Sports Nutrition: Fuel Movement & Sport

Fluid Balance

Water comprises about 60-70% of a person's body weight. Because muscle tissue is 70-75% water, the body weights of lean, muscular athletes may exceed 70% water. Check out the water distribution in this person's body. Except for the digestive juices, which three tissues have the highest water content?

Inadequate fluid intake will have powerful negative effects on all three of these tissues (blood, brain, and muscle.) In contrast, fat tissue is only 10% water. Essential functions of water for athletes include maintaining normal body temperature and normal blood volume. Because these functions are critical to survival as well as to athletic training and performance, athletes need to drink enough fluid throughout the day to replace water lost in sweat, respiration, urine, and feces.

How much is enough? Fluid needs are linked to energy expenditure. The more energy an athlete expends, the greater are his fluid needs. Why? Heat is a byproduct of energy production, and excess heat must be transferred from the body to the environment to maintain normal body temperature. Evaporation of sweat from the athlete's body is the primary way of dissipating excess heat. Sweat loss varies among athletes and can exceed 1.5 liters per hour. To maintain adequate hydration (euhydration), sweat loss must be replaced. Consuming 1.0 to 1.5 ml of fluid for each kilocalorie that is expended is optimal. Thus, an athlete who expends 5,000 calories a day needs 5,000 to 7,500 ml (5.0-7.5 L) of fluid intake a day to maintain fluid balance. Fluid consumption includes all liquids plus water in foods.

Dehydration impairs temperature regulation and both physical and mental performance. The human body cannot adapt to dehydration. Decreased blood volume due to dehydration reduces blood flow to the skin and impairs heat dissipation. When sweating slows, cooling slows. Exercise combined with hyperthermia places an extreme stress on cardiovascular function. As shown in the diagram above, fluid loss as low as 2% of total body weight (three pounds in a 150 pound person) impairs temperature regulation and reduces endurance capacity and aerobic performance. Higher levels of dehydration impair mental concentration, alertness, muscular strength and endurance, physical work capacity, and increase risks for heat injury. Deterioration in thermoregulatory and cardiovascular responses due to dehydration compromises athletic performance and can be life-threatening. How will an athlete know if he needs more water? Maintaining his desired level of exercise intensity will become difficult. He will tire more easily, and athletic performance will decline.

Think to drink. Train to drink. Adequate hydration delays fatigue and prepares athletes to perform well. Because thirst is blunted by intense exercise and fluid balance is critical for optimal athletic performance, athletes need to train themselves to consume fluids before, during, and after exercise. Time and practice are required to enable athletes to increase their consumption of large volumes of fluid. Scheduling fluid intake throughout the day is especially important during periods of heavy training and multiple training sessions per day. Sports Drinks contain specific recommendations regarding oral rehydration fluids.

In 1996 the American College of Sports Medicine (ACSM) issued a position stand on exercise and fluid replacement. The recommendations from ACSM are intended to guide all athletes in maintaining an optimal hydration status. Key elements of the position stand are as follows:

Fluid Consumption Before Exercise

Consume a balanced diet and drink adequate fluids during the 24-hour period before exercise.
Drink about 500 ml (about 16 ounces) of fluid about two hours before exercise.
Goal: promote proper hydration before exercise or competition.

Fluid Consumption During Exercise

Start drinking early.
Drink fluids regularly.
Consume cool, flavored fluids to encourage fluid intake.
During intense exercise lasting longer than one hour: Carbohydrate ingestion at a rate of 30-60 grams per hour is recommended to maintain carbohydrate oxidation and delay fatigue. Electrolyte (sodium chloride) addition in the fluid helps maintain the osmotic drive for drinking.
Goal: replace fluid at a rate that equals water loss from sweating.

Fluid Consumption After Exercise

Drink at least a pint of fluid for every pound of water loss during exercise.
Goal: replace fluid losses during exercise and return to euhydration.

Hydration Considerations

Different considerations for hydration exist in strength/power sports such as ice hockey compared with endurance sports such as cross-country skiing.

Ice Hockey

High intensity activity requiring a great degree of accuracy. Stop-and-go movement: intermittent, unpredictable, random.
Work rates vary among team members and positions on the team.
Heat dissipation is limited by protective clothing; elevated core temperature stimulates excessive sweating. Difficult to anticipate the degree of sweat loss.
Regular opportunities for hydration during games: shift changes, substitutions, pauses in play, time-outs,intermissions.
Tournament play: more than one game may be played per day.

Cross Country Skiing

Prolonged, continuous, moderate to intense activity.
Water loss (2% of body weight) due to sweating can be extensive, even in a cold environment.
Approximate level of sweat loss can be anticipated.
Carbohydrate intake is recommended for long races (>one hour).
Limited opportunities for hydration during races: water stations along the race course, water bottles, portable hydration system carried as a pack on the back (increases the opportunities for hydration).
Races may be scheduled on successive days.