Sibling factors associated with gut microbiota and fecal short-chain fatty acids in preschoolers

BACKGROUND: Sibling factors are linked not only to disease development but also to gut environmental factors (e.g., microbiota, short-chain fatty acids [SCFA]). However, these associations among preschoolers and their underlying mechanisms remain unclear. METHODS: In a sub-cohort study of a nationwide birth-cohort study, using fecal microbiome and SCFA data from 903 children at age 2 or 4 years, we examined associations of sibling factors with gut environmental factors. We also determined enterotypes by clustering and sibling-enterotype associations with potential mediation pathways. RESULTS: Among sibling factors, the number of older siblings showed the strongest associations with gut environmental factors. We identified 3 enterotypes: 1) Mediterraneibacter gnavus group, 2) Faecalibacterium, and 3) Ruminococcus. Compared to enterotype 2, having older siblings was associated with lower odds of enterotype 1, with the lowest Shannon index and propionate level, at age 2 (1 sibling: adjOR 0.49, 95%CI 0.34-0.69; ≥2: adjOR 0.32, 95%CI, 0.19-0.54) and 4 (1 sibling: adjOR 0.45, 95%CI 0.26-0.77; ≥2: adjOR 0.22, 95%CI 0.08-0.53) years. These associations were largely direct, with modest mediation by antibiotic use at age 4 years (1 sibling, adjOR 0.89; 95%CI, 0.79-1.00). CONCLUSION: Our data identified distinct enterotypes of preschoolers and sibling-enterotype association pathways. IMPACT: In this nationwide multicenter birth-cohort sub-cohort, older siblings were strongly associated with gut microbial composition in preschoolers. Having older siblings was associated with lower odds of a potentially less mature enterotype characterized by the lowest Shannon index and propionate level. The sibling-enterotype association was mainly direct, with modest mediation by recent antibiotic use. This study extends sibling-microbiota evidence beyond infancy/toddlerhood by identifying three enterotypes with distinct microbial and short-chain fatty acid profiles. These findings clarify pathways linking family structure to gut environmental development and may inform microbiota-targeted disease prevention strategies.