Half received a daily dose of Lactobacillus reuteri, a beneficial gut bacterium naturally present in many foods and in most people; the other half got an identical placebo.
The intervention lasted three months, and the children were followed for another three months without supplements. The study was published in Pediatrics and was supported by a grant from a manufacturer of probiotic supplements.
During the study, there were 69 episodes of diarrhoea in the placebo group and 42 in the supplement group. The placebo group had 204 respiratory tract infections, compared with 93 in those taking L. reuteri.