This is a group of participants whose everyday dietary pattern was assessed as anti-inflammatory based on the Shivappa et al. (2014) equation, and who will receive probiotics during the first 3 months of the study.

This is a group of participants whose everyday dietary pattern was assessed as anti-inflammatory based on the Shivappa et al. (2014) equation, and who will receive placebo during the first 3 months of the study.

This is a group of participants whose everyday dietary pattern was assessed as pro-inflammatory based on the Shivappa et al. (2014) equation, and who will receive probiotics during the first 3 months of the study.

This is a group of participants whose everyday dietary pattern was assessed as pro-inflammatory based on the Shivappa et al. (2014) equation, and who will receive placebo during the first 3 months of the study.

Each participant will take a probiotic mixture containing Bifidobacterium longum Rosell®-175 & Lactobacillus helveticus Rosell®-52 and excipients: potato starch, magnesium stearate, and the capsule shell of hydroxypropyl methylcellulose. (Sanprobi Stress; Sanprobi sp. z o.o., sp.k., Szczecin, Poland) for 3 months.

• Dosage: One capsule per day, containing 3 × 10⁹ (3 billion) CFU*.
• Instructions: Take before 1:00 PM. Do not consume hot drinks immediately after taking the capsule.

After completion of the first phase, each participant will take a probiotic containing only the excipients, i.e. maize starch, maltodextrins, and the capsule shell for another 3 months.

• Dosage: One capsule per day.
• Instructions: Take before 1:00 PM. Do not consume hot drinks immediately after taking the capsule.
• CFU = Colony Forming Units

California Verbal Learning Test (CVLT) measure the verbal learning and memory. It measures how well individuals learn and recall verbal material, assessing both short-term and long-term memory. The Differentiation score measures the ability to distinguish between correct and incorrect responses. Scores range from 0 to 16, with higher scores indicating better outcome.

The old/new word recognition task is designed to assess episodic memory. It is consisted of two phases: a learning phase and a recognition phase. During the learning phase, participants viewed 30 pictures on the computer. In the recognition phase, participants were presented with 30 "old" and 30 "new" pictures in random order and were asked to determine whether each picture had been shown during the learning phase.

To analyze episodic memory task we followed Macmillan and Creelman (2005) signal detection theory, where d' is the ability to discriminate between signal and noise (between old and new stimuli), H stands for the proportion of correct changes detected (hit rate), and F stands for the proportion of changes incorrectly reported (false alarm rate).

d' = zH - zF

The N400 ERP component will be measured during an old/new word recognition task designed to assess episodic memory. The task involves presenting participants with previously studied ("old") and novel ("new") words while EEG is recorded. The N400 component-typically observed as a negative deflection peaking around 400 ms post-stimulus-is sensitive to familiarity and memory retrieval. A reduced N400 amplitude for old versus new words is interpreted as successful recognition. Changes in N400 amplitude over time will indicate modulation of episodic memory performance across the intervention.

Scores range from 0 to 63, with higher scores indicating more severe depressive symptoms.

Dietary Inflammatory Index assessment based on the Shivappa et al. (2014) equation. It classifies participants' dietary patterns as pro-inflammatory, neutral, or anti-inflammatory. Scores ranged from -8.87 (maximally anti-inflammatory diet) to +7.98 (maximally pro-inflammatory diet).