Journal Title (Medline/Pubmed accepted abbreviation): J. Appl. Physiol.
Page numbers: 972-980
doi (if applicable): 10.1152/japplphysiol.01193.2010.
Background: Training in high heat environments helps the body to adapt to temperature regulation under those conditions. The authors wished to examine how dehydration, as opposed to euhydration, affects the thermoregulatory adaptations to exercise.
Hypothesis: Hypohydration (i.e., decreased plasma volume) will impair the adaptive thermoregulatory response to exercise as compared with euhydration.
Subjects: Seven recreationally active males, age 20.6 ± 2.8 yrs old.
Experimental design: All subjects participated in each of 4 trials in this order: 1) euhydration before training → 2) hypohydration before training → (5-day training period, see below) → 3) euhydration after training → 4) hypohydration after training
Treatments and protocol:
The subjects’ VO2peak
(also known as VO2max
) was determined at least one week before the start of the experiment. To achieve hypohydration, subjects ate a low salt diet (4 g salt, or 1.6 g sodium, per day) as opposed to the normal 12 g salt/4.8 g sodium per day for 2 days before the trial. Also, the day of the trial, they consumed a diuretic about 3 hrs before the thermoregulatory test in order to achieve a total goal of 3% loss of body weight.
The body’s response to high heat and exercise was assessed with a thermoregulatory response test during both euhydration and hypohydration (see Experimental Design above). For this thermoregulatory test, participants cycled at 65% of their VO2peak
for 30 min in a room that was 28.0 ± 0.1°C and 46 ± 1% relative humidity. These measurements were acquired:
- Heart rate (every min)
- Blood pressure (1x/min)
- Esphageal temperature (Tes)
- Skin temperature (Tsk) from a combination of the forearm, chest, and thigh
- Forearm skin vascular conductance (FVC) (2x/min), a marker of cutaneous vasodilation
- Sweat rate (every 5 sec)
- Total sweat rate (SR)
- Cardiac output (2x before and 3x during exercise)
- Total plasma protein concentration (blood samples were acquired 2x before and 3x during exercise)
- Plasma albumin concentration
- Osmolality of plasma
These parameters were acquired in the euhydrated state and the hypohydrated state (performed on different days but at the same time of the day). After the hypohydration trial, there was a > 5 day period before the training period started. The subjects then engaged in a 5-day training period where they cycled on a stationary bicycle for 30 min/day at 30.0 ± 0.1°C and 50 ± 1% relative humidity. (Subjects were euhydrated during training.) The thermoregulatory test was repeated for euhydration and then hypohydration, beginning on the 2nd day after terminating the 5-day training protocol.
The adaptive response of some thermoregulatory variables (e.g., FVC, plasma volume, stroke volume) is more sensitive to the effects of hypohydration than other variables such as sweat rate or cardiac output. The study points to the importance of the maintenance of good hydration status to allow for heat acclimation.
Hypohydration was induced via the use of a diuretic and some degree of dietary sodium restriction in this study. It is not clear if this method of causing hypohydration is representative of exercise-induced hypohydration that is associated with increased sweat losses.