[Absorption and utilization of amino acids infused into the cecum of growing swine. 3. Studies with 15N- and 14C-labeled isoleucine].

In 3 experiments growing female pigs, (live weight 35-56 kg) received continuous infusions of 15N-labelled or 15N- and 14C-doubly labelled isoleucine by means of caecal cannulae. The whereabouts of the infused isoleucine were investigated. Between 8.9 and 12.3% of the infused 15N-excess (15N') was excreted in faeces mainly in the forms of bacteria protein, between 0.7 and 0.9% only of the infused amount unchanged as 15N-isoleucine. The highest quota is microbially decomposed in the colon and absorbed in the form of NH3 or amines. Between 45 and 70% of the infused 15N are excreted in urine, mainly as urea (85-95% of the 15N-amount in urine). Virtually no 15N from the infused isoleucine is incorporated in the organ and tissue proteins (exception: colon wall). These findings are corroborated by the 14C-activity measuring as no intact 14C labelled isoleucine could be detected in the tissue protein. These results show that intact isoleucine is not absorbed in the colon. The nitrogen originating from the absorbed NH3 is nearly completely excreted in urine and thus not available to the biosynthesis of body protein.