Fluid, Carbohydrate, and Electrolyte Replacement During Exercise
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The goal of eating and drinking during exercise should be to maintain or possibly even improve
performance. Not eating and drinking properly during exercise will have consequences ranging from
a slight decrease in performance through, in the extreme case, death.
How much should I drink?
Proper hydration is key to every workout. Exercise generates heat in working muscles, which must
be regulated primarily through evaporation of sweat. By drinking water to offset the water lost
through sweat, a fluid balance is achieved, and the body's ability to cool itself will not be limited
through fluid losses (it still could be limited due to the environment). The amount of fluid lost through
sweat, and the amount of fluid that can be processed by drinking will vary from person to person, and
depends on environmental conditions and exercise intensity.
Personal Hydration Plan
The most basic way to establish a hydration plan is to weigh yourself before and after a workout, and
keep track of how much you drank during the workout. If you lost weight, you didn't drink enough. If
you gained weight, you drank too much. How much? Water weighs 1 kg per liter. So, if you lost 1 kg
of weight, you should have drank an extra liter of fluids. Total the amount of liquid consumed plus any
additional liquid that should have been consumed and divide by the number of hours in the workout.
This is roughly how much fluid you should consume in an hour. This method will get you close, and if
you keep weight loss to less than 2% of overall weight, your performance shouldn't be affected too
much.
A relatively simple experiment can estimate your hydration needs with a bit more accuracy. This will
require a scale to measure body weight, a measuring cup or scale to measure fluid volumes (a
cooking scale works well for this, 1 L of water weighs 1 kg), bottles for drinking water, and a bottle for
urine collection (a container you don't plan to use again, such as an empty milk container, may be a
good choice).
If possible, test at competition intensity and environmental conditions.
1. Empty bladder
2. Take unclothed body weight before exercise (mb, kg)
3. Time the exercise (t, hours)
4. Keep track of fluid intake volume during exercise (vf, L)
5. Keep track of urination volume during and immediately following exercise (vu, L)
6. Take unclothed body weight at the end of exercise (me, kg)
7. vlost = (mb - me)(1 L / 1 kg) - vu + vf
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Fluid, Carbohydrate, and Electrolyte Replacement During Exercise
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Your ideal replenishment rate will be:
vhydration = vlost / t (L / hour)
This is the amount of fluid you must consume each hour to maintain fluid balance. Less than this,
and you will start down the path to dehydration and loss of performance. More than this and you'll be
making more bathroom stops than necessary.
What about electrolytes?
In addition to water, sweat also contains sodium, chloride, potassium, calcium, and magnesium (as
well as other elements). The concentration of the elements lost in sweat also varies from person to
person, and can be dependent on training status and acclimatization level. Loss of these electrolytes
may impact performance and may be related to muscle cramping.
Consuming too much water without electrolytes (sodium specifically) can lead to a condition known
as hyponatremia, where the sodium concentration of the liquid portion of blood becomes too low. An
increase in body mass (due to water retention), extreme fatigue, nausea, disorientation, and
complications leading to death are possible.
General replacement guidelines for sodium, chloride, and potassium are as follows:
Sodium: 460 - 690 mg/L
Chloride: 700 - 1050 mg/L
Potassium: 78 - 195 mg/L
Carbohydrates
Research has shown that consuming carbohydrates during exercise helps maintain blood glucose
levels, and provides an improvement in performance for exercise lasting longer than 1 hour (using up
to 60 g carbohydrates / hour). The upper limit for carbohydrate consumption appears to be related to
"gastric emptying", or, how fast fluids are emptied from the stomach. A slowing of gastric emptying
may leave you feeling bloated and uncomfortable, as well as limiting the effectiveness of your fluid
intake. This emptying is more dependent on the carbohydrate concentration of the fluid rather than
the absolute quantity of carbohydrates. A fluid with a concentration between 6 - 8% (6 - 8 g of
carbohydrates per 100 mL of water) has been found to be tolerable for most people. You personally
might tolerate more or less, but the only way to find out is to experiment.
If you add solid foods to your workout nutrition, be aware of how many carbohydrates are in that food
and how it can affect gastric emptying. Some solid foods may also absorb fluids while in the
stomach, which would further slow gastric emptying.
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Fluid, Carbohydrate, and Electrolyte Replacement During Exercise
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Finally, absorption levels may depend on the type carbohydrate (glucose vs. fructose). Glucose may
be more rapidly absorbed than fructose, and may also be more readily available once in the
bloodstream (fructose requires processing by the liver). Some studies have shown that
supplementing with both glucose (primary) and fructose (secondary) may be more effective than
supplementation with glucose alone. One theory is that fructose and glucose utilize different
transport mechanisms, so using both carbohydrates enables a larger absorption quantity.
Other common carbohydrates found in the exercise world are sucrose, maltodextrin, and galactose.
Sucrose is a combination of glucose and fructose. Maltodextrin is a chain of glucose molecules.
Galactose is a simple sugar (like glucose and fructose).
