Protein digestion and amino acid absorption along the intestine of the common carp (Cyprinus carpio L.), a stomachless fish: an in vivo study.

Protein hydrolysis to peptides and free amino acids and the apparent absorption of amino acids (AAaa) were evaluated in different segments of carp intestine. The AAaa analysed using Cr2O3 as a marker indicated that 73.2% of the amino acids were absorbed in the first 20% of the intestinal tract and 5.3 and 21.5, respectively, in the following segments (20% of gut length). Except for methionine and histidine, essential free amino acid concentration decreased significantly along the intestine. Of the nonessential amino acids, glutamate and aspartate concentrations increased in the hind gut. The absolute amount of the peptide amino acid fraction decreased towards the middle intestine but, expressed as a proportion of the total amino acid content, it changed little along the intestine: 49 and 54% in the anterior and posterior intestine, respectively. The molar concentration of the peptide amino acid fraction was much higher in carp intestine (543.9 mM) than in rainbow trout (147.3 mM) or human (143.9 mM) intestine.