Editorial NexoScience
More than a century ago, the Russian Nobel laureate Élie Metchnikoff noticed something odd about the long-lived peasants of rural Bulgaria: many of them drank enormous amounts of soured milk. He proposed that the lactic acid bacteria in that milk could crowd out the “putrefactive” microbes he blamed for aging, and the idea, however shaky its science, planted the seed for the modern probiotic industry.
That industry now sells everything from yogurt drinks to high dose capsules on the promise of a healthier gut, and in recent years the pitch has expanded well beyond digestion. Researchers increasingly frame the gut and the brain as a single connected system. Bacteria living in the intestine produce neurotransmitters, influence inflammation, and communicate with the central nervous system through the vagus nerve, which has fed growing interest in whether swallowing the right microbes could also sharpen memory and thinking.
A team of researchers led by Nancy Calzada-Gonzales and Joshuan J. Barboza set out to test that idea properly. Published in BMC Complementary Medicine and Therapies, their systematic review and meta-analysis pooled 34 randomized clinical trials involving 2,390 adults, making it the largest synthesis of its kind on probiotics and cognitive function to date. The results point toward a real, measurable effect, at least over the medium term. Participants who took probiotics for twelve weeks scored higher on the Montreal Cognitive Assessment, a standard tool for catching early cognitive decline, with an average gain of about one point compared to placebo. On a categorical fluency test, which measures how many words someone can generate from memory in a set time, probiotic users came out nearly four points ahead.
Those numbers were smaller and less consistent, however, on the eight week Mini-Mental State Examination, another widely used cognitive screening test, where probiotics made essentially no difference at all. Timing appears to matter more than the supplement itself. The improvements only became visible once participants had been taking probiotics for a full twelve weeks, suggesting that any influence on the brain builds up gradually rather than kicking in right away.
The effect also was not evenly distributed across people. In healthy adults with no cognitive complaints, probiotics did nothing measurable. The benefit showed up mainly in older adults and in people already living with mild cognitive impairment or Alzheimer’s disease, hinting that a gut already out of balance may have more room to improve than one that is working fine already. None of this proves that probiotics can prevent or treat dementia. The authors themselves rated the certainty of almost every finding as low or very low, largely because the trials varied wildly in the bacterial strains tested, the doses used, and the tools chosen to measure cognition, and because many studies were small. A finding built on such thin and uneven evidence deserves genuine curiosity rather than a rush to buy supplements.
On safety, at least, the news was reassuring. Across four trials that tracked side effects in 325 participants, probiotics were not associated with any meaningful increase in adverse events, and no serious harms turned up beyond isolated case reports in other research involving people with weakened immune systems.
The authors are explicit that their own work is a starting point rather than a verdict, and they call for larger, longer, and more standardized trials before probiotics could be recommended as a genuine tool against cognitive decline.
The study, “Efficacy and safety of probiotic supplements on cognitive function: a systematic review and meta-analysis of randomized clinical trials,” was led by Nancy Calzada-Gonzales, Joshuan J. Barboza and colleagues, and published in BMC Complementary Medicine and Therapies in 2025. It is available under a CC BY-NC-ND open access license, DOI 10.1186/s12906-025-05149-6.
