2007, Piran, Slovenia

Non-thermal factors
IMPROVED FLUID REGULATION WITH AEROBIC FITNESS
James D. Cotter 1 , Troy L. Merry 1 , Philip N. Ainslie 2
1 School of Physical Education and 2 Department of Physiology,
University of Otago, Dunedin, New Zealand
Contact person: jim.cotter@otago.ac.nz
INTRODUCTION
It is sometimes advised that humans cannot adapt to dehydration, and thus dehydration should
be avoided during competition as well as training. Athletes who undertake endurance exercise
on a daily basis typically dehydrate during this training, and show numerous physiological
adaptations to the multiple stresses of exercise. Because a common feature of these
adaptations is a reduced neuroendocrine response to a given level of stress, and because some
adaptations are related to fluid regulation (e.g., higher resting blood, plasma and extra-cellular
fluid volumes), it is conceivable that adaptations to dehydration might exist. Therefore, the
purpose of this study was to test the hypothesis that aerobically trained people have altered
fluid regulatory responses to dehydration, indicative of adaptation. We examined the osmotic,
endocrine, perceptual and behavioural responses of trained and untrained males to exercise
under maintained, restricted and ad libitum fluid balance.
METHODS
Participants were six untrained and six trained males, unacclimated to heat. Trained
participants had higher training frequency (6.0 ±1.3 vs 1.0 ±0.8 d·wk -1 ) and ~50% higher peak
aerobic power (mean ±SD, cycling VO &
2 peak; 64.4 ±7.7 vs 44.7 ±4.0 ml·kg -1 ·min -1 ). The trained
group was of similar age (31 ± 9 vs 25 ± 6 y), but were lighter (71.8 ±4.3 vs 78.6 ±9.7 kg) and
leaner (7.6 ±1.5 vs 15.7 ±4.5% body fat). The study was approved by the University of Otago
Human Ethics Committee, and participants provided their informed consent in writing after
screening for medical contraindications.
Protocol: After fitness testing and two full familiarisation trials,participants undertook two
80-min exercise trials (Tdb = 24.3 ± 0.6ºC, rh = 50%, va=4.5 m·s -1 ); one whilst euhydrated
throughout (EUH), and one with mild hypohydration (HYPO) by ~2% body mass, in balanced
order, 1-3 weeks apart. Baseline hypohydration was achieved using a 50-min heat+exercise
session on the evening before both EUH and HYPO, with full rehydration for EUH only (but
equivalent carbohydrate replenishment). The exercise trial involved 40-min cycling at
70% VO &
2 peak with full replacement of fluid loss (corrected for CO2 loss) in EUH versus 20%
rehydration in HYPO, before a 40-min self-paced performance trial with continued full
rehydration in EUH versus ad libitum drinking in HYPO. Rehydration was by ingestion of
isotonic saline with artificial sweetener at 10-min intervals. Venous blood was drawn at rest,
and after 10-, 40- and 80-min exercise.
Measures: Respiratory gas was sampled at 15-min intervals throughout familiarisation trials
to determine work rates and to estimate the mass of substrate oxidised in a trial, allowing
calculation of CO2 loss for rehydration. Nude body mass (±20 g, Wedderbrun scales Ltd,
Dunedin, NZ) and urine specific gravity (USG: hand refractometer, Atago, Japan) were
recorded before and after all trials. The cycling work rate was set and recorded using an
electromagnetically-braked Velotron cycle ergometer (v1.5, RaceMate Inc, Seattle). Plasma
was measured for osmolality (Dew point osmometer, Wescor, USA), sodium concentration
(Cobas c111, Roche, Switzerland), and arginine vasopressin concentration (AVP:
commercially, using extracted RIA, Endolab, NZ). Thirst was rated on a 9-point scale.
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