Transcript Carbohydrate Intake and Athletic Performance

Carbohydrate Intake and
Athletic Performance
How much carbohydrate do athletes
need to eat?
What do you need the CHO for?
At rest, brain/nerve cells use about 7 grams/hr= 150g
but 50% from gluconeogenesis so need to eat 75g
Other use (muscle cells, fat cells, etc. using glucose
after a meal) plus general “out and about”= 125-150g
Total = about 200-225 grams
Physical activity:
30 minutes of easy walking = about 25 grams
45 minutes of moderate running = about 75 grams
60 minutes of hard running = 125 grams
120 minutes of interval running = 200 grams
How much CHO do we have in our bodies?
An average 70 kg male has 1500-1600 kcal
of CHO stored in the liver and muscle
glycogen and circulating blood glucose.
Muscle: 1200 kcal = 30g
Liver: 320kcal = 80g
Blood Glucose: 20 kcal = 5g
So, athletes need to consume
approximately:
225g + (100-300g) = 325-625g CHO
The daily recommended intake for athletes
is 5-10 g of CHO per kg of body weight.
Ex: 70 kg = 350-700g CHO (1400-2800 kcal)
This is 60% of a 2500 kcal - 5000 kcal diet
(athlete in training).
Sources of Carbohydrate
Monosaccharide:
Glucose, fructose,
galactose
Disaccharides:
Table sugar: sucrose
Milk sugar: lactose
Corn syrup: glucose/fructose
Complex Carbohydrates:
STARCH-plants store
extra CHO in the form of
starch.
High Glycemic Index CHO = enters the blood
from the gut quickly. They are easy to digest/
absorb and enter the circulation within minutes.
Ex: Source g/serving
Gatorade
8oz=14g
Cornflakes 3/4cup=25g
Honey
1tbsp=15g
Low/Mod GI CHO = enter the bloodstream more
slowly. They must be broken down first by
digestive enzymes before being absorbed.
(generally have a higher fiber content).
MOD
Rice
Pasta
Banana
1cup=45g
1cup=40g
1med=25g
LOW
Apple
Choc milk
Yogurt
1med=20g
8oz=25g
1cup=50g
CHO Intake Fuels Exercise
• The average American consumes 45% of
their daily calories as CHO.
60% seems like a lot!
CHO intake influences liver and muscle glycogen
stores.
• Glycogen levels affect endurance performance.
– Decreased glycogen hasn’t shown a decrease in
performance time but does show increases in the
perception of fatigue.
• Runners consuming 40% of energy from CHO
could not replenish muscle glycogen on a daily
basis whereas runners on a 70% CHO diet
could. (Costill et al.)
Costill et al.
CHO Intake Before an Event
So if CHO is good, is more better?
Is there any evidence in the literature that
increasing CHO consumption will increase
muscle glycogen levels?
And, if you increase your glycogen stores,
does this have any impact on performance?
How do athletes increase their
muscle glycogen stores?
Supercompensation involves “swelling” muscle glycogen by
changing diet and exercise patterns several days before a
major competitive event.
• 2 techniques:
1. depletion/repletion
2. Modified moderate approach
1. Depletion/repletion:
- depleting glycogen reserves as much as possible
through a combination of an extremely long bout of
exercise about 7-10 days prior to a major competition,
followed by a few days on a very low-CHO diet. Another
bout of exercise on the 4th day and then 3 days of a high
CHO diet.
• Muscle glycogen stores can be greatly
increased, often by as much as 50% over
the usual “full” condition (e.g. from 300 
over 400 grams).
• If it costs 100 kcal to run a mile, and 60% of
energy comes from muscle glycogen = 60
kcal of glycogen mile = 15 g glycogen
needed /mile. If you start with 300 grams
you can run 20 miles before running out.
• If increase to 400g, can extend run to 26.5
miles.
2.
More popular approach is to taper training the week
before competition while increasing the %CHO in diet
(e.g. from 50-60% up to 70-75%).
•
Although increases in muscle glycogen are not as
dramatic as with the depletion/repletion approach,
increases of perhaps 20% are possible (300g  350g)
Benefits from CHO loading:
Increases time to exhaustion
Reduces the time to complete a task
(time trial; endurance by 2-3%)
May improve performance in team sports that
involve intermittent exercise and skill executions.
The downside to CHO loading/muscle glycogen
supercompensation:
Only good for a one-time ultra distance event (marathon). A
cyclist riding over several days would not see
benefits...consecutive competition days are not improved
with this protocol.
Every gram of CHO is stored with 3g of water. This means
that the storage of 500g of CHO is accompanied by a water
weight gain of approximately 2kg. This would not be good for
sports where an increase in body mass is not desirable.
