Transcript FN3373, Lecture 2&3 (OWL) – Ch 2 (Carbohydrate)

chapter
chapter
22
Carbohydrate
Carbohydrate as
asaa
Fuel
Fuelfor
forExercise
Exercise
Prof Jennifer Broxterman, RD, MSc
FN3373: Nutrition for Physical Activity
Lectures 2 & 3
Author name here for Edited books
Chapter 2 Introduction
• Carbohydrate as a Fuel for Exercise:
– Well-documented that CHO is important for athletic
performance
– High levels of stored glycogen before endurance
exercise (esp. > 1hr) can help increase performance
& reduce time to fatigue
– High CHO post-exercise enhances recovery
– Many athletes consume inadequate levels of CHO to
support their training
Dietary Carbohydrate
• Optimum dietary CHO levels depend on:
–
–
–
–
Total energy intake
Body size
Health status
Duration, intensity, frequency,
and type of exercise
Function, Classification,
and Dietary Sources
of Carbohydrate
Function of Carbohydrates
• CHO are:
– Primary source of energy (1 of 3 macronutrients)
– Provide the substrate necessary for glycogen
replacement (substrate: glucose)
– When consumed during exercise, help maintain BG
levels & help prevent premature fatigue
• CHO recommendations for active individuals:
– Moderate training: 5-7 g/kg of BW
– Heavy training: up to 10 g/kg of BW (Burke, 2007)
Classification of Dietary CHO
• Different ways to classify CHO
– Type of CHO found in the food
– Level of commercial processing the food has
undergone
– BG or glycemic response to the CHO within the
body
Structural Classification of CHO
• Complex carbohydrates: long complex chains of
sugars linked together
– Initially believed that all complex CHO were digested
more slowly than simple CHO
– The term ‘complex carbohydrate’ only refers to the
structure of the CHO, not to any digestive properties
Food Examples of Complex CHO
• Nutritionists / dietitians
generally consider the
following foods “complex
CHOs” because they are
good sources of vitamins,
minerals, and fibre
–
–
–
–
Vegetables & fruit
Whole grains (breads, cereals, pasta)
Legumes (beans, peas, lentils)
Primarily contain: starch and fibre
Structural Classification of CHO
Structural Classification of CHO
• Simple carbohydrates: primarily refer to
processed foods or foods high in sugar
– E.g. sweetener cereals, breakfast bars, candy,
regular pop, desserts
– Are generally low in vitamins, minerals, and fibre
unless they are fortified
– Primarily contain: mono-, di-, and oligosaccharides (glucose, sucrose, fructose, and highfructose corn syrup)
Primary CHOs & Sugar in the Diet
• Monosaccharides: simplest form of sugar
– Glucose: main CHO in the bloodstream
• Main energy source in the cell
• Stored in the liver, muscles, and other organs as glycogen
• Rapidly absorbed from the gut through sodium-dependent
glucose transporter
– Fructose: simple sugar found in honey & fruit
• Tastes sweeter than table sugar (sucrose)
• Absorbed from the gut through the glucose transporter 5
(GLUT5) and must be transported to the liver for conversion to
glucose
– Galactose: simple sugar found in milk
Primary CHOs & Sugar in the Diet
• Disaccharides: made up of 2 simple sugars
– Sucrose: glucose + fructose
• Common table sugar, extracted from sugar cane and beet sugar
• Most common dietary disaccharide
• Broken down into glucose and fructose in the gut prior to
absorption
– Lactose: glucose + galactose
• Sugar found in milk products
• Lactose intolerant (lacking the lactase enzyme), common in
Asians, Native Americans, Hispanics, and blacks
– Maltose: glucose + glucose
• Primarily formed from the breakdown of starch
• Rapidly digested to glucose and absorbed quickly into the body
Primary CHOs & Sugar in the Diet
• Oligosaccharides: short chains of 3 to 10
monosaccharides linked together
– Maltodextrin:
• Glucose polymer manufactured as long starch units are broken
into smaller groups
• Sugar found in sports drinks and many processed foods
• Rapidly digested to glucose and quickly absorbed
