Dehydration refers both to hypohydration (dehydration induced prior to exercise) and to exercise-induced dehydration (dehydration that develops during exercise). The latter reduces aerobic endurance performance and results in increased body temperature, heart rate, perceived exertion, and possibly increased reliance on carbohydrate as a fuel source. Although the negative effects of exercise-induced dehydration on exercise performance were clearly demonstrated in the 1940s, athletes continued to believe for years thereafter that fluid intake was not beneficial. More recently, negative effects on performance have been demonstrated with modest (<2%) dehydration, and these effects are exacerbated when the exercise is performed in a hot environment. The effects of hypohydration may vary, depending on whether it is induced through diuretics or sauna exposure, which substantially reduce plasma volume, or prior exercise, which has much less impact on plasma volume. Hypohydration reduces aerobic endurance, but its effects on muscle strength and endurance are not consistent and require further study.
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"Imbalance between fluid intake and fluid loss during prolonged exercise may increase the risk for development of dehydration. Previous laboratory studies showed that substantial fluid/body mass losses were related to an impaired exercise performance (Sawka, 1992; Barr, 1999) and to a larger increase in core body temperature (Montain & Coyle, 1992; Sawka, 1992). Accordingly, dehydration may enhance the development of hyperthermia , heat exhaustion, and heat stroke (Carter et al., 2005; Armstrong et al., 2007). "
[Show abstract][Hide abstract] ABSTRACT: Maintaining a proper fluid balance is important during exercise as athletes are prone to develop dehydration during exercise. Although several factors may regulate the fluid balance, little is known about the role of sex during prolonged moderate-intensity exercise. Therefore, we compared body mass changes and fluid balance parameters in men vs women in a large heterogeneous group of participants during prolonged exercise. Ninety-eight volunteers walked 30-50 km at a self-selected pace. Exercise duration (8 h, 32 min) and intensity (69% HR(max) ) were comparable between groups. Men demonstrated a significantly larger change in body mass than women (-1.6% vs -0.9%, respectively, P < 0.001) and a higher incidence of dehydration (defined as ≥2% body mass loss) compared with women (34% vs 12%, respectively, odds ratio = 4.2, 95% CI = 1.1-16.7). Changes in blood sodium levels were significantly different between men (+1.5 mmol/L) and women (-0.4 mmol/L), while 27% of the men vs 0% of the women showed postexercise hypernatremia (sodium levels ≥ 145 mmol/L). Moreover, men demonstrated a significantly lower fluid intake (2.9 mL/kg/h) and higher fluid loss (5.0 mL/kg/h) compared with women (3.7 and 4.8 mL/kg/h, respectively). Taken together, our data suggest that men and women demonstrate different changes in fluid balance in response to a similar bout of exercise.
Full-text · Article · Aug 2011 · Scandinavian Journal of Medicine and Science in Sports
"Dehydration of greater than 3% of body weight further disturbs physiologic function and increases an athlete's risk of developing an exertional heat illness (Casa et al. 2000). Dehydration can bring about an increase in heart rate, core body temperature and oxygen consumption (Barr 1999). When solid food can be ingested, water alone is enough to rehydrate. "
[Show abstract][Hide abstract] ABSTRACT: The objective of the present study was to determine the effects of exercise with water intake, sport drink intake and no fluid on balance performance and recovery. Seventeen physically active men (age, 22.29 ± 1.61 years; height, 176.24 ± 5.18 cm; weight, 69.47 ± 9.20 kg) volunteered to take part in this study. The Biodex Balance System was used to evaluate balance performance and Overall Stability Index (OSI) scores were used to assess balance performance. The exercise protocol consisted of a 1-hour exercise session at 75% and 85% of maximal heart rate. The recovery period was 20 minutes of rest without fluid intake. In all experiment conditions, balance tests were applied three times as pretest, posttest and recovery. In each one of the three experimental conditions, balance tests were consecutively performed with eyes open and eyes closed. All the measurements and exercise protocols were performed in the morning (between 9 AM and 12 PM), in a specially designed and equipped room, with room temperature at 21–24°C. Repeated-measures ANOVA was used to examine all the conditions. OSI post-exercise was significantly higher than pre-exercise (p < 0.01) and recovery (p < 0.05) for exercise with no fluid and eyes open. There was a non-significant difference in OSI between pre-exercise and recovery. No significant differences in OSI for exercise with sport drink intake and water intake were observed among pre-exercise, post-exercise and recovery. The results of this study show that balance performance decreases after prolonged exercise without fluid intake, and that fluid ingested during sport activities could prevent the decrease in balance performance.
Full-text · Article · Dec 2010 · Journal of exercise science and fitness (JESF)
"The adverse effects of acute induction of dehydration on physical performance and exercise have been demonstrated in many studies on adult athletes and outdoor workers (Barr, 1999; Kleiner, 1999). In adults, a 2% loss in body fluids was estimated to result in a decrease of 20% in physical performance (Sawka & Pandolf, 1990). "
[Show abstract][Hide abstract] ABSTRACT: Voluntary dehydration is a condition where humans do not drink appropriately in the presence of an adequate fluid supply. This may adversely affect their physical and intellectual performance. The present study aimed to describe the prevalence of voluntary dehydration among elementary school children of different ethnicities and countries of birth.
Four hundred and twenty-nine elementary school children, aged 8-10 years, from four subpopulations (Israeli-born Jewish and Bedouin-Arab children, and immigrant children who recently arrived to Israel from Eastern Europe and from Ethiopia) were studied. The level of dehydration was determined by noontime urine osmolality, from samples taken over 1 week in mid-summer. Urine osmolality <500 mOsmol kg(-1) H(2)O was considered to be an appropriate level of hydration.
Mean urine osmolality was 862 +/- 211 mOsmol kg(-1) H(2)O. Osmolality above 800 mOsmol kg(-1) H(2)O was detected in 67.5% of the urine samples; among these, 25% were above 1000 mOsmol kg(-1) H(2)O. The most dehydrated group was that of Israeli-born Jewish children, whereas the Bedouin-Arab children were the least dehydrated.
A high proportion of children who reside in a hot and arid environment were found to be in a state of moderate to severe dehydration. Bedouin ethnicity was associated with better hydration, whereas Israeli-born Jews were most severely dehydrated. Educational intervention programmes promoting water intake should start in early childhood and continue throughout life.
Full-text · Article · May 2009 · Journal of Human Nutrition and Dietetics