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 Lactobacillus helveticus and Lactobacillus fermentum (e.g., NS8 and NS9 originally isolated from Mongolian fermented koumiss). 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.