Psychobiotic foods and the challenge of reproducible mental health effects

The concept of psychobiotics emerged from recognition of the microbiota–gut–brain axis as a relevant modulator of mental health. Despite increasing scientific and commercial interest, translating this concept into foods that produce reproducible neuropsychological effects remains limited. This critical review examines the major biological, technological, and regulatory barriers that constrain the development of psychobiotic foods with consistent and scientifically sustainable outcomes. Emphasis is placed on microbial functional activity rather than taxonomic composition, highlighting strong host dependency, particularly baseline inflammatory status and habitual dietary patterns, as key determinants of responsiveness. Central technological challenges are discussed, including the frequent misinterpretation of cellular viability as functional viability, the impact of food processing and matrix composition on metabolic competence, and difficulties in defining effective dose, stability, and intestinal residence. From a regulatory perspective, the review addresses obstacles specific to mental health claims, such as reliance on subjective endpoints, the lack of validated biomarkers, and ethical risks associated with overstating benefits. Finally, emerging stratification-based and precision nutrition approaches are discussed as realistic strategies to improve reproducibility while balancing technological feasibility, regulatory constraints, and equitable access. Collectively, this review proposes a food-science-oriented critical framework to advance psychobiotics from experimental promise to responsible, evidence-based applications. • Psychobiotic effects depend on host inflammation, diet, and microbiota context. • Functional viability, not cell survival, limits psychobiotic food performance. • Processing and food matrix strongly constrain neuroactive microbial functions. • Mental health claims face major regulatory and ethical barriers in food. • Precision and stratification models improve reproducibility of psychobiotic effects.