Gut-Brain Axis

Because the two serotonin pools are separate, a product cannot raise brain serotonin by raising gut serotonin, and any label implying otherwise is worth treating as a warning sign about the rest of the packaging. The routes that are actually under investigation are indirect - vagal signalling, precursor and metabolite availability, immune mediators and microbial metabolites - and they are considerably less tidy than the headline.

So when comparing products here, the useful questions are procedural rather than mechanistic. Which specific strains are named, at what CFU count, and are those the strains the cited research used? Is viability guaranteed at the end of shelf life or merely at the time of manufacture? Was the study in humans, and did it measure an outcome you would notice? A supplement that names its strains precisely is making a checkable claim; one that says only probiotic blend is not.