This invention relates to the use of hydroxybutyrate and poly hydroxybutyrate as components of animal feed or feed additives, as well as to compositions, feed additives and feed containing them. The inventors surprisingly found that hydroxybutyrate and poly-hydroxybutyrate, preferably 3-hydroxybutyrate and poly-3-hydroxybutyrate, have a great potential for use in animal feed for modulation of the gut flora. More precisely, it has been found that poly-3-hydroxybutyrate or a microbial strain capable for producing poly-3-hydroxybutyrate can be used as a potential growth promoter or gut flora modulator by releasing SCFA, preferably 3-hydroxy butyric acid, in the gut micro flora. Further, the inventors found that 3-hydroxybutyrate and poly-3-hydroxybutyrate have a great potential for suppressing or inhibiting pathogenic bacteria in the gastro intestinal tract, e.g. have a antimicrobial activity against strains of Vibrio, E. coli and Salmonella.