The researchers do not know why these particular changes occurred, but they say it could be because Firmicutes die when oxygen is present, and shortening the gastrointestinal tract means that oxygen that is normally consumed in the small intestine reaches the colon. Alternatively, the changes could occur because food is being digested faster. (The group did not test microbial make-up in individuals before surgery, but is now working on a follow-up study that compares before and after.) A similar shift in gut flora has been reported in rats undergoing a gastric bypass