Lactate and glucose responses to exercise in the horse: influence of interval training and dietary fat

Abstract

Eight mature Arabian horses were assigned randomly to two groups. The two groups were fed either a control diet or a diet similar in which 10 % corn oil was substituted for cracked corn after a baseline SET. Horses were interval trained in a climate controlled barn 4 days per week on a high speed equine treadmill at a slope of 6 %. An SET was performed at 0, 6 and 11 wk of training. The horse walked at 1.6 m/s, then speed was increased every 3 min until the horse showed signs of fatigue. Blood samples were taken every 3 min prior to each speed increment and analyzed for glucose, lactic acid, cholesterol, triglycerides, total protein, and muscle glycogen. Muscle biopsies were taken before and after exercise and analyzed for glycogen. Lactate and glucose were plotted against speed and fitted to a broken line model. Resulting parameters were tested for significance by analysis of variance. Diet had an effect in SET 3 and SETs 2 and 3 combined, but not SET 2 alone. There were 3 effects of the fat diet on the lactate curve. The high fat diet tended to increase the lactate threshold (x-intercept at point of inflection) during SET 3 (P = 0.16) and SETs 2 and 3 combined (P = 0.08). Lactate threshold ranged from 3.88 to 6.53 m/s. The high fat diet also increased the plasma lactate level prior to the lactate threshold (y intercept) during SET 3 (P = 0.11) and SETs 2 and 3 (P = 0.10). The third effect was an increase in slope after the point of inflection in SET 3 (P = 0.03). Training increased the lactate threshold from 4.77 ± 0.22 m/s in SET 2 to 5.82 ± 0.22 m/s in SET 3. Plasma glucose and lactate concentrations were correlated (R² = 0.848, P < 0.0001). Mean values for SET 2 and 3 combined were 4.42 ± 0.29 and 5.09 ± 0.20 m/s for glucose and lactate thresholds, respectively, in the control group, and 4.13 ± 0.33 and 5.50 ± 0.23 m/s, respectively, in the fat group. There was a strong diet * speed interaction for triglycerides (P < 0.0001). Cholesterol tended to be higher in the high fat group during SET 3 (P = 0.15). Muscle glycogen decreased with exercise, however there was no significant difference between diets. These results show that full metabolic adaptation to a high fat diet was achieved in 11 wk but not 6 wk. The higher speeds of the lactate thresholds with both training and high fat diet were small but consistent.