The invention provides the use of an agent comprising glucose isomerase for (a) reducing the bioavailability of fructose in the body of a human or animal or (b) reducing the amount of fructose available to the human or animal body or to intestinal bacteria colonizing therein after the ingestion of a fructose-containing food or substance, but excluding the use of an agent containing 5-D-fructose dehydrogenase.