Randomization and masking
Participants were randomly assigned (1:1:1) to single dose probiotics, double dose probiotics, or placebo using a fixed block size of nine participants and stratified for sex. Participants were randomized to (a) four capsules of probiotics per day or (b) two capsules of probiotics per day and two capsules of placebo, or (c) four capsules of placebo. Hence, all participants were to take four capsules per day, two before breakfast and two before dinner. The funder, which produced the product, packed the probiotics and placebo into pouches for morning and evening respectively, and labeled them group A–B (morning and evening), C–D, and E–F. At delivery of the study product, the labeling key was in a sealed envelope and kept in a locked space at the study center. The study was blinded during the first phase for both the researchers and the study participants. Type of received treatment was revealed to researcher and study participant at the 3-month follow-up, before statistical analyses. During the second phase of study, all participants received six capsules of probiotics per day (triple dose). This phase was non-blinded. Any participant that required extra capsules of probiotics/placebo due to prolonged follow-up time (e.g. due to current Covid-19 restrictions) were provided so by regular mail.