– Corn syrup:
• Sweet syrup made up of glucose and short-chain
glucose polymers produced by enzymatic hydrolysis
of corn starch
• Rapidly digested and absorbed
Primary CHOs & Sugar in the Diet
• Oligosaccharides: short chains of 3 to 10
monosaccharides linked together
– High-fructose corn syrup:
• Especially sweet corn syrup
• 45% to 55% of the CHO is enzymatically hydrolyzed to glucose
and fructose (has nearly 2x the concentration of mono- and
disaccharides found in regular corn syrup
• Predominant sweetener found in commercially sweetened foods
Primary CHOs & Sugar in the Diet
• Polysaccharides: contain starch and fibre
(“complex carbohydrates”)
– Starch: found in plants, seeds, and roots
• Made up of straight chains of glucose polymers called amylose
and some branching chain polymers called amylopectin
• Starch is digested into glucose
• Starches high in amylopectin are more rapidly digested and
absorbed than starches high in amylase
– Dietary fibre: part of the plant that cannot be digested
by human gut enzymes
• Goes from the small intestine into the colon, where it is expelled
as fecal material or fermented and used by gut bacteria as food
• Soluble vs. insoluble fibre
Glycemic Response
to Carbohydrates
Glycemic Response
• Glycemic response:
– Classify foods as producing a high, moderate, or low
glycemic response
– Glycemic response to both simple and complex CHO
foods can vary greatly
– Some complex CHO (i.e. high in starch) can be
hydrolyzed and absorbed as quickly as simple sugars
Glycemic Response
• High glycemic response:
– Foods that produce a
large and rapid rise in
blood glucose and insulin
– Can increase muscle
glycogen more than
foods that produce a
low glycemic response
Glycemic Response
• Glycemic index (GI): scale that ranks CHO-rich
foods by how much they raise blood glucose
levels compared to a standard food
– Determined by feeding 50 g of a particular food and
watching the blood glucose response over a 2 hr
period
– BG response is compared to a reference food
(usually white bread or glucose), with a GI = 100
GI = BG area of test food
x 100
BG area of reference food
Glycemic Response
• Glycemic load (GL): accounts for both the
amount and source of CHO in a meal
– GL = (GI of a food or meal) x (g of available CHO in
the food or meal)
Video: GI vs. GL
Carbohydrate Metabolism
During Exercise
Carbohydrate as a Fuel Source
• Muscles use of CHO during exercise:
– Amount of CHO required depends on the:
• frequency, intensity, duration, and type of exercise
• environmental conditions
– CHO used during exercise comes from the following
sources:
• Endogenous production of glucose by the liver
(gluconeogenesis)
• Blood glucose
• Muscle and liver glycogen stores
• CHO consumed during exercise (exogenous CHO)
Figure 2.1
Crossover concept of fuel
use during exercise:
– Low-to-moderate intensity:
CHO + lipids play major
roles as energy substrates
– Higher intensity (relative
aerobic power = 60-65%):
CHO becomes increasingly
important
– Lipids become important
energy sources during
recovery
Gluconeogenesis
• Gluconeogenesis: endogenous glucose
production
– Metabolic pathway that results in the generation of
glucose from non-carbohydrate carbon substrates
– One of the main mechanisms humans use to keep
BG levels from dropping too low (hypoglycemia)
– Main substrates during exercise: lactate, alanine,
glycerol, pyruvate
• Primarily come from the muscle
• Small amounts of glycerol come from adipose tissue
• Are transported to the liver for glucose production
Figure 2.3 Gluconeogenesis Pathway
Gluconeogenesis
• Amount of gluconeogenesis that occurs
during exercise is impacted by:
–
–
–
–
–
Available CHO reserves prior to exercise initiation
Amount of CHO provided during exercise
Type, duration, and intensity of the exercise bout
Exercise environment (e.g. temperature, altitude)
Level of endurance training
Gluconeogenesis Substrates
• Lactate:
