Simulation of the metabolism of absorbed energy-yielding nutrients in young sheep: efficiency of utilization of lipid and amino acid.
A previously described mathematical model that simulates the metabolism of absorbed nutrients in a sheep weighing 25 kg was used to examine the effects of altering lipid and amino acid absorption, and the potential of the animal to deposit protein on the efficiency of utilization (kf) of metabolizable energy. The predicted kf of energy from lipid added to several diets ranged from 0.65 to 0.74 and was influenced by both the protein and glucose content of the diet. The highest efficiency occurred when body protein deposition was limited by amino acid absorption. Lower values occurred with high protein diets when the proportion of body energy deposited as fat declined and, with low protein diets, when NADPH supply limited the synthesis of fatty acids from acetyl-CoA. Predicted kf of energy from amino acid added to several diets ranged from 0.003 to 0.47. Low values occurred when protein synthesis was limited by amino acid absorption; an increase in amino acid supply increased ATP utilization for protein synthesis and substantially reduced fat deposition with little resulting change in energy retention. The higher values occurred when most of the additional amino acid was oxidized. Glucose supply increased the kf of added amino acid primarily by reducing the synthesis of glucose from amino acids. Predicted kf of a whole diet generally declined as the proportion of protein in the diet increased. However, when the balance of absorbed nutrients was such that fat synthesis was limited by the supply of NADPH, kf increased with increasing dietary protein. Predicted kf of a diet also declined when the proportion of body energy deposited as protein increased, except when fat synthesis was limited by the supply of NADPH. The predictions suggest that kf is determined primarily by the energetic efficiency of biochemical reactions for maintenance and growth. However, most kf values less than 0.5 were associated with a flux of more than 1 g mol/d of ATP through the degradation pathway (representing substrate cycles) that occurred when NADPH supply limited the synthesis of fatty acids from acetyl-CoA.
D000042:Absorption; D000596:Amino Acids / Q000378:metabolism; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D004032:Diet; D004044:Dietary Proteins / Q000494:pharmacology; D004734:Energy Metabolism; D050356:Lipid Metabolism; D008954:Models, Biological; D011506:Proteins / Q000378:metabolism; D012756:Sheep
D000596:Amino Acids; D004044:Dietary Proteins; D011506:Proteins