Alteration of microbial populations in the gastrointestinal tract

A method of altering the number, activity, or number and activity of a resident population of microorganism in a selected site of the gastrointestinal tract of an animal, the method comprising providing to the animal a selected modified or unmodified resistant starch or mixtures thereof in the form of high amylose starch selected from the group consisting of maize starch having an amylose content of 50% w/w or more, rice starch having an amylose content of 27% w/w or more, and wheat starch having an amylose content of 27% w/w or more, in combination with one or more probiotic microorganism such that upon ingestion the resistant starch passes through the gastrointestinal tract substantially unutilized until reaching the selected site of the gastrointestinal site where the resistant starch is utilized by the resident population of microorganism, the probiotic microorganisms or both the resident population and probiotic microorganisms thereby causing an alteration in number, activity, or number and activity of the resident population of microorganism.