Tannin-containing compositions and methods of using same to enhance or maintain immune function during simplified nutrition feeding. Pharmaceutical compositions, including enteral nutrition compositions, are provided. The compositions comprise such tannins as proanthocyanidins and/or hydrolysable tannins. Administering the tannins to the gastrointestinal tract of a subject receiving simplified nutrition, such as with enteral nutrition therapy or parenteral nutrition therapy, attenuates or prevents deleterious effects on the gastrointestinal immune system that would otherwise occur with the simplified nutrition.