There are two commonly-cited ideas: enzymes and anti-methanogenic compounds. I think it works too fast for enzymes to be the full story. From https://journals.

There are two commonly-cited ideas: enzymes and anti-methanogenic compounds. I think it works too fast for enzymes to be the full story. From https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0247820 https://journals.plos.org/plosone/article?id=10.1371/journal... >Some red seaweeds are anti-methanogenic, particularly the genus Asparagopsis, due to their capacity to synthesize and encapsulate halogenated CH4 analogues, such as bromoform and dibromochloromethane It cites studies that suggest the mechanism that's so effective in cattle, and presumably the same in humans, is that the seaweed contains a good combination of compounds which inhibit methane production in anaerobic bacteria in the gut.