NEW METHODS FOR ASSESSING SUBSTRATE UTILIZATION IN HORSES DURING EXERCISE

There are two major goals in designing diets and feeding regimens for athletic horses: 1) to meet the caloric requirements for maintenance and the work performed; and 2) to optimize substrate availability and utilization during exercise. Although there is a substantial body of knowledge pertaining to the digestible energy requirements for working horses, the effects of different diets and pre-exercise feeding strategies on substrate utilization during exercise are less well understood. Several important questions arise when considering the effects of a dietary manipulation on substrate metabolism during exercise. First, do such manipulations alter substrate availability during exercise? (e.g. greater availability of muscle glycogen or circulating free fatty acids). Second, does the manipulation alter the mix of substrates utilized during exercise? (e.g. greater use of fat when fed a fat-supplemented diet). Finally, is there a change in the relative contribu-tions by muscle (glycogen, triacylglycerols) vs. non-muscle (blood glucose and fatty acids) fuel sources to energy production? Particularly during prolonged athletic activities, such as endurance rides and the speed and endurance test of