Chapter 9: Water Requirement and Fluid Balance Lecture 6.1

Fluid Requirements for Athletes Athletes must be fully hydrated before they train or compete Cannot adapt to dehydration Performance will diminish as the athlete becomes dehydrated during competition

Ensuring Adequate Hydration Before Exercise Adequate hydration can be accomplished in the last few days leading up to competition Methods to check: Urine should be pale in color - not reliable as athletes may be taking supplements that contain B vitamins that color urine yellow Measuring urine osmolality – most reliable > 900 mOsmol/kg – dehydrated 100 – 300 mOsmol/kg – well hydrated Measuring body weight after rising before and after voiding – moderately reliable Sudden drop in body mass any given day can indicate dehydration

Ensuring Hydration During Exercise Thirst Relying on thirst as a signal to drink is unreliable Dehydration can occur before the thirst sensation Goals Consume enough fluids to keep body weight constant before and after Athletes need to acclimatize to drinking and regular intervals during exercise American and Canadian Dietetic Associations recommendations Before Exercise: 15 minutes before consume ~ 500 ml of fluid Hot and humid environments: frequent consumption, every 15 to 20 minutes of small volumes of 120 – 180 ml Exercise lasting between 30 – 60 minutes water is sufficient to maintain hydration

Fluid Ingestion During Exercise Benefits Supplies exogenous fuel substrate – usually in the form of carbohydrates Helps maintain plasma volume Prevents dehydration Gastric emptying Availability of fluids ingested is limited by the rate of gastric emptying (intestinal absorption) The faster gastric emptying = decreased absorption Addition of carbohydrates to fluids decreases gastric emptying = increased absorption

Water Absorption in Small Intestine Osmosis Promoted by coupled transport of glucose and sodium

Composition of Sports Drinks Dependent upon relative needs to replace water and provide fuel substrate Main goal is rehydration i.e. heat Carbohydrate (i.e. glucose) = 20-60 g/L Sodium = 20-60 mmol/L Not exceed isotonicity of 290 mOsmol/L Substrate provision to maintain endurance i.e. cooler environment Carbohydrate (i.e. glucose polymers) = 100-150 g/L Gastric emptying can be minimized by adding glucose polymers Minimize osmolarity Keep volume of the fluid in stomach as high as is comfortable for the athlete

Composition of Sports Drinks cont. Commercially available sports drinks Carbohydrate (i.e. glucose, glucose polymers, and some fructose) = 60-80 g/L, Sodium = 20-25 mmol/L Ideal fluid Replacement Tastes good Does not cause GI discomfort when consumed in large volumes Promotes rapid gastric emptying and fluid absorption to maintain extracellular fluid volume Provides energy in the form of carbohydrates for working muscles Presence of sodium promotes consumption by maintaining thirst

Jeukendrup, Gleeson. Sport Nutrition: An Introduction to Energy Production and Performance 2e. Human Kinetics Inc, 2010.

Drinking During Training Often neglected Practice will accustom athletes to feeling of fluids in the stomach Opportunity to experiment with different volumes and flavorings Measure fluid consumption and body mass changes before and after Provides an idea of athlete’s sweat rate under different environmental conditions Help determine athlete’s requirement for fluid intakes during competition

Rehydration after Exercise Progressive Dehydration with repeated bouts of exercise Imparied thermoregulation Increased cardiovascular strain Loss of thermoregulatory advantages conferred by heat acclimation and high aerobic fitness Loss of intracellular and extracellular volume during recovery Reduced intracellular volume reduces rates of glycogen and protein synthesis Main factors influencing effectiveness of postexercise rehydration Volume and composition of fluid consumed

Sodium in Sports Drink Major cation in intracellular fluid Euhydration is achieved when sodium intake is greater than sodium loss (Schirreffs et. al, 1996) Promotes rapid fluid absorption in small intestine Allows plasma sodium concentration to remain elevated during rehydration proces Maintains thirst Delays stimulation of urine output

Potassium in Sports Drink Enhance replacement of intracellular water Promote rehydration Further studies need to be done to confirm hypothesis

Optimal Rehydration Beverage Carbohydrates Glucose Glucose polymers Stimulates fluid absorption in stomach

Fluid Consumption Post Exercise Post Regular Exercise Any fluid deficit incurred during one exercise can compromise the following exercise session Ingestion of 150 % of weight loss for every kilogram of weight loss during exercise Intake of caffeine and alcohol is discouraged Diuretic actions Consume solid fluids Replace sodium and potassium losses

Recommendations on Exercise and Fluid Replacement American College of Sports Medicine Guidelines on Fluid Intake for Exercise (2002) Adequate fluid replacement maintains hydration and promotes health, safety and optimal performance Consume a nutritionally balanced diet and drink adequate fluids 24 hours before an event Consume approximately 6-8 ml of fluid per kg of body weight about 2 hours before exercise

Recommendations on Exercise and Fluid Replacement Cont. During exercise drink early and at regular intervals to prevent excesses of 2 % of body weight Fluids consumed should be below ambient temperature Add carbohydrates and sodium when exercising longer than 60 minutes Exercise bouts > 60 minutes Add carbohydrates at a rate of 30-60 g/hour to maintain oxidation of carbohydrate and delay fatigue

Recommendations on Exercise and Fluid Replacement Cont. Addition of sodium during exercise bouts lasting > 60 minutes 500 – 700 mg/L water Enhance palatability Promote fluid retention Prevent hyponatremia (in people who drink excessive amounts of fluid during prolonged exercise) Rapid recovery from excessive dehydration 1.5 L of fluid for each kilogram of body weight loss Addition of sodium will assist rapid and complete rehydration by stimulating thirst and fluid retention