Exercise-induced dehydration with and without environmental heat stress results in increased oxidative stress


Journal Title (Medline/Pubmed accepted abbreviation): Appl. Physiol. Nutr. Metab.
Year: 2011
Volume: 36
Page Numbers: 698-706
doi: 10.1139/H11-080

Summary of background and research design:

Background: Dehydration can cause stress on the body leading to sub-optimal performance. Oxidative stress is a result of production of reactive oxygen species (ROS). Although some ROS are naturally formed during the act of exercising itself, excessive ROS production can lead to unhealthy amounts of inflammation and compromised cell function.  

Hypothesis: Dehydration will increase oxidative stress, especially when dehydration occurs in hot environments.

Subjects: Seven male cyclists, competitive yet not used to training in the heat, age 36 ± 6 y

Experimental design: randomized, double-blind, cross-over design 

Treatments:
Environment: warm environment (W): 33.9 ± 0.9°C or 93.0 ± 1.6°F or an environment with a neutral temperature (T): 23.0 ± 1.0°C or 73.4 ± 1.2°F.
Hydration: Dehydrated (DE): no fluid consumed during the trial or euhydrated (EU): fluid was consumed to match the mass lost during exercise (1 kg body weight lost = consumption of 1 L of water)

Protocol: The participants reported to the laboratory 5 times; the first time was to familiarize themselves with the experimental protocol and determine their lactate threshold (LT), and the other 4 times were to complete each of the 4 treatments. At least 1 wk separated the exercise trials. On the day of the trial, the participants arrived not having eaten for 2 hrs and having drunk 500 mL (about 2 cups) of water. They rested for 20 min while pre-test blood samples were collected, hydration status was confirmed, and body temperature and weight were measured. The subjects then cycled for 90 min at 95% of their previously determined LT. This exercise has been shown previously to cause ≥ 2% dehydration. Blood was collected at 45 min and 90 min. After a 15 min break, they completed a 5-km time trial. Afterwards, body weight and a final blood sample were collected. Blood was analyzed for oxidative stress with thiobarbituric acid reactive substances (TBARS) and total and oxidized glutathione serving as markers of oxidative stress.


Summary of research findings:
  • There were comparable increases in oxidized glutathione from pre- to postexercise in the dehydration trials vs. the euhydration trials in both the thermally neutral and warm environmental conditions. Thus, dehydration appeared to be the major factor governing the increase in oxidized glutathione.
  • There was no effect of time or treatment on TBARS, indicating that this assay did not detect increased oxidative stress from the exercise.
  • Expression of heat shock proteins (HSP) is enhanced under conditions of stress. HSP in white blood cells was increased with training but was not different between temperature or hydration treatments.
  • Performance was significantly hindered during the dehydration-warm temperature trial (p< 0.02). This was reflected in both reduced power output and distance covered over a period of time in the during 90 min of exercise and the 5-km time trial.

Key practice applications:

Dehydration and heat stress are both stressful situations for the body, especially during exercise, and the combination was shown to greatly hinder performance. In this study, dehydration caused an increase in the oxidative state of the blood. Assure that athletes keep hydrated during sporting events. This can be done by weighing the athlete before training and at intervals during training. Some weight loss is okay, but losing more than 2% of body weight puts an athlete at risk for compromising performance. Sports beverages and/or water should be consumed during exercise, especially in warm environments.


 

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