My bottom line recommendation is to look for a carbohydrate source that contains mainly glucose or
maltodextrin listed before any fructose, sucrose, or galactose. This source could be in the form of a
powder added to water, or in the form of a gel.
Example - Hydration Plan
Here's a real world example. I picked a very hot and humid day to determine my personal hydration
needs through a run experiment. On a twelve mile run, at a blazingly fast pace of just under 10
minutes per mile, I kept track of my pre and post run weight, along with the amount of fluids
consumed.
Pre-run weight: 68.31 kg
Post-run weight: 66.86 kg
I consumed 4 water bottles with approximately 540 mL of water each.
4 x 540 mL = 2160 mL = 2.16 L
I did not measure urine output during the run.
How much fluid deficit did I have?
68.31 kg (pre-run) - 66.86 kg (post-run) = 1.45 kg, and since 1 L weighs 1 kg, = 1.45 L
I should have drank 1.45 L in addition to the 2.16 L, for a total of 3.61 L
Over the duration of 2 hours, my intake rate should have been 3.61 L / 2 h = 1.8 L / h
That's just over 60 oz per hour! The equivalent of three small bike bottles, or two and a half large
bike bottles PER HOUR!
(c) 2011 FxD Coaching
Fluid, Carbohydrate, and Electrolyte Replacement During Exercise
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Example - Adding in the extras
Great, now I know on a really hot, humid day running at a low intensity requires 1.8 L / h if my goal is
to maintain fluid balance. I might back that down a bit for a "normal" day, and estimate two large
water bottles (48 oz, or 1.4 L) per hour.
I want to take in 6 - 8% carbs, plus the required electrolytes. For running, I prefer to hydrate strictly
with water and supplement with gels. Any fluid I take running will splash, and a sticky mess tends to
be the result if I use a drink mix. Plus, in a race you're almost guaranteed that water will be available.
There's no guarantee any sports drink will be offered, or of the quality of the drink.
How many carbohydrates can I take in? At 1.4 L / h, a 6 - 8% solution would contain 84 to 112 g.
That's approximately 3 to 4 GU gels an hour. Wow - that's a lot of GU.
How do 4 GU gels stack up for the electrolytes?
We're looking for:
Sodium: 460 - 690 mg/L = 644 - 966 mg (1.4 L)
Chloride: 700 - 1050 mg/L = 980 - 1470 mg (1.4 L)
Potassium: 78 - 195 mg/L = 109 - 273 mg (1.4 L)
and 4 GU gels provides:
Sodium: 55 x 4 = 220 mg
Chloride 86 x 4 = 344 mg (determined through a little math)
Potassium: 45 x 4 = 180 mg
Not very good. The potassium is fine, but sodium and chloride are low. One solution is to take in a
little table salt (sodium chloride) during the run. Or, use a commercial product such as Saltstick
capsules (215 mg sodium per capsule).
What if I was using a drink mix, such as GU Electrolyte Brew (formerly GU2O)? Using the product as
directed (3 scoops per 24 oz bottle, 6 scoops per hour), I would be taking in:
Carbohydrates: 78 g (5.6% concentration)
Sodium: 750 mg
Chloride 1170 mg (determined through a little math)
Potassium: 120 mg
Now we're in the ballpark. The carbohydrates are a little low (78 g versus 84 - 112 g). Add in a GU
gel each hour (25 g) and I'm up to 103 g (25 g + 78 g).
(c) 2011 FxD Coaching
Fluid, Carbohydrate, and Electrolyte Replacement During Exercise
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Funny how this is working out - GU recommends hydrating with their electrolyte brew and
supplementing with 1 GU gel every 45 minutes (33 g / hour). Following this strategy results in:
Carbohydrates: 111 g (8% concentration)
Sodium: 823 mg
Chloride 1285 mg (determined through a little math)
Potassium: 180 mg
Perfect - you'd almost think the folks at GU planned this!
FxD General Recommendations
For light to moderate exercise of an hour or less under normal heat and humidity conditions,
consuming water alone is adequate.
For intense exercise of an hour or less, consume a GU Energy Gel approximately 15 - 20 minutes
prior to exercise. The gel flavors with caffeine may provide some additional benefits. Hydrate with
water alone, or with water plus GU Electrolyte Brew.
For exercise lasting over an hour, consume at least one GU Energy Gel per hour and hydrate with
GU Electrolyte Brew. Inclusion of solid foods may be a benefit for durations longer than 2 to 3 hours.
References
American College of Sports Medicine et al. American College of Sports Medicine position stand.
Exercise and fluid replacement. Medicine and Science in Sports and Exercise (2007) vol. 39 (2) pp.
377-90
Armstrong, L. E. Performing in Extreme Environments. Champaign, IL: Human Kinetics, 2000.
McArdle, W. D., F. L. Kartch, and V. L. Katch. Essentials of Exercise Physiology. Baltimore, MD:
Lippincott Williams and Wilkins, 2006.
Wilmore, J. H., and D. L. Costill. Physiology of Sport and Exercise. Champaign, IL: Human Kinetics,
2004.
GU: www.guenergy.com
SaltStick: www.saltstick.com
(c) 2011 FxD Coaching