A biological mechanism can make a research idea plausible. It cannot, on its own, show that a supplement changes how people feel.
Three questions keep the distinction clear
Where was the result found? Cell and animal studies can explore early mechanisms but do not directly establish a human outcome. What intervention was tested? A specific strain or combination is different from an entire category. What changed for participants? A laboratory marker is not the same as a validated symptom measure or a clinically meaningful improvement.
A strong article or product page should state which kind of evidence it is citing. When a mechanism is presented as proof of a clinical effect, look for the human trials that connect the two steps.
The NIH Office of Dietary Supplements notes that probiotic effects may depend on the strain. That makes ingredient identity and study context especially important when evaluating a claim.
Educational information only; not medical advice. Mechanistic research does not establish that a probiotic treats a mental-health condition.
NIH Office of Dietary Supplements: Probiotics — Health Professional Fact Sheet