The invention relates to immobilized or encapsulated enzyme and/or cells to lower bile acids and cholesterol. The invention also relates to methods of quantitatively measuring bile acids. The invention provides a composition for decreasing the amount of a target compound in the gastrointestinal tract of an animal, comprising: a) a biologically active agent which decreases the amount of the target compound; b) a retainer for retaining the biologically active agent by contacting the agent to limit movement of the agent; and c) a carrier.