Sex differences in fluid balance responses during prolonged exercise

Journal Title (Medline/Pubmed accepted abbreviation): Scand. J. Med. Sci. Sports
Year: 2013
Volume: 23
Page Numbers: 918-206
doi: 10.1111/j.1600-0838.2011.01371.x

Summary of background and research design:
Background: Men tend to sweat more and women tend to ingest more fluid during exercise. This observation leads us to believe that there may be differences in the way males vs. females balance fluids during exercise.

Hypothesis: During a long walk (about 8½ hours), men will exhibit a greater incidence of dehydration. On average, men will also show larger changes in markers of dehydration (ex. greater sodium concentration in the blood, greater change in plasma volume) compared to women.

Experimental Design: repeated measures design, no intervention

Subjects: Males (n = 57) and females (n = 42), age 21 – 82 y

Protocol: The participants were assigned to walk 30 km, 40 km, or 50 km. At baseline, participants were evaluated for height, weight, body composition (skinfold), waist circumference, resting heart rate, and blood pressure. Immediately after exercise, body mass was measured again. Fluid ingestion was noted for 12 hours preceding the walk and during the walk. Blood was collected before and after the walk to measure sodium concentration and both hematocrit and hemoglobin (from which plasma volume is calculated). A urine sample was provided before and after the walk; urine specific gravity (USG, essentially urine "concentration") is commonly used to assess hydration. Core body temperature was monitored throughout the walk via an ingestible thermometer that was swallowed the night before the walk. Heart rate was measured periodically during the walk and was used to measure exercise intensity: (mean heart rate during exercise) ÷ (maximal predicted heart rate) where maximal predicted heart rate is [208 – 0.7 × (age)].

Summary of research findings:
  • Men and women both performed at 69 ± 11% of their predicted maximal heart rate.
  • On average, men lost about 1.6% of their body weight and women lost about 0.9% of their body weight during the course of the walk (p < 0.001). A loss of more than 2% of one’s body weight categorizes him or her as dehydrated. Of men, 34% were dehydrated while only 12% of women were dehydrated.
  • Fluid intake did not differ between genders during the 12 hours before the walk. During the walk, men drank 2.1 ± 1.2 L of fluid and women consumed 2.2 ± 0.8 L of fluid. When correcting for body mass, men consumed a lower amount of fluid per kg body weight. Both genders preferred a little more than half of the fluid to be water, about 11-16% to be a sports beverage, and 32% to be "other".
  • Men exhibited a decrease in plasma volume (average -1.5%) while women demonstrated an average increase in plasma volume (+1.4%).
  • A USG of ≥1.030 g/mL is considered a biomarker for dehydration. At the end of the walk, 45% of men and 26% of women exhibited high USGs.
  • During the walk, men demonstrated an increase in blood sodium concentrations while women showed no significant difference between pre- and post-exercise. "Hypernatremia" is clinically high blood sodium at ≥145 mM. None of the females were afflicted but 27% of men had hypernatremia.

Key practice applications: There were clearly more incidences of dehydration in men in this study than in women. This result is likely caused by differences in men and women in regard to sweat threshold and sweat rate; men tend to sweat at a lower core body temperature, tend to have a different morphology (more muscle, less fat), and have different levels of hormones that regulate fluid balance. It is important to remind all people to stay hydrated while performing moderate exercise, especially outdoors such as on a long hike. Because men may be more susceptible to dehydration than women, it is important to be aware of this and assure adequate intake. Monitoring body mass is the easiest way to assure that one does not lose more than 2% body mass during exercise.
Be cautious as it is possible to drink too much water and experience hyponatremia or too low blood sodium concentrations. Do not drink if fluid intake is uncomfortable.
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