Many Dexalytics users know that adequate hydration will enhance athletic performance and have asked whether it is possible to calculate hydration status based on body weight or body composition. It is possible to determine approximate basal needs since water makes up approximately 75% of lean muscle tissue and about 25% or less of fat mass (Kraemer et al., 2016).  However, there is a wide variation for hydration needs for athletes depending on the intensity of exercise and duration of activity that the athlete is involved in, environmental temperature and humidity that the athlete is working out in, and the sweat rate of the athlete.  All of these variables make the calculation of hydration needs highly variable and individual.

Water and Athletic Performance We all know that water is essential for overall health. Lesser known, is that water loss of as little as 2% loss of body weight can affect performance (Cheuvront et al., 2007; Dougherty et al., 2006; Sawka & Noakes, 2007). For a 200 pound athlete, this means a 4 pound sweat loss can result in decreased mental sharpness, decreased recovery, decreased metabolic rate and increased risk of injury. 

Assessing Hydration Status
It is important to evaluate hydration status before, during, and after exercise.  There are several ways to measure hydration status and a hydration plan should be determined for each athlete.

The simplest method for measuring adequate hydration and fluid needs during exercise is to measure body weight difference.  A change in body weight from pre-exercise to post-exercise typically indicates the amount of fluid lost during exercise (correcting for weight of fluid, food intake, urine and fecal losses, and sweating) (Armstrong, 2007).

Another hydration indice is the 24-hour urine volume.  To calculate 24-hour urine volume one must collect all urine that is produced throughout a day in a clean plastic jug.  A healthy woman produces about 1.13 liters (1.2 quarts) of urine a day whereas a healthy male excretes about 1.36 liters (1.44 quarters) per day.  Children produce proportionately less urine each day as do older adults (>90 yrs) (Armstrong, 2007). 

Measuring specific gravity is another method to measure hydration status. This requires a refractometer to measure the density of urine relative to the density of water.  In dehydrated states, the specific gravity of urine exceeds 1.03.  Normal urine specific gravity ranges from 1.013 to 1.029 (Armstrong, 2007).

Urine color can be another method to determine hydration status.  A “pale yellow” color or “straw-colored” visual reading indicates that an athlete is within 1% of baseline body weight and a darker color (i.e., deep yellow, tan) indicates an increasing level of dehydration (Armstrong et al., 1998). However, taking B-complex vitamins or a multivitamin can cause urine to have a bright yellow or even orange color and may not indicate dehydration.

Thirst can also be an indice of hydration status.  If an athlete is a little thirsty or moderately thirsty than they are typically mildly dehydrated (approximately 1% or 2% of body weight) (Armstrong, 2007). 

Too much water
Can you drink too much water?  It is possible to drink too much water too fast and develop a condition called hyponatremia.  It should be noted that this happens very rarely in athletes and dehydration is more common.  
The bottom line is hydration is an important determinant of an individual’s performance and several indices should be used to determine hydration status. Finally, hydration is highly variable from athlete to athlete necessitating the need for individualized hydration plans.

References:
Armstrong LE. Assessing hydration status. The elusive gold standard. J Am Coll Nutr. 2007;26(5):575S-584S.

Armstrong LE, Herrera Soto JA, Hacker FT, Casa DJ, Kavouras SA, Maresh CM. Urinary indices during dehydration, exercise, and rehydration. Int J Sport Nutr Exerc Metab. 1998;8:345-355.

Cheuvront SN, Montain SJ, Sawka MN.  Fluid replacement and performance during the marathon. Sport Med. 2007;37:353-357.
Dougherty KA, Baker LB, Chow M, Kenney, WL. Two percent dehydration impairs and six percent carbohydrate drink improves boys basketball skills. Med Sci Sports Exerc. 2006;38:1650-1658.

Sawka MN, Noakes TD. Does dehydration impair exercise performance? Med Sci Sports Exerc. 2007;39:1209-1217.

Kraemer WJ, Fleck SJ, Deschenes MR. (2016). Exercise physiology: integrating theory and application, 2nd edition. Baltimore, MD: Lippincott Williams & Wilkins.

About the Author: Donald Dengel, Ph.D., is a Professor in the School of Kinesiology at the University of Minnesota and is a co-founder of Dexalytics. He serves as the Director of the Laboratory of Integrative Human Physiology, which provides clinical vascular, metabolic, exercise and body composition testing for researchers across the University of Minnesota.