Dietary protein level and energy metabolism during treadmill exercise in horses

Six conditioned Quarter Horse mares were used in a crossover design to assess the effect of the dietary protein level on intramuscular and hepatic glycogen and lactate, oxygen uptake and blood lactate, pyruvate and free fatty acids. After a 2-wk adaptation period to either a 9.0% (control) or an 18.5% crude protein diet, each horse performed an exercise test. The horses were exercised for 15 min on an 11% grade treadmill at 4.5 m/sec. The exercise test was performed 3-4 h after a meal. Venous, arterial and mixed-venous blood samples were taken simultaneously at rest and during exercise. Muscle and liver samples were taken at rest and after exercise. The dietary protein level did not affect hepatic or intramuscular glycogen utilization (P > 0.05). Exercise resulted in increased (P < 0.001) lactate in venous blood, muscle and liver; however, dietary treatment did not affect (P > 0.05) lactate level. Venous blood lactate:pyruvate ratio was higher (P < 0.05) in the control horses. Dietary protein level did not affect (P > 0.05) oxygen uptake or plasma free fatty acids; however, exercise increased (P < 0.01) both. These results indicate the dietary protein did not affect substrate utilization during the absorptive phase of digestion in exercising horses; however, the higher lactate:pyruvate ratio in the control horses suggests that the intraconversion of lactate and pyruvate may be influenced by diet composition.