Abstract: Use of a seaweed blend to modify gastrointestinal (GI) microbiota of an animal host. Modifying the GI microbiota may comprise modifying a community of bacteria, wherein the community of bacteria comprises Firmicutes and Bacteriodetes and the seaweed blend modifies the ratio of Firmicutes to Bacteriodetes, such as increasing the ratio of Firmicutes to Bacteriodetes. Further, the seaweed blend modifies specific families of commensal bacteria families including Ruminococceae and Lachnospiraceae within the Phyla Firmicutes. The seaweed blend may comprise (i) 65-80 wt % Ulva; (ii) 3-8 wt % Gracilaria; and (iii) 15-25 wt % Sargassum and/or Ascophyllum.