Metabolic Response to Exercise in Arabian and Thoroughbred Horses

Many people tend to think of Thoroughbreds as racehorses that excel at high-intensity exercise, and of Arabians as endurance horses whose strengths lie with low-intensity exercise. Are these perceptions based simply on the most common use of each breed, or on particular innate characteristics or abilities that set the breeds apart? A study conducted at Kentucky Equine Research was designed to test the hypothesis that Thoroughbreds are better suited to high-intensity exercise because they have greater aerobic and anaerobic capacities than Arabians.

Arabians have a higher proportion of oxidative muscle fibers (types I and IIa) than Thoroughbreds. For that reason, it was hypothesized that Arabians would make greater use of fat for energy. The study tested this hypothesis as well.

The study compared selected measures of exercise capacity and metabolism in a small group of Thoroughbreds and Arabians of similar age, training background, and diet. Five Thoroughbred and five Arabian geldings were placed on identical exercise programs and fed identical diets for two months. Each horse then performed three exercise trials on a high-speed treadmill set at a 3-degree incline.

Trial 1 was an incremental test in which each horse ran at a steadily increasing pace until he could no longer maintain speed. This trial determined each horse’s aerobic capacity.

Trial 2 was a sprint test for estimation of maximal accumulated oxygen deficit in which horses ran to fatigue at 115% of maximal oxygen usage. In trial 3, horses exercised at 35% of maximal oxygen usage for 90 minutes. Oxygen consumption and carbon dioxide production were determined during exercise. Blood was sampled during and after exercise in trial 2 for the measurement of plasma glucose and free fatty acids.

Thoroughbreds reached higher speeds than Arabians, and maximum oxygen consumption was also greater in Thoroughbreds. Thoroughbreds were able to maintain their top speed for a greater distance than Arabians. During trial 3, free fatty acids were higher in Arabians than in Thoroughbreds between 60 and 90 minutes, while respiratory exchange rate was lower during this period. These results indicate greater use of fat for energy by the Arabians.

This study supported both hypotheses: that Thoroughbreds are better suited to high-intensity exercise because they have greater aerobic and anaerobic capacities than Arabians, and that the respiratory exchange ratio would be lower in Arabians than in Thoroughbreds during low-intensity exercise, showing that the Arabians made greater use of fat for energy.

Results were consistent with those of other studies that have indicated better speed performance by Thoroughbreds and better endurance performance by Arabians. The researchers explained that the apparent breed difference in aerobic and anaerobic capacity might reflect breed variation in muscle fiber types and the muscle concentrations and activities of enzymes involved in the conversion of glucose to energy.

It is tempting to interpret these findings to mean that Arabians used for endurance riding should be given high-fat diets, and many endurance horses actually do perform well on diets with a relatively large proportion of fat. However, many factors (genetics, exercise program, age, dietary variations, time of feeding before exercise, training, general health, rider characteristics) affect the way a horse performs. This study was able to control some, but not all, of these factors. Further studies may help to determine the exact mechanisms of energy metabolism during exercise of different intensities.