Certain sources of energy may effect the metabolic responses of exercising horses subjected to feed restriction. Eating causes a positive response in thyroid hormones, and exercise may help maintain thyroid hormone production during negative caloric balance.
A study conducted at the University of Kentucky was designed to determine if horses consuming a restricted diet, with 70% of the calories coming from the concentrate source, were better able to maintain thyroid hormone concentrations during exercise when fed prior to exercise.
Initial body weights and condition scores were obtained for eight Thoroughbred geldings with normal thyroid function. The horses were randomly assigned to one of two treatments: RHR, a calorie-restricted diet that provided 70% of the caloric need from the roughage source, or RHC, a calorie-restricted diet using the concentrate source to provide 70% of the caloric need. The diets were adjusted weekly so that all horses maintained an approximate 1 kg (2.2 lb) weight loss per day.
The horses were exercise-conditioned for 55 days prior to exercise testing. At the end of the conditioning period, each horse performed two exercise tests. One test followed a 12-hour fast and the other was conducted two hours after a meal of 2 kg (4.4 lb) of concentrate. This time period had previously been determined to give an adequate thyroid hormone response to feeding. Blood samples were taken pre-exercise, during exercise, and during a 10-minute recovery period after exercise. The samples were analyzed for glucose, insulin, free fatty acids (FFA), lactate concentrations, and the thyroid hormones T4 and T3. Heart rates and time to fatigue were also recorded. The exercise tests were separated by a week, during which all horses continued their conditioning program.
Initial and final body condition scores were 5.5 and 4.75 for the RHR group and 5.2 and 4.5 for the RHC group, respectively. Horses in both treatment groups lost 8% of their initial body weight by the end of the study. Those receiving the RHR dietary treatment had the greater T4 response and a decline in glucose concentrations. In response to fasting, horses receiving the RHR diet had a greater FFA response and numerically longer times to fatigue. Horses receiving the RHC dietary treatment tended to show an increase in T3 concentrations during the test and tended to have lower heart rates when fasted. Insulin concentrations declined during the test when horses were fed but were not affected by diet. Plasma lactate concentrations increased in response to exercise but were not affected by diet or feeding state.
These data suggest that exercise and calorie source may be important to hormonal regulation and energy metabolism in horses subjected to long-term regimens of calorie-deficient diets.
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