Older siblings associated with distinct gut microbiota profiles in preschool children

An observational analysis within the Japan Environment and Children’s Study examined associations between sibling factors, gut microbiota and fecal short-chain fatty acids in 903 children, using samples collected at ages 2 and 4 years. Researchers profiled bacterial composition, measured fecal metabolites and used clustering to identify enterotypes, then assessed potential pathways linking sibling number to these profiles.

The number of older siblings showed the strongest associations among sibling factors. Three enterotypes were identified: Mediterraneibacter gnavus group, Faecalibacterium and Ruminococcus. Compared with the Faecalibacterium enterotype, children with older siblings had lower adjusted odds of the M. gnavus group enterotype, which had the lowest Shannon diversity index and propionate level. At age 2, adjusted odds ratios were 0.49 (95% CI 0.34–0.69) for one older sibling and 0.32 (0.19–0.54) for two or more. Corresponding estimates at age 4 were 0.45 (0.26–0.77) and 0.22 (0.08–0.53).

The associations were largely direct in the mediation analysis, with modest mediation by recent antibiotic use at age 4. The findings extend evidence linking family structure to gut microbial development beyond infancy. However, the observational design does not establish causality, and the potentially less mature enterotype is a microbial profile rather than a demonstrated adverse clinical outcome. These results do not establish a microbiota-targeted prevention strategy.