2007, Piran, Slovenia

Sweating
THE SWEAT SECRETION AND SODIUM LOSS QUADRANT: A
CONCEPT FOR PREDICTING HYDRATION REQUIREMENTS.
Nigel A.S. Taylor, Anne M.J. van den Heuvel, Lara S van den Wijngaart
Human Performance Laboratories, University of Wollongong, Wollongong, Australia
Contact person: nigel_taylor@uow.edu.au
INTRODUCTION
It is well known that, for some physically-demanding occupations and some long-duration
athletic pursuits, individuals must consider not just fluid replacement, but also electrolyte
replacement. Of the solutes contained within sweat, sodium and chloride are dominant
(Patterson et al., 2000). However, whilst one can easily determine water replacement on the
basis of mass changes, it can be difficult to determine, in the absence of sophisticated analysis
equipment, how much sodium an individual should replace. It occurred to us that one may be
able to predict this simply on the basis of the steady-state exercise intensity (heart rate), and
herein we report how one may achieve such a prediction.
Our predictive model uses a novel application, and the interaction, of four well-established
physiological relationships. First, it has long been known that oxygen uptake increases
linearly with increments in work rate and heart rate (Astrand and Ryhming, 1954). Second,
since exercise results in the predictable liberation of thermal energy, then body core
temperature is intrinsically linked with oxygen uptake (Saltin and Hermansen, 1966). Third,
sweat rate increases curvilinearly with core temperature elevation (Wyndham, 1967). Finally,
sweat sodium loss displays a positive linear relationship with sweat rate (Cage and Dobson,
1965). From this knowledge, it becomes possible, once one has determined the typical
operating heart rate, to predict the oxygen uptake, core temperature, and eventually the sweat
rate and sodium loss associated with various exercise and thermal steady states.
Of course, such a prediction, like all mathematical models, is inherently imprecise. However,
its predictive precision can be increased if one applies the physiological characteristics of the
population sample that is most closely related to the individuals in question. For example, to
more effectively define these physiological characteristics, one must consider the gender, age,
endurance fitness and heat adaptation status of the individual, and also the environmental
conditions in which the individual is working. Even with these characteristics defined, the
prediction of sweat rate and sodium loss will remain somewhat imprecise. Nevertheless, one
hopes that such a prediction may have some utility, particularly when applied to homogenous
groups of individuals. If we are proven to be incorrect on all counts, then at least we had fun
doing the experiments.
METHODS
Six fully-hydrated and unacclimatised, young women participated in two cycling trials,
conducted at the same time of day, in a heated climate-controlled chamber (36 o C, 50%
relative humidity). Within each trial, subjects were exposed to a series of three step increases
in work rate, designed to elicit steady-state heart rates between 120-170 beats.min -1 . The first
work rate was set to obtain a heart rate of 120 beats.min -1 (30 min), while the order of next
two work rates was randomised within each trial (25 min): trial 1: 120, 140, 170 beats.min -1 ;
trial 2: 120, 150, 160 beats.min -1 . Subjects rested for 5 min between successive steps.
Trials were scheduled during the follicular phase of the menstrual cycle. Subjects were asked
to refrain from strenuous exercise and the consumption of alcohol and tobacco during the 12 h
before to each trial. Prior to testing, subjects were instructed to drink 15 mL.kg -1 of additional
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