IN BRIEF: Inadequate fluid replacement after physical activity is associated with dehydration, a decrease in physical performance, heat exhaustion, and even death. Self-hydration has been shown to be unreliable in rehydrating after exercise, and may put as many as 50% of exercisers at risk for dehydration. This study shows that predictive equations can prove valuable in estimating actual fluid loss, thereby reducing the risk for thermal injury.  Many examples have been reported:
- 2002 A firefighter falls from a ladder during training breaking his back. He sues the city claiming that he was pushed into a dehydrated state that compromised his safety.
- Paul Silas the Ex-Seattle Sonic and coach of the Charlotte Hornets could lose as many as 17 pounds in one basketball game.
- While writing her thesis in exercise physiology in 1982, a 23 year-old student, Julie Moss crawls to the finish line of the Ironman Triathlon, losing her lead in the last 100 meters of the race. She suffered from severe dehydration.
- Michael Chang wins the French open in 1989 at the age of 17, suffering from dehydration. The Mercer Island native serves the last set underhanded and wins the tournament.
- Gabriel Anderson suffering from heat exhaustion and severe dehydration drops from 23rd to 37th place in the last half mile of the 1984 Olympic Marathon in Los Angeles. She staggers around the last lap in over 4 minutes and collapses at the finish line.
Many of these athletes are well coached and are keenly aware of the dangers of dehydration, but still find may find themselves in potentially life threatening situations. It is well known that fluid losses as little as 1 to 2% of body weight can adversely affect athletic performance. The purpose of this study is to examine:
- The efficacy of using predictive equations for the estimation of actual fluid loss.
- Will the equations tested work on a wide variety of body types.
- Are there significant differences between self-hydration and what can be done with predictive equations?
- What equation has the highest degree of accuracy and diversity of the equations evaluated?
By developing an equation to predict fluid loss associated with physical activity, applications could range from:
- Programming the equation into a wristwatch, and using the display to quantify similar exercise intensities in extreme conditions.
- Program equations into exercise equipment consoles, (i.e. treadmills, bicycle ergometers, elliptical walkers, etc…)
- Program equation into monitors for firefighters, law enforcement bicycle patrol officers, as a teaching tool to instruct them into the quantities of fluid replacement needed for physiological requirements of the job.
- Educating and providing adequate amounts of water for employees based on physical demands of the job, thereby increasing productivity through optimal hydration minimizing the deleterious effects of dehydration.
- Use of hydration monitors for congestive heart failure patients on fluid restriction, providing guidelines for fluid intake to cover losses only.
- Use of the equation to teach athletes and exercisers about adequate hydration in an effort to optimize performance.
These are only a few examples of what can be done with predictive equations that may prove to correlate well with actual fluid loss associated with activity. It is hoped that this evaluation Estimating Fluid Loss During Sub Maximal Work on a Treadmill will answer these questions as well as create new ones, in an effort to minimize the effects of fluid loss on performance.
Brief Overview and Procedures
Twenty subjects, representing a physiological diverse sample of regular exercisers at an exercise facility, were recruited for a dehydration study. Each subject was weighed nude both before and after a two-hour exercise bout on a motor driven treadmill at 4.0 mph and a 1% grade. The actual fluid loss was obtained as a difference between the pre and post weights of the subjects with exercise. Actual fluid loss was correlated with the predictive equation copyrighted by Fluid Check, and two other equations which are commonly used to predict fluid loss, (Ashrae and Nieman equations). The Fluid Check System has the added benefit of accounting for different environmental conditions associated with differing humidity and temperatures in the prediction of fluid loss with exercise. For the purposes of this study it should be noted that the environmental conditions (60-65 degrees, 50-60% humidity) were conducive to all three equations.
At the conclusion of the study, the subjects were required to answer how much water they would rehydrate with on their own volition, and results were analyzed to measure the effectiveness of self-hydration. Body fat, BMI and heart rate data was collected during the study to identify the possibly of significant relationships with fluid loss.
Sample Population
Purposive samplings of subjects were selected to achieve a diversity of sexes, ages, weights, heights, % body fats, and BMI’s, (10 males, 10 females). All subjects were members of an exercise facility, and were capable of completing the prescribed exercise intensity and duration, (4 mph, 1% grade, 2 hours). All subjects were regular exercisers for at least six months prior to the study. Subject’s descriptive statistics are presented in Table 1 below:
Table 1
Descriptive Statistics of Study Population (N=20)
| Males (n=10) | Mean | Std Dev | Females (n=10) | Mean | Std Dev |
| Age (yrs) | 42.4 | 13.1 | Age (yrs) | 41.8 | 10.2 |
| Weight (lbs) | 184.9 | 26.2 | Weight (lbs) | 142.9 | 34.5 |
| Height (in) | 69.8 | 2.2 | Height (in) | 66.2 | 2.9 |
| %Body Fat | 17.1 | 4.9 | %Body Fat | 26.3 | 3.5 |
| Body Mass Index (BMI) | 26.8 | 4.3 | Body Mass Index (BMI) | 22.4 | 4.5 |
Actual losses were measured as the difference between the pre and post weights associated with the exercise session. Five subjects had to urinate during the exercise session. Urine was collected in Styrofoam cups, and carried during the post weigh in. When the subjects were asked at the completion of the post exercise how much water they would typically drink on their own volition after similar exercise, they were provided a sixteen ounce bottle of water to use as a reference.
