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Horses can use dietary carbohydrates, fats, and protein to generate energy for exercise. In general, carbohydrates are used first, with fats being used as a secondary energy supply and protein being the last fuel used.

Using this sampling technique, scientists can quantify changes in tissue substrate storage and use resulting from dietary and exercise interventions.Various techniques are available to scientists assessing substrate metabolism in the horse during exercise. Tissue sampling is one method that yields information about what energy sources are being drawn upon and the efficiency of the horse’s metabolic processes.

Samples of skeletal muscle are used to quantify various aspects of metabolism. In horses, use of percutaneous muscle biopsies (most commonly, sampling of the middle gluteal muscle) allows scientists to examine effects of the intensity and duration of exercise on rates of muscle glycogen utilization. This technique has been used to determine the effects of different feeding strategies on storage and mobilization of muscle glycogen during exercise. Horses fed a carbohydrate-rich diet can have a 12% increase in the resting muscle glycogen concentration compared to traditional or fat-supplemented diets<sup>1</sup>.

Using this sampling technique, scientists can quantify changes in tissue substrate storage and use resulting from dietary and exercise interventions. However, interpretation of biopsy data in relation to whole-body metabolism is limited by the static and local nature of the sample. As samples can only be obtained at fixed time points (e.g., before and after a period of exercise), it is not possible to determine the dynamics of muscle glycogen use during exercise. Rather, the net rate of glycogen utilization can be estimated in these circumstances.

Furthermore, the change in glycogen content measured in one muscle (e.g., middle gluteal muscle) may not be representative of glycogen utilization in other working skeletal muscle. Even within a single muscle, differences in the fiber-type composition (percentage of type I vs. type II fibers) of samples collected before and after exercise may confound interpretation of the data.

Finally, measurement of substrate concentrations in muscle samples does not allow for differentiation of the contributions to energy production by intra- vs. extra-muscularly derived substrates. Clearly, this is a major limitation for studies of substrate metabolism during sustained exercise when a substantial proportion of fuel substrate will be derived from non-muscle sources (i.e., liver, adipose tissue, gut).

  1. Essen-Gustavsson B., E. Blomstrand, K. Karlstrom, et al. Influence of diet on substrate metabolism during exercise. In:  Third International Conference for Equine Exercise Physiology, ICEEP Publications, Davis, CA, 1991: pp 289-298.
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