– Primary source of lactate during exercise is from the
metabolism of glucose to lactate (through gylcolysis)
– Lactate is transporated to the liver for glucose
production through the Cori cycle, or it may be used
directly by adjoining cells as an energy source
– As glycogen is depleted in the working muscles,
non-working muscles can give up some of their
stored CHO by releasing lactate
• Ahlborg & Felig (1982)  showed that lactate released
from the arms increased both during and after 3-3.5 hr of
leg exercise (cycling)
Figure 2.6 Cori Cycle
Gluconeogenesis Substrates
• Alanine:
– Primary amino acid released by working muscles
during exercise
– Alanine is synthesized as nitrogen (released from
the breakdown of aa in the muscles) and is
combined with pyruvate
– Alanine is transported to the liver, where it is broken
down into pyruvate and nitrogen
• Pyruvate can be used as a gluconeogenic substrate
• Nitrogen is converted into urea and eliminated through the
kidneys
– This pathway is called the glucose-alanine cycle
Figure 2.7 Glucose-Alanine Cycle
Gluconeogenesis Substrates
• Glycerol:
– Is the 3-carbon backbone
of a triglyceride
– Adipose tissue or muscle
triglycerides can be broken
down to yield 3 FAs and glycerol
– FAs transported to the muscles for energy
production
– Glycerol transported to the liver for gluconeogenesis
Gluconeogenesis Substrates
• Pyruvate:
– Final substrate used for gluconeogenesis
– 3-carbon compound
– Can leak from working cells into the blood and is
transported to the liver to make glucose
Glycogenolysis
• Glycogenolysis: the chemical process by
which glucose is freed from glycogen
– Liver glycogenolysis: Another source of BG during
exercise is the breakdown of liver glycogen
– Glucose from the liver can be released directly into
the bloodstream helping to maintain BG levels
during exercise (unlike muscle glycogen)
– Liver glycogen can be depleted if exercise is
strenuous and of long duration
• Gluconeogenesis and consuming exogenous CHO (e.g.
sports drinks, gels) become increasingly important to
maintain BG levels
Hormonal Control of
Carbohydrate Metabolism
During Exercise
Hormones & Exercise
• Hormonal changes:
– Signal the body to break down stored energy for
fuel, which can then be used by the working muscles
for energy
– Hormonal responses depend on 2 main factors:
• Intensity and duration of the exercise
• Individual’s level of physical fitness
Hormones & Exercise
• Norepinephrine & Epinephrine:
– Blood levels rise dramatically within minutes of the
initiation of exercise
– Stimulate the breakdown of stored fat (both adipose
& muscle tissue) and CHO (both liver & muscle
glycogen), making these fuels available to the
working muscles
• Insulin:
– Levels decrease or are maintained at a low
concentration during exercise
• Acute & chronic exercise increases the sensitivity of the
skeletal muscle to the action of insulin
Hormones & Exercise
• Glucagon:
– Released from the pancreas in response to the low
BG levels that may occur with exercise
– Potent stimulator of glycogenolysis and
gluconeogenesis
– Helps to maintain BG levels by increasing the
release of glucose into the bloodstream
• Cortisol:
– Also stimulated gluconeogenesis and helps to
mobilize free FAs and amino acids
Carbohydrate Reserves
and Dietary Intake
Carbohydrate Reserves
• Primary fuel sources during exercise:
– Carbohydrate (glucose) & fat (fatty acids)
– Relative amounts used depend on the exercise intensity
and duration
• CHO reserves:
– Compared to fat & protein, the body’s CHO reserves are
severely limited
– Total amount of energy stored as glycogen ranges from
800-2000 kcal
• Depends on the diet, size of athlete, fitness level, and time of
day
• CHO consumed during exercise can supplement these reserves
Total Body Glycogen Reserves
• Total CHO storage:
– Total glycogen found in the liver, muscle, and other
organs is not much greater than the amount of CHO
