An apparatus comprises a sleeve, a seal, and an anchor assembly. The sleeve is flexible and non-permeable and is configured to fit in a duodenum. The seal is positioned at the proximal end of the sleeve and is configured to seal the proximal end of the sleeve against the stomach mucosa, the pylorus, or the duodenum mucosa. The anchor assembly comprises a pair of atraumatic anchor ends and a resilient member positioned between the anchor ends. At least a portion of the resilient member extends along at least a portion of the length of the sleeve. The resilient member is configured to bias the anchor ends generally toward each other, to thereby substantially anchor the sleeve in the duodenum. The sleeve is configured to prevent chyme from contacting the mucosa of the duodenum; and to prevent enzymes excreted in the duodenum from mixing with chyme in the duodenum.