Water Balance and
Exercise Performance
NFSC 303
Nutrition and Fitness
“While it is possible to live for
several weeks or even months
without food, our bodies can feel
the effects of water deprivation
within as little as 30 minutes.”
Water comprises 60-70% of body weight
• gives structure to cells
• body fluids:
– saliva, blood, digestive juices, etc.
• solvent for nutrients and waste products;
transport of these
• padding/lubrication:
– joints, spinal cord, GI tract
• thermoregulation
• Intracellular water (~2/3)
– inside cells
• Extracellular water (~1/3)
– interstitial (between and around cells)
– in blood
Water weight varies w/ body composition:
• 1g protein (lean tissue) is associated with 3g
water
• 1g glycogen –
• 1g fat –
• So lower body fat and higher
muscle/glycogen =
• Low kcal/low CHO diet
• decreased glycogen stores (metabolized for
energy)
• decreased body protein (metabolized for
energy or gluconeogenesis)
• weight loss from decreased body water
Water Balance
• Water “in”
– water in food, beverages, and water produced
during metabolism of food
• Water “out”
– skin, urine, feces, lungs
During heavy exercise: losses up to 4 lb/hour
possible
Thermoregulation is affected after:
Electrolytes
• Charged atoms (ions) that conduct electrical
current
• Involved with fluid balance:
– Sodium (Na+) extracellular fluid
– Potassium (K+) intracellular fluid:
important in:
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the generation of electrical impulses to nerves
muscle contraction
transport of glucose into the muscle cells
glycogen storage
Electrolytes
• Chloride (Cl-) major negative ion in both
body fluids and in foods.
• Works w/Na+ in the regulation of both acidbase balance and fluid balance.
Water dissolves salts and follows
electrolytes
Regulation of Body Water
• Osmotic pressure:
– membrane with solution on either side (intra- and
extracellular water)
– Water can pass through freely, but not solutes
– The concentration differs according to the # of
particles per unit volume
– Water is drawn from the compartment with low
concentration to the one w/ high concentration
Factors affecting water balance
• Diuretic: substance that causes the body to
excrete water
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Factors affecting water balance
• ADH: Anti-diuretic Hormone
– Secreted when blood concentration of ions rises
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• Thirst:
****RDA for Water****
• 1 ml/ kcalorie expended
– (your kcals = your ml water recommended via
food and liquids)
Thermoregulation
• Water absorbs heat from muscles  blood to skin
 blood cooled by evaporation of sweat.
– humidity: less evaporation of sweat/higher body temp.
– hot, dry climate: more sweat/water loss
• Excessive body temperature:
– nausea, dizziness, cramps (heat exhaustion)
– can  heat stroke (can be fatal)
Exercise and Water Balance
• During exercise: water lost through breathing and
sweat
• Effects of a 1% body weight water loss:
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–  risk of injury
Signs of Dehydration
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Mild Dehydration
Thirst
Sudden wt. Loss
Dry mouth, throat,
body linings
Rapid pulse
 B.P.
Weakness
Dark, infrequent
urination
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Severe Dehydration
Pale skin
Bluish lips and
fingertips
Confusion
Rapid, shallow
breathing
Weak, rapid, irregular
pulse
Shock; seizures
Coma; death
• During exercise, sodium and chloride are
lost in sweat
• But, kidneys respond by reabsorbing more
Na+ and Cl• Unnecessary to supplement in most cases
(too much K+ can be dangerous)
• Losses of Na, K, calcium magnesium and
phosphorus are easily replaced in the diet
• Sports drinks supply electrolytes in the
event of heavy losses
Preventing dehydration:
• Consume adequate fluids daily
– clear urine every 2-4 hours
– dark colored, infrequent urination indicates dehydration
Preventing dehydration:
• Before exercise:
– 7ml/kg (~1 oz/10lb) water or sports drink about 2 hours
before exercise
• More for profuse sweaters: weigh pre- and post- training
session
– kidneys need 90 min to process fluids
– then 1-2 cups a few minutes prior to exercise
– cold beverages are absorbed more quickly
• During exercise
– Don’t rely on thirst alone (mechanism blunted during
ex)
– Drink early and regularly
– Fluid replacement enhances performance
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– 1 - 1.5 cups fluid every 15 to 20 minutes during
endurance activity to keep gastric emptying up.
Sports Drink
• contain CHO to help maintain glycogen
• more than 8-10% CHO
– water drawn osmotically into the intestine
– bloating, cramping, and diarrhea (osmosis)
Sports Drink
• 6-8% CHO solution is best (most sports
drinks)
– encourages more consumption and is absorbed
quickly (12-20g CHO/cup)
– fruit juice diluted 1:2 with water
• Glucose polymers in sports drinks are quickly
absorbed
• Exercise >60 min, CHO drinks enhance
endurance
• After exercise:
– CHO repletion after exercise to replete glycogen:
1g/Kg within first 2 hours, then high CHO diet
thereafter (65-70%)
– Post exercise fluids should be high glycemic index
fluids (low fructose)
– Protein-CHO combination may increase glycogen resynthesis (3g CHO:1g prot)
– Salt supplement tablets are not generally
recommended (gastric irritation and excess fluid
retention)
Acclimatization
• Exercising in a different climate over 5-10 days
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• Regular exercise also produces
acclimatization-type effects:
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