Abstract: Methods and compositions for treating medical conditions or improving health in which the composition is ingested in an amount sufficient to modify the proportion of bacteria in a gut microbiome of the individual ingesting the composition. The composition may comprise polydextrose or soluble com fiber, for example. The proportion of Coriobacteriaceae may be decreased to treat conditions such as elevated serum triglycerides, nonalcoholic fatty liver disease, metabolic syndrome, obesity, or type-2 diabetes. The proportion of V eillonellaceae may be increased to decrease serum cholesterol. The proportion of Faecalibacterium may be increased to treat inflammatory bowel disease or Chrohn's disease.