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In designing diets and feeding regimens for athletic horses, the two main goals are to meet the caloric requirements for maintenance and the work performed, and 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 contributions by muscle (glycogen) 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 a three-day event, both the availability of fuel substrates and the efficiency with which these substrates are utilized can affect exercise performance. Therefore, recommendations for feeding these animals must be based on a sound knowledge of the effects of diet and feeding on substrate availability and utilization during exercise.

Measurement of the plasma concentrations of glucose, NEFA, glycerol, and various amino acids have been widely employed in equine studies. There are several advantages of this method. In laboratory studies involving treadmill exercise, blood samples can be readily obtained at frequent intervals. Furthermore, laboratory analysis of these samples is straightforward and relatively inexpensive. However, it is important to recognize that plasma concentrations of a substrate result from the difference between the rate of release of a substrate into plasma and its rate of removal from the plasma compartment (tissue uptake).

For example, various studies conducted at Kentucky Equine Research (KER) and other institutions have clearly demonstrated that the nature and timing of a pre-exercise meal markedly alter plasma glucose concentrations during exercise. In one KER study, consumption of corn 2.5 to 3 hours before exercise resulted in a large glycemic response. At the start of a standardized exercise test, plasma glucose concentrations in the fed horses were 1-2 mmol/l higher when compared to trials in which no grain was fed. Conversely, during exercise there were marked decreases in plasma glucose concentrations in trials preceded by corn feeding, whereas plasma glucose was unchanged in the control (no pre-exercise meal) trial. These data provide some evidence for an increase in utilization of plasma glucose under circumstances of increased substrate availability (pre-exercise feeding of carbohydrate).

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