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Sweat rate and the loss of electrolytes in sweat in conditions of heat stress

Bates, Graham P (1994) Sweat rate and the loss of electrolytes in sweat in conditions of heat stress. PhD thesis, Murdoch University.

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In hot working conditions, high sweat rates with excessive loss of body fluids may result in dehydration and electrolyte imbalance. It is well established that dehydration and/or electrolyte disturbances will impair work performance, and if prolonged or severe can pose a serious risk to health. The lesser condition of hypohydration is undoubtedly widespread in the workplace, and may be indirectly responsible for less than optimal performance and accidents.

One of the principal aims of this study was to develop and validate a new sweat collection method. Following validation of this method, fluid and electrolyte loss from a population of male workers of varying fitness and body composition was documented. Based on sweat rate and electrolyte loss in sweat, guidelines for fluid replacement when working or exercising in conditions of heat stress were formulated. Using these same measurement methods, the minimum duration of heat exposure required to trigger acclimatisation was sought using sweat sodium as the indicator.

Twenty nine males, aged between 18 and 50 years, volunteered to exercise in the heat for 35 mins on two consecutive days in winter and summer (40% of maximal aerobic power; 30 °C; 50% RH). Prior to heat exposure, all subjects had their body composition and fitness assessed. Subjects were weighed before and after exposure to record sweat rate. During exercise, after sweat onset, sweat was collected by attaching 4 small collecting devices (9cm diameter) to the upper arms and legs. Following exercise, the sweat collected in these collection devices was evacuated and weighed, and the sodium and potassium concentrations analysed.

Linear regression analysis provided validation of the collection method, by satisfactorily comparing electrolyte concentrations from both the right and left limbs. A very high degree of linearity bi-laterally for arms and legs but significant differences between arms and legs in summer was demonstrated. The same strength of linearity was reproduced in winter. Statistically significant relationships in decreasing strength were noted between sweat rate and fitness, sweat sodium and core temperature increase, and between sweat potassium and sweat rate.

Based on the results of this study, rehydration at the rate of 500 mL/hr (250 ml every 30 mins) is recommended for people working in all but extreme heat (>45°C). Electrolyte supplements to avoid hyponatremia (sodium and potassium), are not required in the workplace, but are warranted in endurance sporting events (>3 hrs). The ability to predict the susceptibility of an individual to fluid and electrolyte disturbances, cannot be made from age, body composition, ethnicity or V02 max, although a high VO2 max appears to enhance heat tolerance. Sodium loss in sweat is extremely varied and is not statistically related to sweat rate. Acclimatisation to heat exposure accounts for a significant decrease in sweat sodium loss, and an increased sweat rate during summer compared with winter. This advantageous physiological adaptation requires a minimum of nine hours heat exposure to initiate.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Cena, Kris
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