Study links older sibling count to preschoolers’ gut microbiome types

Three microbiome types at ages two and four

A joint research group analyzed stool samples and living-environment information from participants in an additional survey of the Japan Environment and Children’s Study. It classified gut microbiota—the communities of bacteria living in the intestine—into three types.

The group reported that children with older siblings had lower odds, relative to type 2, of being classified as type 1, the type with the lowest bacterial diversity and fecal propionate, a short-chain fatty acid. The reported adjusted odds ratios for one older sibling were 0.49 at age 2 and 0.45 at age 4; for two or more older siblings they were 0.32 and 0.22, respectively. These are associations, not measures of disease risk.

Earlier epidemiological research had examined links between older siblings and the gut environment mainly during infancy or before age 3; whether such associations continued into the preschool years and what might account for them was not well understood.

The analysis extends observations of sibling-related differences in the gut environment to ages 2 and 4, while finding that the examined pathways did not explain most of the association. Recent antibiotic use was identified as a possible contributor to part of the association at age 4.

The odds ratios do not measure disease risk

The reported finding follows children into the preschool years and considers both microbiome composition and fecal metabolites. It does not show that older siblings prevent disease, that these gut differences lower disease risk, or that any observed association is causal.