Regulation of macronutrient metabolism by the gastrointestinal tract of ruminants

Abstract

We set out to test the hypothesis that the gastrointestinal tract (GIT) of ruminant animals catabolizes amino acids (AAs) preferentially. We sought to determine whether this catabolism represents an obligate requirement, and whether this requirement stems from the need to generate energy or support other metabolic demands. The aim was to determine the composition of macronutrients (AAs, short chain fatty acids, and glucose) utilized by the GIT, and the influence of general and specific nutrient supplies on their routes of metabolism. Increasing protein supply to the small intestine did not alter the total amount of glucose removed by the GIT indicating, that glucose removal and therefore utilization is obligatory. In contrast, the net removal of AAs occurred at a constant proportion of arterial and luminal supplies. This translated to larger amounts of AAs removed from blood circulation, and from the lumen of the small intestine in response to increased small intestinal and blood supplies. In this respect, the net absorption of branched chain AAs was, unlike other essential AAs lower than 100%. Further, glutamate and glutamine net appearance across the whole GIT and small intestine was unaffected by protein supply. The disproportionate utilization of BCAA, glutamate, and glutamine as compared to other AAs suggested that their metabolism occurred toward specific metabolic requirements, possibly energy production. When Krebs cycle metabolism was investigated using individual AAs, glucose, and short chain fatty acids, leucine and valine did not contribute to the flux of Krebs cycle intermediates. Conversely, α-ketoglutarate flux originated mainly from glutamate, and to a lesser extent from glutamine. Though glucose was metabolized to pyruvate and lactate, glucose did not contribute to Krebs cycle intermediates. Overall, these results indicated that glutamate plays an important role in energy metabolism, and in insuring replenishment of Krebs cycle intermediates that leave the cycle via cataplerosis. Yet, the results raised new questions that ought to be addressed in future studies. The fate of glutamine carbon, the metabolic significance of leucine and valine deamination, and the role of glucose partial catabolism to lactate need to be investigated.