The secondary analysis of a recent randomized controlled trial supported the significance of the gut microbiota-brain axis in major depressive disorder and emphasized the potential of microbiota-linked regimens as an accessible, non-stigmatizing, and holistic therapy to combat cognitive symptoms in individuals suffering from depression. Else Schneider et al. sought to determine how short-term, high-dose probiotic supplementation affected cognition, associated brain processes, and brain-derived neurotrophic factor (BDNF) in depressed individuals.

Overall, 60 subjects suffering from major depressive disorder were incorporated, and 43 of them joined the modified intention-to-treat analysis. Over a period of 31 days, a probiotic supplement or an identical placebo containing maltose was given in addition to the regular course of depression. At three separate time points—before, immediately after, and four weeks post-intervention—participant scores on the Verbal Learning Memory Test (VLMT), both versions of the Trail Making Test, brain-derived neurotrophic factor levels, as well as Corsi Block Tapping Test were measured.

Alterations in working memory processing during brain activity were looked into both prior to and right away after treatment. Immediately post-treatment, the probiotic group's VLMT showed considerably better instantaneous memory and a trend for a time × group interaction when all time points were taken into account. A remediated hippocampus function was witnessed in the probiotic group, as revealed from a time × group interaction in hippocampus activation during the processing of working memory. Other measurements did not show any appreciable differences.

The probiotic group displayed a slight but nonsignificant rise in BDNF levels while the placebo group illustrated a decrease in BDNF. Additional supplementation of probiotics improves verbal episodic memory and influences the brain processes underlying cognitive impairment in major depressive disorder. Hence, by balancing the altered hippocampus function during the 2-back task, probiotics exert a direct impact on the neural mechanisms underlying cognitive disability in depression.