Articles Doug Theis on 02 Jul 2003 06:36 am

Hydrate!: Water Intake/Outtake in the Field

Water is the major component of the human body, comprising approximately 73% of lean body mass and 60% of total body mass. Body water is transported within and between cells and in the blood plasma. During rest, 30-35% of the water is maintained inside the cells (intracellular fluid), 20-25% is maintained outside the cells of the body (interstitial fluid) and the remaining 5% is in the blood plasma.

When the body’s core temperature increases, water movement between the intracellular and extracellular compartments occurs due to hydrostatic pressure and electrolyte gradient shifts. Sweat is hypotonic relative to body water, which causes an elevation in the extracellular tonicity, resulting in a water gradient shift from the intracellular to the extracellular spaces. This means that water is being drawn out of the cells (especially muscle cells) to the surface of the skin to dissipate heat from the body, which is the process known as perspiration.

During exercise, perspiration is the primary mechanism of heat dissipation for the body. The evaporation of sweat from the skin’s surface assists the body in the regulation of its core temperature. If the body cannot effectively evaporate sweat to the surface of the skin to dissipate heat, the body’s core temperature will rapidly rise. The degree to which the core temperature rises directly relates to a decline in physical performance. The rate at which fluids are lost from the body through surface skin evaporation is a function of exercise intensity, genetic differences, environmental conditions, clothing, acclimatization ability and baseline hydration status.

If sweat loss exceeds fluid intake during physical activity, an athlete will be at risk for developing dehydration. If an athlete begins an event inadequately hydrated, then dehydration can develop in less than an hour. If the body’s warning signs of dehydration are ignored, the consequences can be life threatening. As the body’s core temperature increases and water is shifted from inside to outside the muscle cells, the rate of glycogen breakdown increases causing an increase in lactate levels. As lactic acid builds, the muscle’s physical performance declines. The buildup of lactic acid directly affects the muscle’s ability to contract. This results in a decrease in time to exhaustion even when muscle strength is not compromised.

Accompanying the body’s thermal strain is a greater cardiovascular strain, as characterized by a decreased stroke volume, increased heart rate and an increased blood flow resistance due to a higher blood viscosity. Similar to the body’s physical changes with an increase in core temperature, the degree of cardiovascular changes is proportional to the body’s fluid loss. For example, one’s heart rate will rise an additional 3 to 5 beats per minute for every 1% loss of body weight. Aerobic capacity usually decreases with more than 3% loss of body weight regardless of physical fitness level and heat acclimation. Dehydration greater than 3% can lead to heat cramps from lactic acid formation, heat exhaustion, impaired mental function, heat stroke and death due to cardiac arrest (heart attack) and cerebral edema (swelling of the brain).

Last year in Kernville temperatures reached 105 degrees and the only way to stay adequately hydrated was to supplement a camelback with a water filtration system. One could not physically carry enough water to stay adequately hydrated between the transitions without rehydrating by filtering water from the rivers and streams along the course. Heeding your body’s warning signs of dehydration is crucial to your physical performance. Remember that your team is only as fast as your slowest person; if one member of the team becomes dehydrated and has to stop, the whole team stops. The most common signs of dehydration are thirst, irritability, headache, weakness, cramps, dizziness, nausea, increased heart rate and decreased aerobic performance.

To beat the heat during training events, every athlete should practice matching fluid intake with sweating rate and urine losses. Calculating an athlete’s sweat rate can be quite tedious, as represented by the following equation: [sweating rate = (pre-exercise body weight – post-exercise body weight) + (fluid intake – urine volume/exercise time in hours)]. This equation can be calculated much easier by weighing the athlete before an intense 1-hour exercise session and then reweighing them again at the end of the 1-hour session. The athlete cannot rehydrate or urinate during the 1-hour session in order to obtain an accurate sweat rate.

This calculation is crucial in order to determine your body’s fluid replacement needs so that your physical performance will not be compromised. During training events it is easy to inaccurately estimate fluid intake based on sips of water. An easy training tip in measuring fluid intake is to use a clear sports bottle in 100-ml (3.4 fluid oz.) increments to provide a visual reminder to drink beyond thirst or a few large gulps. To ensure proper pre-exercise hydration, the athlete should consume approximately 500-600 ml (17-20 fluid oz.) of water or a 6% carbohydrate-containing beverage 2 to 3 hours before the event, and then another 200-300 ml (7-10 fluid oz.) 15-20 minutes before the start. During the event, for fluid replacement to approximate sweat and urine losses, it is recommended to consume 200-300 ml (7-10 fluid oz.) every 15-20 minutes.

Post-exercise hydration should aim to correct fluid losses accumulated during practice or the race. Ideally, rehydration should be completed within the initial 2 hours immediately following the event. The rehydration fluid should be composed of water to restore hydration status, carbohydrates to replenish depleted glycogen stores (glucose), and electrolytes to increase the rate of rehydration. Fluid temperature influences the rate of absorption and a cool beverage of 50 – 59 degrees Fahrenheit is recommended. The carbohydrate concentration of the beverage consumed can have a negative impact on the body’s ability to adequately rehydrate if the concentration is greater than 8%. Carbohydrate concentrations greater than 8% (fruit juices, carbohydrate gels, sodas and some sports drinks) decrease the rate of fluid emptying from the stomach and absorption from the intestine. Most carbohydrate forms (i.e., glucose, sucrose, glucose polymers) are well tolerated. Substances to be limited include fructose (which may cause stomach cramping and diarrhea), and caffeinated beverages, which may increase urine output and reduce fluid retention.

During continuous exercise like an adventure race the body uses 30 – 60 grams of carbohydrate per hour that needs to be replaced to prevent glycogen depletion and fatigue. Therefore, including 60 grams of carbohydrate in 1 liter of fluid will not hinder fluid absorption and will provide an adequate amount of carbohydrate for sustained endurance.

Inclusion of salt in fluid-replacement beverages or in the form of salt tablets requires special consideration. If salt is ingested when it is not required, an osmotic shift will occur, drawing an excess amount of water into the stomach and depleting the body’s other compartments (lean body mass) of essential water, resulting in a decrease in physical performance. Adding a modest amount of salt (0.3 – 0.7 grams/liter) under the following conditions should be considered: when food has not been consumed; physical activity exceeding 4 hours in duration under a hot climate; in extremely hot weather with profuse sweat loss (observable salt crystals excreted on the surface of the skin or clothing). Adding salt when required by the body can offset salt loss in sweat and minimize life-threatening events associated with electrolyte imbalances (hyponatremia, cerebral edema).

In conclusion, do not underestimate the importance of adequate hydration. Start observing your hydration behavior during training sessions. Using the tips provided, develop fluid-replacement practices that will optimize your hydration status before, during and after training or competitive events. Optimal fluid intake is directly related to physical performance output.

Drink up!!!


Tricia Bland, R.D., C.P.T, M.P.H. is a graduate of UC Davis in Nutrition and Dietetics.

She is a Certified Personal Trainer through the American Council on Exercise and I also train clients in physical fitness.

She has her own private practice called L.E.A.N. (Lifestyle of Exercise and Nutrition) Consulting.

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