Hydration and Sports Drinks
Heat Removal
• Exercise increases core temperature.
For every L of O2 consumed during
exercise ~5 kcal is produced but only
1kcal is used for mechanical work – the
rest is heat
• Increases in core temperature:
– Increase blood flow to the skin
– Initiate sweating
Sweating and Fluid Loss
• Mild to moderate work and routine physical
training typically result in whole-body sweat
losses of about ~0.8-1.4 L/h.
• Evaporation of 1L of water from the skin will
remove 573 kcal of heat from the body.
• Heat loss via evaporation of sweat is largely
dependent on the humidity (proximate to the
skin) in which the athlete is working.
• Individual sweat rates and fluid losses vary
widely between athletes and across sports.
Dehydration and Performance
• If sweating is not compensated for with fluid
intake performance is affected.
– Dehydration may explain some of the fatigue
at the end of a long sporting event.
• Exercise performance is impaired by a fluid loss
of 2% (this is 1.5L in a 155lb male athlete),
Losses in excess of 5% can decrease work
capacity by 30% (water weight loss of ~8lbs)
In cool laboratory conditions, VO2max has been shown to
decrease with fluid losses > 2% BW...
In the heat, sweat rates increase even more such that fluid
intake must be even more closely monitored.
Exercise Capacity
100%
80%
60%
0
1
2
3
Body weight loss (%)
4
5
How does sweating (fluid loss) work to decrease performance?
increase in plasma osmolarity (higher
concentration of solute in solution)
release of vasopressin (ADH) from the
posterior pituitary gland.
ADH causes the kidney to re-absorb water
Urine volume goes DOWN dramatically to preserve
blood osmolarity, volume and pressure
As exercise progresses, the transfer of body
water into the plasma cannot keep up with
sweat losses so...performance decreases.
How can I tell how much I’m sweating?
Pre exercise weight
Post-exercise weight
Volume of fluid consumed during exercise
Exercise duration
55 kg
53.5 kg
1 kg
2h
Calculations
• Fluid deficit (L) = 55 kg – 53.5kg = 1.5 kg
• Total sweat loss (L) = 1.5 kg + 1 kg = 2.5 kg
• Sweat rate (L/h) = 2.5 kg / 2hr = 1.25 L/h
Hydration & Rehydration
1
2
3
4
5
6
7
8
• Urine Color Chart
• Assess Hydration
• 1, 2, or 3: Well
Hydrated
• 7 or Darker:
Dehydrated
Daily Water Balance
Athletes need to be fully hydrated before training/competition
The average sedentary person excretes about
2700 milliliters of water/day (1400 from urine, 1100
sweat/breath)
To replace that 2700 ml, get:
• 300-400 ml from metabolism,
• 1000 ml from food and
• 1300-1400 ml from beverages
2700 ml
Athletes and Daily Water Balance
Athletes and Fluid Intake
• Athletes have to consider their daily water
balance and sweat loss during exercise
when they construct a hydration plan.
– Choices to maintain/increase fluid intake:
•
•
•
•
Water
CHO drinks
Choc Milk
Coca-Cola
• The maximum amount of fluid that empties
from the stomach during exercise is ~0.81.2 L/h.
– Remember, sweating during exercise can
cause fluid losses of 0.8-1.4l/hr
• Up to 10% CHO solution empties from the
stomach at the same rate as pure water
(Coke is 11%)
• Glucose and sodium both stimulate the
absorption of fluid across the membrane of
the small intestine.
• Athletes should consume enough water
throughout exercise that their weight
remains constant (if possible)
• Guidelines for fluid intake are VERY
general due to the variability of sweat
rates among individuals.
– Good rule of thumb is to consume something
every fifteen minutes (8-12 oz.)
Before
ACSM
NATA
AIS
Gatorade
NCAA
500 ml : 2 hours
500-600 ml : 2-3 hours
Ind
17 - 20 oz : 2 to 3 hours
17 - 18 oz : 2 hours
200-300 ml : 10-20 min
7-10 oz : 10 - 15 pre warm-up
Regular Intervals
200-300 every 10-20 min
Ind
7-10 oz every 10-15 min
8 oz every 10-15
min
After
Equal to loss
150% of weight lost
Ind
20-24 oz / lb
20-24 oz / lb
Temp
59o - 72o F
50o - 59o F
Ind
Cooled
50o - 59o F
CHO and Sodium
CHO and Electrolytres
Ind
CHO (6-7%) and Sodium
-
During
Contents
After Exercise
• Drink 150% of water weight loss to achieve
normal hydration 6hrs post-exercise
(this accounts for reduced urine output)
• Pure water may not be the ideal recovery drink:
– Causes rapid fall in plasma sodium and thus plasma
osmolarity which reduce the stimulation to drink and
also increases urine output. This can delay the rehydration process
• What to drink then?
– Plasma volume is rapidly restored if some sodium
chloride is added (77mol or 0.45g/L)
Sodium:
– Helps maintain thirst, delays urine production
Potassium:
– Thought to promote intracellular hydration (little
evidence to support this, but it sounds good)
CHO (glucose, glucose polymers)
– More likely to continue drinking fluid if beverage is
sweetened (taste) vs. pure water
– Presence of glucose stimulates fluid absorption
– We know it helps with muscle glycogen re-synthesis
CHO Sports Drinks
• The goal of the carbohydrate content is to provide the
appropriate CHO concentration while preventing
gastrointestinal disturbances.
– At 6-8% carbohydrate concentration, sports drinks have
can be absorbed 30% faster than water.
• The composition of the carbohydrates in sports drinks is
another variable that affects the effectiveness of the drink
The 3 things a sports drink should accomplish:
• Hydrate effectively.
• Replenish electrolytes.
• Provide energy for working muscles to enhance
performance and speed recovery.
Gatorade
• Gatorade
– 6% Carbohydrateelectrolyte solution.
– 14 g of CHO (sugar) in 8 fl
oz.
– 110 mg of Na+ in 8 fl oz.
– 30 mg of K+ in 8 fl oz.
– 90 mg of Cl- in 8 fl oz.
– Retail Price $1-2.
Powerade
• PowerAde
– 7% Carbohydrateelectrolyte drink.
– 18 g of CHO in 8 fl oz with
15 g as sugar.
– 53 mg of Na+ in 8 fl oz.
– 34 mg of K+ in 8 fl oz.
– Retail Price $1-2.
Flavored Water
• Propel Fitness Water
–
–
–
–
–
–
–
–
–
–
Calories 10 in 8 fl oz.
Sodium 35 mg
Potassium 40 mg
Total Carbohydrate 3 g Sugars
2g
Vitamin C 10%
Vitamin E 10% Niacin 25%
Pantothenic Acid 25%
Vitamin B12 4%
Vitamin B6
Retail Price $1.25 per bottle.
• From the textbook to the track:
– Don’t wait until thirsty... Thirst signal is the
result of being slightly dehydrated.
– Monitor urine output and weight before and
after exercise (approximate fluid loss)
– In hot weather athletes need to consume
more fluid during/after exercise compared to
colder weather.
– For recovery, supplement with a carbohydrate
drink that contains some sodium to aid in
replenishment of glycogen stores and
electrolytes. (remember the protein)...
Gatorade, Choc milk or coke?