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Equine Vet J Suppl. 1996 Jul;(22):54-62.

Sweat fluid and ion losses in horses during training and competition in cool vs. hot ambient conditions: implications for ion supplementation.

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Department of Pathobiology, Ontario Veterinary College, University of Guelph, Canada.


The objectives of this study were to: 1) determine incremental and total sweat fluid and ion losses during and following (a) exercise training and (b) a treadmill Speed and Endurance exercise test (SEET) which simulated running speeds and distances required for each phase of an Olympic level (CCI****) 3-day-event in cool and hot ambient conditions and 2) determine the requirement for ion supplementation based on the calculated ion losses associated with these activities. Six exercise-trained Thoroughbred horses completed 2 weeks of exercise training in each of 2 ambient conditions: cool, dry (CD, room temperature [T] = 20-22 degrees C, relative humidity [RH] = 45-55%), or hot and humid (HH, T = 33-35 degrees C, RH = 80-85%). Following the 2 week period of training in either CD or HH conditions, horses completed a SEET under similar conditions (either CD, or hot and dry (HD, T = 33-35 degrees C, RH = 45-55%). Sweating rate and sweat ion composition for each 5 min interval was determined from sweat samples collected from a sealed pouch attached to the lateral thorax. Total sweat fluid losses during training in the heat were 2- or 3-fold greater when compared to CD. Similarly, sweat fluid losses associated with the SEET in HD were almost double (19.2 litres) the losses in CD (11.7 litres). Total calculated ion losses associated with 2 h of training in HH (3724 mmol; 115.2 g) were significantly greater when compared to CD (1413 mmol; 43.5 g). Following the SEET and a 30 min recovery period, total ion losses in CD were 3636 mmol (112.2 g) compared with 6519 mmol (200.6 g) in HD. The differences in ion losses represent the increased sweating rates stimulated by higher core temperatures during moderate to high intensity exercise in warmer ambient conditions and increases in sweat ion concentrations associated with higher sweating rates. Extracellular fluid (ECF) ion losses during daily exercise training and the SEET were also calculated from changes in plasma ion concentrations and ECF volume. Calculated ECF ion losses were significantly higher in hot ambient conditions but were approximately 50% less than calculated sweat ion losses. The calculated sweat ion losses incurred during daily exercise training in hot and humid ambient conditions are > 3-fold higher than losses measured following exercise training in cooler conditions. Whereas fluid regulating hormones may have reduced urinary and faecal losses of ions during 2 weeks of training in HH, the quantity of sodium, potassium and chloride calculated to have been lost in sweat during the SEET in HD exceeded the daily dietary intake of these ions and suggests the need for appropriate ion supplementation during training and competition in hot ambient conditions.

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