Condition the water before adding the fish: a hypothesis on probiotic selection without prior gut assessment.

Introduction: Current clinical practice often assumes that gut microbiome assessment before selecting probiotic species, assuming a 'test-then-match' strategy yields better outcomes. However, routine testing has limited predictive value for probiotic efficacy, and many commercial probiotics fail to achieve sustained clinical improvement. Methods: In this Hypothesis and Theory article, we propose a novel, testable hypothesis: prioritize remodelling the gut environment using metabolically potent probiotics, (i.e., strains with high capacity to produce beneficial metabolites such as short-chain fatty acids, bioactive peptides, and organic acids) without prior microbiome assessment-a 'condition-then-attract' strategy. Our hypothesis is motivated by preclinical studies demonstrating the metabolic potency of specific strains of Results: We derive several directly testable predictions from this framework, including that a randomized controlled trial comparing 'test-then-match' versus 'condition-then-attract' strategies would show that prior gut assessment does not improve outcomes, and that metabolically potent probiotics will outperform less active strains regardless of species identity. Discussion: We contend that refocusing from species-level matching to strain-level metabolic potency may offer a more effective and simpler alternative. Future research should prioritize metabolic potency over colony-forming unit counts and reconsider the necessity of routine pre-intervention gut assessment. Our framework does not rely on a single metabolite but embraces diverse mechanisms-proteolysis, carbohydrate fermentation, pH modulation-that collectively restore gut homeostasis.