Effects of hypohydration on thermoregulation during exercise before and after 5-day aerobic training in a warm environment in young men


Journal Title (Medline/Pubmed accepted abbreviation): J. Appl. Physiol.
Year: 2011
Volume: 110
Page numbers: 972-980
doi (if applicable): 10.1152/japplphysiol.01193.2010.

Summary of Background and Research Design

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.


Summary
  • The authors were successful in achieving hypohydration for the hypohydration trials. This was confirmed by decreases in plasma volume and body weight.
  • When comparing the after versus before training tests, the rise in plasma volume was much greater for the euhydrated as opposed to the hypohydrated condition. This indicated that the hypohydration limited the adaptive increase in plasma volume that normally occurs as the result of regular exercise training.
  • The authors also plotted the relationship between Tes (x-axis) and FVC (y-axis). The first observation was that FVC values were lower in hypohydration condition, both before and after training, compared with the euhydration condition. In addition, the euhydration condition allowed for a larger exercise adaptation (i.e., greater increase in FVC at lower Tes, or greater degree of shifting the curve to the left) compared with the hypohydration condition.
  • The authors plotted Tes vs. SR in the same way as Tes vs. FVC. However, in contrast to the results with Tes vs. FVC, the Tes vs. SR relationship was much less influenced by hydration status.
  • The adaptation in stroke volume in response to training was more pronounced in the euhydrated vs. hypohydrated condition. In addition, stroke volume was lower in the dehydration condition vs. the hydration condition both before and after training.
  • The adaptation in cardiac output was largely unaffected by hydration status. As with stroke volume, cardiac output both before and after training was lower in the hypohydrated vs. euhydrated condition.

Interpretation of findings/Key practice applications

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.

Limitations

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.
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