Rate and composition of sweat fluid losses are unaltered by hypohydration during prolonged exercise in horses.

This research explores how sweat rate and composition in horses remain consistent during prolonged low-intensity exercise, even with moderate hypohydration. It also looks at how evaporative heat loss is processed through respiratory and cutaneous avenues.

Investigating Sweat Rate and Composition in Horses

The study tracks sweat rate and the ionic composition of sweat fluid losses in six horses. The experiment was conducted during three 15-km phases of exercise, with a targeted intensity of about 40% of maximum oxygen uptake.

• The sweat rate increased rapidly during the initial 20 minutes of exercise, but throttled to a consistent 24-28 ml per square meter per minute for the remainder of each phase.
• The concentrations of Sodium (Na) and Chloride (Cl) in sweat also surged within the first 30 minutes of exercise, remaining stable thereafter.
• The osmolality of the sweat, along with the concentrations of Sodium and Chloride, showed a positive correlation with sweat rate. This means that higher sweat rates resulted in higher Sodium, Chloride, and osmolality levels.
• Interestingly, the concentration of Potassium (K) in the sweat decreased during exercise, but there was no correlation found between this decrease and the sweat rate.

Fluid Losses and Hypohydration Effects

In relation to fluid losses, a key finding was that there was an average fluid loss of roughly 33.8 liters during the experiment period.

• This resulted in about a 21% reduction in plasma volume and about an 11% decrease in total body water.
• The ensuing hypohydration, approximately 6%, didn’t show any changes in sweat rate, sweat composition, or heat storage in the horses.

Evaporative Heat Loss

The researchers also sought to understand how evaporative heat loss was divided between respiratory and cutaneous components during the exercise phases.

• They found that respiratory and cutaneous evaporative heat loss accounted for approximately 23% and 70% respectively, of the total heat dissipated during exercise.
• The division of heat loss remained uniform in each exercise phase, despite the hypohydration.

The conclusions drawn indicate that sweat rates and the relative proportions of respiratory and cutaneous evaporative heat loss remain constant in horses, even under prolonged low-intensity exercise and moderate hypohydration.