5.3 Hydration and fluid balance

Hydration is crucial for athletic performance. It maintains cardiovascular function, regulates body temperature, and supports muscle function during exercise. Proper hydration ensures optimal blood volume, oxygen delivery, and waste removal, while preventing fatigue and cognitive decline.

Dehydration negatively impacts performance, even at just 2% body weight loss. It reduces blood volume, increases heart rate, and impairs heat dissipation. Athletes should monitor hydration through urine color, body weight changes, and thirst levels to maintain peak performance and avoid heat-related illnesses.

Hydration for Athletic Performance

Importance of Hydration

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• Hydration maintains cardiovascular function, thermoregulation, and muscle function during exercise
• Dehydration of as little as 2% of body weight negatively affects athletic performance, particularly in endurance events and high-intensity activities lasting longer than 30 minutes (marathons, triathlons)
• Proper hydration maintains blood volume, which delivers oxygen and nutrients to working muscles and removes metabolic waste products
• Adequate hydration allows the body to produce sweat and dissipate heat during exercise, preventing increased core body temperature and heat stress
• Dehydration causes fatigue, reduced cognitive function, and impaired decision-making, decreasing athletic performance and increasing the risk of injury (heatstroke, muscle cramps)

Effects of Dehydration on Performance

• Dehydration leads to decreased blood volume, increased heart rate, and reduced skin blood flow, impairing performance
• Cardiovascular strain increases as dehydration worsens, making it more difficult for the heart to pump blood to working muscles and skin for cooling
• Reduced blood flow to the skin impairs heat dissipation, leading to increased core body temperature and risk of heat illness (heat exhaustion, heatstroke)
• Dehydration impairs muscle function by reducing muscle blood flow and increasing muscle glycogen use, leading to early fatigue
• Cognitive performance declines with dehydration, affecting decision-making, reaction time, and motor control, which can increase the risk of injury (concussions, sprains)

Dehydration and Hyponatremia

Signs and Symptoms of Dehydration

• Dehydration occurs when fluid losses exceed fluid intake, decreasing total body water
• Thirst, dark-colored urine, dry mouth, fatigue, headache, dizziness, and decreased urine output are signs and symptoms of dehydration
• Severe dehydration causes additional symptoms such as rapid heartbeat, rapid breathing, sunken eyes, skin that remains "tented" when pinched, irritability, confusion, and fainting
• Athletes should monitor hydration status by checking urine color (aim for pale yellow), monitoring body weight changes before and after exercise, and being aware of thirst and other dehydration symptoms

Hyponatremia

• Hyponatremia is a condition characterized by low blood sodium levels, which can occur due to excessive fluid intake or inadequate sodium replacement during prolonged exercise (ultramarathons, Ironman triathlons)
• Symptoms of hyponatremia include nausea, vomiting, headache, confusion, fatigue, muscle cramps, seizures, and coma
• Hyponatremia can be life-threatening if not treated promptly, as severe cases can lead to brain swelling and respiratory arrest
• Athletes should avoid overdrinking during prolonged exercise and consume electrolyte-containing beverages to replace sodium losses

Fluid Intake for Exercise

Pre-Exercise Hydration

• Before exercise, consume 16-20 ounces (475-590 ml) of fluid 2-3 hours prior to the event and an additional 8-12 ounces (235-355 ml) 15-30 minutes before the event to ensure adequate hydration
• Pre-exercise hydration helps maintain blood volume and delays the onset of dehydration during exercise
• Athletes should consume beverages that are well-tolerated and familiar to avoid gastrointestinal discomfort

Hydration During Exercise

• During exercise, consume fluids at regular intervals to replace sweat losses
• A general recommendation is to consume 3-8 ounces (90-240 ml) of fluid every 15-20 minutes, depending on individual sweat rates, exercise intensity, and duration
• Athletes should monitor their sweat rate by weighing themselves before and after exercise to determine fluid losses and develop an individualized hydration plan
• Consuming small amounts of fluid frequently during exercise helps maintain hydration status without causing gastrointestinal discomfort

Post-Exercise Hydration

• After exercise, replace 150% of fluid losses within 2 hours of completing the activity (if an athlete loses 1 pound (0.45 kg) of body weight, they should consume 24 ounces (710 ml) of fluid)
• Post-exercise fluid intake should include both water and electrolyte-containing beverages to replace losses and promote optimal recovery
• Consuming fluids with carbohydrates and electrolytes after exercise helps replenish glycogen stores and replace electrolyte losses (sports drinks, chocolate milk)
• Monitoring urine color and volume after exercise can help assess hydration status and guide post-exercise fluid intake

Electrolytes and Fluid Balance

Role of Electrolytes

• Electrolytes (sodium, potassium, chloride, and magnesium) regulate the movement of water between intracellular and extracellular spaces, maintaining fluid balance
• Sodium is the primary electrolyte lost in sweat and is essential for maintaining extracellular fluid volume and stimulating thirst
• Potassium maintains intracellular fluid balance and plays a role in muscle and nerve function
• Chloride works with sodium to maintain extracellular fluid volume and is also lost in sweat
• Magnesium is involved in muscle contraction, relaxation, and energy production, although losses during exercise are relatively small

Electrolyte Replacement

• Adequate sodium intake is necessary to replace losses and prevent hyponatremia
• Potassium losses during exercise are typically lower than sodium losses but should still be replaced
• Electrolyte-containing sports drinks can be beneficial for replacing electrolyte losses during prolonged exercise (>1 hour) or in hot and humid conditions when sweat rates are high (Gatorade, Powerade)
• For shorter duration or lower-intensity activities, water is sufficient for hydration
• Consuming a balanced diet that includes fruits, vegetables, and whole grains can help replenish electrolyte losses after exercise