Gastrointestinal absorption of peptides was examined in sheep fed a forage-based diet. Peptide concentrations were determined in arterial, portal, and mesenteric blood and plasma by quantification of amino acid concentrations before and after acid hydrolysis of samples that had been first deproteinized then subjected to Sephadex G-15 gel-filtration to remove residual protein. In contrast to other studies of ruminants, peptide concentrations for individual amino acids were lower than for the corresponding free amino acids with peptide (expressed as a proportion of total nonprotein amino acid) representing not more than .25 to .3 of total amino acid. Peptide concentrations in arterial, mesenteric, and portal blood and plasma were similar, indicating that on this diet there was no net uptake of peptides from the small intestine (mesenteric-drained viscera, MDV) or the whole tract (portal-drained viscera, PDV). Increasing the intake of alfalfa pellets from 800 to 1,200 g/d, while increasing the absorption and net flux across the MDV and PDV of free amino acids, had no effect on peptide absorption. Preparation of blood and plasma samples for peptide analysis with methods used in studies in which substantial peptide absorption has been reported indicated no net MDV or PDV flux of peptide. Such conflicting data on the extent of gastrointestinal peptide flux are discussed in the context of methodological differences and the importance of diet and physiological state of the animal.

It is known that uptake of di- and tripeptides by distinct peptide transport systems is an important biological process by which amino acids are absorbed from the gastrointestinal tract (Rubino et al 1971; Webb and Bergman, 1991). However, until recently it was generally accepted that absorbed peptides had to be completely hydrolysed in to free amino acids within the enterocyte (Krzysik and Adibi, 1977) prior to their transport into the circulation. Nevertheless, it has been suggested that, in ruminant species at least, a substantial proportion of the total amino acid uptake from the gastrointestinal tract may be in the form of peptides (Webb, 1990; Seal and Parker, 1991; Webb and Bergman, 1991) and that a large part of this absorption may occur across the stomach region (i.e. rumen and omasum) (Webb, 1990). Consequently, the present study was designed to assess the uptake of amino acids in the form of low molecular weight peptides across the gastrointestinal tract of growing lambs.