Objective: To quantify total fluid loss in sweat of Thoroughbreds during >3 hours of low-intensity exercise in controlled conditions and to calculate and compare estimated ion losses in sweat, according to 3 methods.
Animals: 6 exercise-trained Thoroughbreds.
Procedure: Fluid and ion losses in sweat were measured in 6 horses exercising at 40% of the speed that elicited maximum oxygen consumption for 45 km. Horses were given a 15-minute rest period at the end of three 15-km exercise phases. Horses completed 2 exercise trials. Ion losses in sweat were calculated, using measurements of local sweating rate and sweat ion composition (SWT), change in net exchangeable cation content (CAT), and change in extracellular ion content (PLAS) derived from plasma total solids and ion concentrations.
Results: Measurement of SWT revealed a mean (+/- SEM) fluid loss in sweat during 45 km of exercise of 27.5 +/- 1.6 L. Total ion loss in sweat was approximately 241 g or 7.8 mol with higher sodium losses in the second and third phases of exercise compared with the first phase. Losses of sodium and potassium calculated by SWT or CAT were not significantly different from each other, whereas losses of these ions as determined by PLAS were significantly lower.
Conclusions and clinical relevance: Calculation of ion losses from a mean whole body sweating rate extrapolated from either local sweating rate and sweat ion composition or from change in net exchangeable cation content provide similar results, whereas ion losses determined by changes in extracellular ion content derived from plasma total solids and ion concentration results in underestimation of actual losses.