Abstract: A process for feeding an animal a diet which alters the function and morphology of the gastrointestinal tract (GIT), a large lymphoid organ in the animal and which improves glucose metabolism, satiety, and nutrient absorption. The process involves feeding a companion animal such as, for example, a dog or cat a diet of a pet food composition containing fermentable fibers which have an organic matter disappearance (OMD) of 15 to 60 percent when fermented by fecal bacteria for a 24 hour period, the fibers being present in amounts from about 1 to 11 weight percent of supplemental total dietary fiber. The animal is maintained on the diet for a sufficient period of time to allow the fermentable fibers to ferment in the GIT of the animal.