Although adaptations of the gastrointestinal tract to diet have been widely documented, statistical evaluations of corresponding correlations between diet and morphology are scarce. We use a dataset that scores the complexity of the stomach, caecum and colon in 599 Eutherian species, with a higher complexity score indicating a higher degree of macroanatomical differentiation, and link these data to information on body mass and the estimated fibre content of the natural diet, controlling for phylogeny. The results indicate that dietary niche differentiation occurs along, rather than within, major Eutherian lineages, and that stomach and large intestine complexity are often positively correlated. Large intestine complexity represents a convergent adaptation to high-fibre diets across lineages (although not all species ingesting high-fibre diets have complex large intestines). By contrast, stomach complexity cannot be linked to diet, but represents a taxon-specific signal in the sense of homoplasies. In particular, the functions of stomach complexity in the smallest and the largest Eutheria with complex stomachs (the Myomorpha and the Cetacea) remain to be elucidated.