Fungal and Bacterial Dysbiosis in Attention-Deficit/Hyperactivity Disorder: Implications for Candida, Diet, Probiotics, and Quality of Life—A Narrative Review

Attention-deficit/hyperactivity disorder (ADHD) is a complex neurodevelopmental disorder increasingly discussed within the microbiota-gut–brain axis. This narrative review synthesizes evidence on bacterial and fungal dysbiosis in ADHD, with emphasis on Candida spp., diet, probiotics, synbiotics, and health-related quality of life. A structured narrative search of PubMed/MEDLINE, Scopus, and KoBSON-accessible sources was performed for studies addressing ADHD, gut microbiota, mycobiome, Candida, nutrition, microbiome-targeted interventions, and quality of life. Evidence was synthesized thematically because of methodological heterogeneity. Available studies suggest that ADHD may be associated with altered gut microbial diversity, changes in taxa such as Faecalibacterium, Blautia, Odoribacter, and Enterococcus, and immune–metabolic alterations. However, findings are heterogeneous and do not support a single ADHD-specific microbial signature. The fungal component remains insufficiently investigated, although evidence indicates increased Candida, particularly Candida albicans, in children with ADHD and a possible link with intestinal permeability. Dietary quality, micronutrient status, probiotics, and synbiotics may modulate microbiota–gut–brain pathways, but should be considered complementary and individualized, particularly in patients with gastrointestinal, dietary, immune, or metabolic vulnerability. Bacterial and fungal dysbiosis may represent biologically plausible, primarily associative components of ADHD-related pathophysiology. Evidence remains preliminary, exploratory, non-causal, and requires cautious interpretation in future research and clinical settings.