Jun 24, 2021Assessing Proper Hydration in Athletes
Dehydration is a condition in which the body loses more fluids than what is taken in. This is of particular concern during exercise as dehydration impairs physiologic and cognitive function, increases the risk of heat illness, and decreases exercise performance.
Exercise results in heat production that the body balances through conduction, convection, evaporation, and radiation. Heat dissipation through sweat evaporation is the primary mechanism in which the body cools itself during exercise in most sports. However, there are large differences between sport types.
For example, sports like aquatics that train primarily indoors with minimal clothing tend to have lower average sweat rates in comparison to sports that train outdoors in all types of weather with lots of equipment, such as football. Sweat rates are highly variable among athletic populations in general and are impacted by environmental conditions, acclimatization state, clothing, exercise intensity, fitness, baseline hydration, and individual differences. Therefore, hydration assessment and education protocols should be individualized and adjusted throughout training and competition.
There are primarily three practical field assessment methods used to assess hydration status in athletic populations: body weight change, urine color, and urine specific gravity (USG).
Body Weight Change
In order to assess hydration status via body weight changes, normal baseline body weight must be known. This can be determined by measuring body weight for three consecutive days (with daily variability of 0.51 ± 0.2 kg). Best practices for this include standardized clothing, measuring on the same scale, and taking measurements at the same time of day. Euhydration, or well-hydrated status, can be determined if an individual’s body weight is within ±1% of the baseline value.
Urine color can also be used to monitor hydration status throughout the day. Urine color should be maintained at “pale yellow” or “straw-colored” for euhydration. A urine color chart for comparison has been developed and validated by Armstrong and colleagues (Armstrong et al., 1994). Urine color should be assessed midstream and environmental lighting should also be taken into consideration when using urine color to assess hydration status. It is also important to note that clear urine is not the goal for hydration as this may indicate overhydration, increasing the risk for hyponatremia which can have potentially fatal consequences.
Urine Specific Gravity
Urine-specific gravity is a less subjective method to quickly assess pre-exercise hydration status, however, a refractometer is required. Refractometers are simple to use and some are somewhat inexpensive. USG values <1.020 g/mL-1 indicate euhydration. The best practice is to test the first-morning urine when possible as it is the most accurate reflection of hydration status. This may pose logistical obstacles such as early morning practice times, athletes voiding urine before arriving at facilities, or lack of resources and space to store urine samples during the measuring.
Hydration Before Exercise
The goal of fluid intake before exercise is to start activity hydrated with normal electrolyte levels. For example, fluid intake of 16-20 fl. oz. 2 hours before exercise should ensure proper hydration. Hydrating early should be another consideration to allow for urine output to return to normal before exercise. However, hyperhydration, or over-hydrating, is not recommended due to unclear benefits and the risk of dilutional hyponatremia.
Hydration During Exercise
The goal of fluid intake during exercise is to prevent excessive dehydration, indicated by >2% body weight loss from water deficit. Fluid needs during activity are highly individualized and dependent on sweat rates, exercise duration, and opportunities to drink. It is recommended to monitor body weight changes during training and competition to estimate sweat rates for fluid replacement. As a starting guideline, 7-10 fl. oz. every 15-20 minutes may be sufficient for some individuals. However, events with longer durations or different environmental conditions may necessitate different fluid replacement protocols. Therefore, fluid replacement during exercise should be determined on an individual basis.
Hydration After Exercise
After exercise, the goal is to correct any fluid deficits that occurred during activity. For events with long recovery periods (>24 hours), drinking fluids and consuming foods with sodium is generally sufficient to achieve rehydration. During periods of rapid recovery (<24 hours) or in response to substantial fluid loss, ingestion of 1.5 L/kg (23 fl. oz./lb) of body weight lost is often acceptable to achieve complete rehydration. The additional 0.5 L volume should be included to offset increased urine production during rapid rehydration. These fluids should be consumed over several hours when possible to allow for proper fluid retention. If recovery periods are even shorter (<12 hours), then more aggressive rehydration plans may be warranted.
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Other beverages besides water can also be good choices to hydrate such as sports beverages, fruit juice, milk, and tea. Palatability of fluids can help promote fluid consumption. Therefore, flavor, temperature, and sodium content should also be considered with athlete preferences to promote good hydration habits.
To learn more about sports nutrition and CPSDA, go to www.sportsrd.org.