Possibly the most  underemphasized aspect of sports nutrition, hydration can make or break an athlete’s performance.  While fuel depletion during exercise can impair performance, inadequate water not only impairs exercise capacity, but can create life threatening disturbances in fluid balances and core temperature.  With as little as 1% of body weight lost in fluid, athletes can experience thirst, fatigue, and weakness.(1)  Thirst drives a person to drink, but it can actually lag behind the body’s need.  When too much water is lost from the body and not replaced, dehydration develops.  Dehydration refers to an imbalance in fluid dynamics when fluid intake does not replenish water lost.  On the flip side, water intoxication occurs with excessive water intake or kidney disorders that can reduce urine output.  In healthy individuals, dehydration is more common than water intoxication, but appropriate water balance is crucial to optimal performance and ultimately survival.  

Water and Body Fluids

Water constitutes about 60 percent of an adult’s weight and it is the medium in which all life processes occur, including:

• Carrying nutrients and waste products through the body
• Maintaining the structure of large molecules
• Acting as a solvent for minerals (i.e., sodium and potassium) and other small molecules
• Acting as a lubricant and cushion
• Maintaining blood volume
• Helping the body regulate temperature

Every cell in the body contains fluid specific for that cell called intracellular fluid, and around it is extracellular fluid.  These fluids continually lose and replace their components, but the composition of each compartment remains remarkably constant.

Because an imbalance can be devastating, the body continually adjusts water intake and excretion as needed.  This type of balance is referred to as homeostasis.  

The body must excrete a minimum of about 500ml (about 2 cups) of water each day as urine to eliminate waste products that the body makes through metabolism.  Above this amount, the body adjusts by excreting excesses to balance intake.  In addition to water lost in the urine, the body also loses water from the lungs as vapor and the skin as sweat.  The amount of fluid lost depends on the environment (heat and humidity), and physical conditions such as activity. On average, the body loses about 2.5 liter/day.

Fluid maintains blood volume, which influences blood pressure. The homeostatic balance of fluids and solutes in the body is carefully regulated by the kidneys with assistance from several hormones including antidiuretic hormone (ADH) and aldosterone.  The primary solutes that are involved in maintaining fluid balance are sodium, potassium, proteins, and glucose.  Adequate intakes of water and electrolytes maintain hydration levels and regulate fluid shifts between the intra- and extra-cellular compartments.

To maintain water balance, intake from liquids, foods, and metabolism must equal losses from the kidneys, skin, lungs, and GI tract.

The Needs of the Athlete

One of the primary functions of water for all individuals, especially athletes, is thermoregulation.  A person can tolerate a drop in body temperature of 10 degrees (c), but an increase of only 5 degrees (c).  Heat generated by active muscles can raise core temperature to levels that would incapacitate a person if caused by heat stress alone.(2)

The body uses three mechanisms to dissipate heat: 

• circulation
• evaporation
• hormonal adjustment. 

The circulatory system works to deliver warm blood to the body’s shell.  This produces the typical “flushed face.”  Sweating begins within several seconds of the start of vigorous exercise and this evaporative cooling controls heat dissipation during exercise.  Lastly, because sweat contains water and electrolytes, the body makes hormonal adjustments to help prevent the loss of salts and fluid.

When the fluid lost in thermoregulation results in dehydration and a drop in blood volume, the end result produces circulatory failure and core temperature levels can increase to lethal levels.

Water Replacement

Total water intake at the reference level of 3.7 liters for adult men and 2.7 liters for adult women per day covers the expected needs of healthy, sedentary people in temperate climates.

The obvious dietary sources of water are water itself and other beverages, but most foods contain some water as well.  Most fruits and vegetables are almost 90% water, and many other foods like meats and cheese are approximately 50% water.  The body also makes water during metabolism, when energy-yielding nutrients break down and their carbons, hydrogens, and oxygens combine to make carbon dioxide (CO2) and water (H2O). 

According to the Dietary Reference Intakes: The Essential Guide to Nutrient Requirements, published by the Institute of Medicine (3), most people get adequate fluids by drinking when they’re thirsty.  However, the report does add that prolonged physical activity and heat exposure will increase water losses and therefore may raise daily fluid needs.  Very active individuals, who are continually exposed to hot weather, often have daily total water needs of six liters or more.   Both heat acclimatization as well as nutrition intervention are indicated for this population.  

According to the American College of Sports Medicine, general guidelines for fluid replacement are as follows: (5) Consume a nutritionally-balanced diet and drink adequate fluids during the 24-hr period before an event. Drink about 500 ml (about 17 ounces) of fluid about 2 hours before exercise to promote adequate hydration and allow time for excretion of excess ingested water. During exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating or consume the maximal amount that can be tolerated. It is recommended that ingested fluids be cooler than ambient temperature [between 15 degrees and 22 degrees C (59 degrees and 72 degrees F])] to enhance palatability and promote fluid replacement. Addition of proper amounts of carbohydrates and/or electrolytes to a fluid replacement solution is recommended for exercise events of duration greater than 1 h since it does not significantly impair water delivery to the body and may enhance performance.

Water loss by sweating peaks at about 3L per hour during intense exercise in heat.  However, just about any degree of dehydration can impair performance.  Adequate fluid replacement sustains the body’s potential for evaporative cooling.  Rehydration protocols are often based on water lost as measured either by urine color, urine specific gravity, or changes in body weight.(4) If collecting urine is not feasible, sweat loss as reflected by weight loss can be used.  An athlete’s pre and post workout (or event) weights are takenand whole body sweat rate can be calculated by dividing the sweat loss by the time period of collection. The following equation can be used to determine volume of fluid lost: (5)

Sweat loss = (body weight before – body weight after) + amount of fluid intake – toilet loss.   

Alternatively, taking a simple measure of body weight each morning after emptying the bladder can show a pattern of hydration over time, provided gains or losses of fat and muscle tissue are not also taking place.

Fluid balance, electrolyte homeostasis, cardiovascular function, and thermoregulatory control are intimately linked to fluid consumption and each has a major impact on health and performance.  Fluid replacement helps maintain hydration and, therefore, promotes the health, safety, and optimal physical performance of individuals participating in regular physical activity.  Athletes and their coaches should be aware of fluid replacement needs, and develop strategies and protocols to insure athletes drink enough to keep pace with sweat loss.

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Tags: energy, fluid replenishment, hydration, Nutrition

This entry was posted on Thursday, March 11th, 2010 at 12:44 pm and is filed under Nutrition. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.