Body fluids and exercise: Replacement strategies

Athletic performance is initially enabled but then becomes limited by chang es in body fluids. This review deals with potentially beneficial interventi ons replacement strategies. It is the second of a trilogy on exercise with a focus on body fluids. The first part (published previously in this Journa l) concerned physiological changes in body fluids likely to affect performa nce. These changes are certain to tax the homeostatic capacity of a competi tive horse, which may be supported by supplementation with water and electr olytes, before, during or after an event. The simplest replacement strategy is to completely offset depletion, so as to maintain or restore the restin g status. A preferred replacement strategy emphasizes more frequent and sma ller doses during an event, so as to avoid over-riding the initial benefici al changes in body fluids. Because the turning points from enabling to limi ting are not well established, effective electrolyte,replacement currently depends as much on the rider's development of a specific dosage regimen (am ounts and times), for an individual horse as on any scientist's rational de sign of a flexible formula for all horses. Marketed formulas are based on s weat loss and changes in plasma. Most contain Na, K and Cl; some also have Ca, Mg, P, glucose and nutraceuticals. Attention to hyperkalemia at speeds over 4 m/s suggests the need for two formulas - one without potassium for u se during an event, the other with potassium for use before and after. Illu strative studies are examined for the validity of their physiological inter pretations and their practical implications. They indicate various advantag es from. the administration of hypertonic mixtures of electrolytes in paste s or slurries a few hours before as well as during and after an event, prov ided that the horse has ample opportunity to drink. Nevertheless, the physi ology of absorption suggests that hypertonic mixtures should be regarded as unsuitable for a clinically hypohydrated horse until shown otherwise. The third part of this series will discuss the influence of nutrition and feedi ng management on body fluids and performance.