The microbial community of the human gut possess “carbohydrate active enzymes”, or CAZymes, that are absent from the human genome. These CAZymes are used to digest and metabolise polysaccharides that our own enzymes are incapable of affecting. Metagenomic analyses of the gut microbiomes of American, Spanish and Japanese populations has shown the presence of CAZymes from marine bacteria, obtained via horizontal gene transfer, that confer the ability to digest algal polysaccharides such as those found in sushi.
Hehemann, J. H., Kelly, A. G., Pudlo, N. A., Martens, E. C., & Boraston, A. B. (2012). Bacteria of the human gut microbiome catabolize red seaweed glycans with carbohydrate-active enzyme updates from extrinsic microbes. Proceedings of the National Academy of Sciences, 109(48), 19786-19791.