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Simulation of the Metabolism of Absorbed Energy-Yielding Nutrients in Young Sheep: Efficiency of Utilization of Acetate¹

John L. Black, Margaret Gill^*, David E. Beever^*, John H. M. Thornley^* and John D. Oldham^,2

CSIRO, Division of Animal Production, Prospect, P.O. Box 239, Blacktown, New South Wales 2148, Australia, ^* Grassland Research Institute,^,3 Hurley, Maidenhead, Berks, SL6 5LR, and National Institute for Research in Dairying, Shinfield, Reading, Berks, RG2 9AT, United Kingdom

A previously described mathematical model that simulates the metabolism of absorbed nutrients was used to examine factors influencing the efficiency of utilization of acetate in a sheep weighing 25 kg. A significant component of the model was a function representing substrate cycles that degraded ATP when its concentration reached twice initial levels. Acetate utilization was simulated when up to 2 g mol/d were added to four diets (representing forages and concentrates) that supplied varying proportions of absorbed volatile fatty acids, glucose and protein. The basal diets provided either 4.375 (near energy balance) or 8.75 MJ/d absorbed energy. The predicted net efficiency of utilization (k_f) of the energy in added acetate was high (0.58–0.70) for all combinations of absorbed nutrients at the low level of energy absorption, whereas at the higher level, it ranged from 0.16 (forage) to 0.49 (high protein concentrate). Low k_f values were always associated with a considerable flux of ATP through the degradation pathway. The increase in ATP concentration that caused this pathway to operate was linked to an inhibition in the utilization of acetyl-CoA for fatty acid synthesis and an increase in its oxidation. The simulated addition of exogenous NADPH or its precursors (particularly glucose and propionate) to these diets repartitioned acetyl-CoA flux towards fatty acid synthesis instead of oxidation, decreased the flux of ATP through the degradation pathway and increased the k_f of added acetate. Although a negative relationship was predicted between k_f of added acetate and NADPH production for diets with increasing protein content, k_f still depended on ATP flux through the degradation pathway. Addition of glucose to the high protein diets decreased this flux by decreasing acetyl-CoA oxidation and increasing fatty acid synthesis. The predictions suggest that the efficiency of acetate utilization in ruminants may be influenced by NADPH availability when a considerable amount of absorbed energy is derived from acetate. Increased absorption of glucose or propionate enhanced NADPH production, but the major effect on acetate utilization of increasing dietary protein was not through NADPH production.

KEY WORDS: • acetate utilization • ruminants • fatty acid synthesis • NADPH production • futile cycles • computer simulation

¹ J. L. Black received financial assistance from the Underwood Fund and MG from the British Society of Animal Production. This support is gratefully acknowledged. The Grassland Research Institute and The National Institute for Research in Dairying are financed by the Agricultural and Food Research Council; the work was in part commissioned by The Ministry of Agriculture, Fisheries and Food.

² Present address: East of Scotland College of Agriculture, Animal Production Advisory and Development Department, Bush Estate, Nr Penicuik, Midlothian, E26 00E.

³ Present name: Animal and Grassland Research Institute.

Manuscript received 18 February 1986. Revision accepted 3 September 1986.