Abstract :
Athletic performance is initially enabled but then becomes limited by changes in body fluids. This subject is reviewed in three parts: physiological responses, replacement strategies, and dietary influences. The first part provides a physiological basis for understanding body fluids and typical changes during exercise, with emphasis on water and electrolytes. It describes the volume and distribution of body water, and the water shifts and changes in concentrations of electrolytes that occur during strenuous exercise, whether brief or prolonged. These changes may relate to muscle activity or reflect losses via sweat or expiration. Changes in electrolyte concentrations influence acid-base status, with decreasing concentrations of strong ions in the horse (but not most species) tending to exacerbate the acidogenic effect of exercise. Electrolyte changes during exercise also tend to exert many effects on neuromuscular excitability, with the extracellular potassium concentration likely to play a dominant role. Relative losses of water and electrolytes lead to hypo-, iso- or hyper-osmotic hypohydration. Loss of equine hypertonic sweat is conducive to hypotonic plasma, and this hypotonicity inhibits voluntary water intake. Reduction in plasma volume initially increases oxygen carriage per unit volume so favors oxygen transport but, if more extreme, risks hypotension or acute heart failure. Reduction of cellular water initially sustains plasma volume but eventually compromises performance. Hypohydration reduces the sweating rate and compromises heat elimination. Increasing muscle temperature at first improves performance (“warming up”) then induces fatigue. The regulation of body water volume is controlled to a large extent by conservation of sodium. Total water turnover is related to energy expenditure, and both variables are dependent largely on the same internal and external factors. The horseʹs homeostatic capacity may be influenced by diet in general or be supported by supplementation with water and electrolytes before, during or after an event. Potentially beneficial replacement strategies and dietary designs will be discussed in parts two and three of this review.
