10^13 bacteria representing hundreds to thousands of species, with composition varying substantially between individuals and within individuals over time. The microbiome is not merely a passive inhabitant but an active metabolic organ: producing short-chain fatty acids (SCFAs), vitamins, and neurotransmitter precursors; modulating immune function; and communicating with the central nervous system via the gut-brain axis.

Gut dysbiosis, defined as reduced diversity or altered composition associated with disease, is implicated in multiple conditions. In ADHD, prospective evidence shows gut dysbiosis at age 1 precedes ADHD diagnosis by years (Ahrens et al., 2024, n=16,440 Swedish children). Adults with ADHD show reduced SCFA-producing bacteria (Faecalibacterium, Roseburia) and elevated inflammatory species. Depression is associated with reduced Faecalibacterium and Coprococcus. Inflammatory bowel disease involves reduced microbial diversity and increased pro-inflammatory species.

Interventions to modulate the microbiome vary in evidence quality. Diet (particularly Mediterranean pattern, high in fiber and fermented foods) produces measurable shifts within days to weeks and shows clinical benefit exceeding most supplement interventions. Specific probiotic strains have clinical evidence (L. rhamnosus HN001 for perinatal mood, B. longum NCC3001 for IBS-related depression) but strain specificity is critical: generic products labeled “L. rhamnosus” are not interchangeable with validated clinical strains. Antibiotic exposure, particularly in early life, reduces diversity for months to years and should be avoided unless clearly indicated. Prebiotics (fibers that feed beneficial bacteria) complement probiotic approaches.