CHO intake before exercise
Goal: maxmize exercise performance
3-5 hours: this is usually • Remember that the
breakfast 
liver glycogen is used
to:
- Large meal to
– Maintain blood
replenish liver
glucose levels (it’s the
glycogen
brain food)
- Increase muscle
– It’s decreased after an
glycogen stores
overnight fast.
– It’s decreased by 50%
after an hour of
exercise at 75%
VO2max.
60-30 minutes before exercise...
• Top off liver glycogen
• Increase blood glucose levels, can be
used as a substrate during exercise...or if
intensity is low enough it will be used to
synthesize muscle glycogen.
Common belief: ingesting High GI CHO right
before exercise will create rebound
hypoglycemia and decrease performance.
Why?
Rebound Hypoglycemia
Hypoglycemia:
- Dizziness, nausea,
cold sweat...
-
-
Insulin release stimulated by glucose
promotes skeletal muscle glucose uptake.
Exercise (muscle contractions) also stimulate
glucose uptake...both are working...
Galactose
Decreased mental
alertness
Decreased motor skills
Placebo
9
Glucose
8
- Increased HR
- Hunger
- Disorientation
Glucose (mM)
7
6
5
4
3
2
1
0
0 15 30 45 60 70 80 90 100 110 120 135 150 165 180
TIME (min)
There is little evidence to support this
hypothesis that performance is
decreased...
However, two strategies are widely used to
prevent rebound hypoglycemia in
athletes.
1.
2.
Consuming lower GI CHO one hour before exercise
If high GI CHO is consumed right before or at the
onset of exercise, then there seems to be a benefit to
maintaining carbohydrate intake during exercise.
“once on gatorade-stay on gatorade”
CHO during Exercise
CHO consumption during exercise >45min can
improve performance.
How?
1. Maintain blood glucose and CHO oxidation rates.
2. Spares liver glycogen.
3. Synthesizes muscle glycogen during LOW intensity
exercise
4. Increases motor skills in prolonged periods of play.
5. “Feel better” later into the exercise bout.
How much should I consume?
- 1.2g per minute, 70g/hour during endurance exercise...
What should I drink?
glucose polymer (gatorade=good)
galactose,fructose-not so good, must first be converted
to glucose by the liver to be used as a substrate.
The delivery rate from the gut to the muscle tissue is
1.0g/min, as intensity increases blood flow to the
digestive system decreases and there is a greater
reliance on muscle glycogen as a fuel source.
Choose drinks ranging from 4-8% CHO. Drinks with too
much CHO (coke has 11.25% CHO) will not be absorbed
as efficiently and could cause stomach upset.
Remember that CHO replacement during exercise is
not generally needed for activities lasting less than
90 minutes.
CHO after exercise...
GOAL: To replenish muscle glycogen
The rate of glycogen synthesis depends on glucose
transport to the muscle and the enzymes involved in
glycogen synthesis.
Exercise increases:
- GLUT 4 (transporters)
- glycogen synthase
There are two phases of muscle glycogen replenishment
after post-exercise CHO consumption:
Rapid (not dependent on insulin-but enhanced)
Slow (>3hrs, insulin dependent)
CHO after exercise...
• Factors affecting glycogen synthesis:
– Timing
• When CHO is delayed two hours muscle glycogen
is 45% lower compared to immediate consumption
of the same amount.
– Rate of ingestion (amount): 0.7-1.0g
CHO/kg consumed every two hours
– Type (High GI, vs. Low GI): High GI, large
insulin release, more CHO storage
– Combo with protein
Glycogen Synthesis
Once CHO needs are
met, the effect of
protein
supplementation does
not enhance muscle
glycogen storage.
Glycogen
synthesis relative
to control
300
200
100
%
0.8 g/kg/hr
0.8 g/kg/hr
1.2 g/kg/hr
1.2 g/kg/hr
CHO
CHO+PRO
CHO
CHO+PRO
What do I do if rode 35 miles and left my $2
drink and $3 bar at home?
The key to optimal recover is to create a hormonal environment that is
anabolic and provides the necessary raw materials.
Eating foods high in CHO provides glucose and also stimulates a
large insulin response that facilitates entry of glucose into cells
and its storage as glycogen.
Maximizing the amount of glycogen stored depends on both the
quantity of CHO and the insulin response: in the short-term (first
2-6 hours) there does appear to be an advantage to consuming
simple sugars rather than complex CHO because the insulin
response is greater.
Strength and Power Athletes
• We know CHO is great for increasing endurance
performance but what about strength and power
athletes, do they need CHO the same way?
– Higher intensity activities use muscle glycogen as a
fuel source so adequate glycogen is necessary for
performance.
– Glycogen super-compensation does not appear to
increase performance once a certain intensity is
reached.
– Supplementation is most advantageous for athletes
involved in high intensity training regimes where time
between sessions is limited <20hours.