Statistical Procedure
Descriptive statistics, which included mean and standard deviation, were run on the sample subject population, (Table 1). The Pearson r coefficient correlation statistic was utilized to compare the three equations used to predict fluid loss, with Actual fluid losses for each of the study subjects. A Pearson r was also used to compare the subjects self reported fluid rehydration amounts with Actual fluid loss.
Summary of Results
Results showed that all three predictive equations showed a significant correlation with Actual fluid loss. The Pearson r for the Fluid Check System was the highest at .80, with the Ashrae and the Nieman Equations at .78. Their was no significant correlation attributed with self reported hydration amounts and Actual fluid losses for the subjects tested, (r=.11), See Figure 1 below:
All three predictive equations overestimated Actual fluid loss, but mean losses predicted by the Fluid Check System was the only equation within one standard deviation of Actual mean losses. Self-hydration mean fluid loss values were similar to actual losses, but varied widely between subjects, with large standard deviations (Std Dev. = 12.7 ounces), and a non-significant poor correlation with Actual fluid loss, (r=.11).
Interpretation of Findings
Although underestimation by all three predictive equations were evident from Actual fluid losses measured, (Appendix E), the American College of Sports Medicine (1996) recommends “to minimize the potential for thermal injury, it is advocated that water losses due to sweating during exercise be replaced at a rate equal to the sweat rate.â€Â Many investigators recommend amounts of rehydration exceeding fluid loss shown on the scale. Maughan (1996) states “ To surmount ongoing obligatory urine losses, the volume consumed should be greater that the volume of the sweat loss.†and recommends up to 150% of actual fluid loss as seen on the scale. The Gatorade Sports Institute (1999) general guidelines state “The goal of fluid intake during exercise should be to fully replace sweat losses†but also states, “or consume the maximal amount that can be tolerated.â€
Research seems to be unclear regarding recommendation of fluid replacement. The danger of over consumption of fluids is with stomach discomfort and hyponatremia that can result with over-consumption of fluids˛. It is well documented that researchers agree fluid replacement should at least cover losses on the scale, and that there is evidence that fluid replacement in excess of this amount may be warranted.
Conclusions and Recommendations
The data presented in this evaluation showed that good correlations can be derived through equations for the prediction of fluid loss. One of the equations in the evaluation, the Fluid Check System was superior in its ability to predict fluid loss in a diverse sample population. It should also be noted the Fluid Check equation has the added benefit of an ability to compensate for the impact of temperature and humidity effects on fluid loss during activity. Preliminary data suggests that the Fluid Check system may also serve well in more extreme environmental situations with higher risk for dehydration.
Lastly, it was also shown that thirst and self-rehydration in this group of subjects was not a good predictor of fluid replenishment. Higher workloads and harsher environmental conditions may put as many as one half of the subjects at risk of dehydration. The study also noted that a high correlation was seen with Body Mass Index (BMI) as a possible variable in the refinement an equation for the prediction of fluid loss. Future research centering on equations that can be used in a variety of extreme physical and environmental conditions should be undertaken. Further research is also needed to determine optimal levels of rehydration at or in excess of fluid losses through activity.
Bibliography
American College of Sports Medicine Position Statement (1996). Exercise and Fluid Replacement. Med. Sci. Sports Exercise 28(1):i-iiv.
Ashrae, Ashrae Handbook – Fundamentals of Air Conditioning and Refrigeration Physiologic Principles. Ashrea Pub., ISBN 883413-56-7, Atlanta, GA 1998.
Murray, Robert, Gatorade Sports Institute: Position Statement: ACSM position stand on Fluid Replacement, http://www.gssiweb.com/reflib/sresults.cfm
Maughan, R.J., J.B. Leiper, and S.M. Shirreffs, (1996). Restoration of Fluid Balance After Exercise-Induced Dehydration: Effects of Food and Fluid Intake. Eur.J.            Appl. Physiology. 73:317-325.
Matthews, Donald, K., and Fox, Edward L., The Physiological Basis of Physical Education and Athletics. W.B. Saunders Co, Philadelphia, 1976.
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Nieman, David, Fitness and Sports Medicine, Bull Publishing Company, ISBN 091595091, (May 1990).