The present invention is directed generically to a means for altering the ability of the mammalian body to absorb nutritive content from ingested foodstuffs, and more specifically to an apparatus and method of use for an endolumenal sleeve (referred to also as an "intragastrointestinal device" or "gastrointestinal device") positioned in the mammalian gastrointestinal (GI) tract. A suitable endolumenal sleeve is comprised of an anchor element and an opening at a proximal end, an elongate lumen or hollow open-ended tube having a transverse dimension, and a distal orifice. Optionally, an exterior aspect of the elongate lumen may include additional modes of attachment to the tissues walls of the GI tract through the use of one or more means for promoting tissue in-growth. The endolumenal sleeve is retained in the GI tract such that a substantial fraction of the food and liquids passing through the GI tract is channeled into the proximal opening and through an interlumenal space defined within the interior space of the endolumenal sleeve. Within the endolumenal sleeve there may be one or more restrictive means to constrain, impede or otherwise control the operative flow of material through the device. An individual restrictive means can either be of a fixed geometry or such means may include one or more elements which are adjustable in nature or function. The elongate lumen of the endolumenal sleeve is formed of a polymer composition suitable for controlled ingress of biological secretions, egress of certain selected nutritional elements, and may comprise either a single tubular structure or a multi-section (i.e. articulated and/or multiple lumen) assembly. When the endolumenal sleeve is in situ within the mammalian gastro-intestinal system, ingested foodstuffs are conveyed from the proximal end to said distal orifice. In typical applications, the proximal end of the endolumenal sleeve is positioned within the physiological region extending from the lower esophagus to the duodenum and the distal orifice is positioned within the physiological region extending from the upper duodenum to the lower jejunum, though further extension into the lower intestine is possible. Through proper selection of position for the endolumenal sleeve proximal and distal ends, combined by selection of the composition used in the fabrication of the elongate lumen, it is possible to finitely control the degree of nutritive absorption performed by the gastrointestinal tract.