consumed on average each day
– 2000 kcal/day  50% of kcal from CHO offers
~250 g of CHO
– After a typical meal, approximately 25-33% of CHO
consumed is converted to liver glycogen; about 3350% is converted to muscle glycogen; and the
remainder is oxidized for energy in the hours after
eating
Liver Glycogen
• CHO Reserves:
– Primarily liver and muscle glycogen
– Glycogen concentrations are highest in the liver
• Amount in a typical liver weighing 1.5 kg after an
overnight fast is ~4% of the liver’s total weight, or 60 g
• After a meal, amount of glycogen can double to ~8% of
the liver’s weight, or 120 g glycogen
• Liver glycogen plays a major role in maintaining BG
levels throughout the night
– morning meal containing CHO is important to replenish
glycogen stores
Muscle Glycogen
• Storage of glycogen in the muscle:
– Lower than that in the liver
– Deliberate CHO loading is required to increase the
amount to more than 2% of fresh weight of rested
muscle (~400 g)
– Absolute amount of glycogen stored in the muscle
can range from ~300 to 400 g (1200 - 1600 kcal) in
a 70 kg athlete
Muscle Glycogen
• Use of muscle glycogen during exercise:
– Depends on amount of glycogen available before
exercise begins
– Exercise intensity and duration
– Environmental conditions
– Whether or not exogenous CHO is consumed
Dietary Carbohydrate Intakes
of Active Individuals
• Dietary intake of CHO:
– Active men & women usually report CHO intakes
similar to weight-matched inactive individuals
– 45-55% of total energy from CHO or ~5-6 g/kg BW
per day
– Appropriate for recreational athletes who exercise
for 1 hr or less per day
– May be too low for endurance athletes who engage
in daily intense training and whose glycogen stores
need to be replenished rapidly
• May require up to 10 g CHO/kg BW for men and
6-8 g CHO/kg BW for women
Carbohydrate Feeding
Before Exercise
Pre-Exercise & BetweenCompetition Meals
• Goals of pre-exercise meal:
–
–
–
–
Promote additional glycogen synthesis
Supply the body with glucose for use during exercise
Minimize fatigue during exercise
Replenish liver glycogen, especially after an overnight fast
• Timing: pre-exercise meal usually consumed
2-4 hours prior to the exercise event
– Often can be safely eaten at late as 1 hour before
exercise
Pre-Exercise & BetweenCompetition Meals
• Pre-exercise meal should be:
– Small, easy to digest
– Familiar to the individual
– Contain foods that do not cause gastrointestinal
distress (e.g. fibre, fat, carbonation)
– Provide CHO to improve glycogen reserves and BG
• Glycemic index:
– Low GI foods may offer better satiety and produce
more stable BG concentrations than high GI meals
Pre-Exercise & BetweenCompetition Meals
• Nerves & appetite:
– Nervousness before an exercise event can cause GI
distress and loss of appetite
– Can use fruit juices, sport drinks, or glycogen
replacement products to provide the energy and CHO
needed
• Multiple exercise bouts within a 24 hr period:
– What to eat depends on athlete’s preferences, type of
event, and amount of time between exercise
– If time is short, water, fruit juices, or sport drinks are
most appropriate (CHO can be rapidly absorbed)
Effects of Pre-Exercise Feeding
on Performance and Fatigue
• A high-CHO pre-exercise meal 3-4 hours
before exercise can improve performance
– If this meal is then combined with CHO intake during
exercise (e.g. sport drink), the performance
improvements are even greater (Wright, 1991)
– May be especially helpful for those who:
• pay little attention to their diet
• or who have had a poor diet during the 24 hour period
before an exercise event
Carbohydrate Consumption
Immediately Before Exercise
• Controversy:
– Does CHO eaten immediately before exercise cause
hypoglycemia during exercise?
– Hypothesis: the high blood insulin levels resulting
from CHO consumption immediately before exercise
(~30-60 min) may cause a decline in BG
(hypoglycemia) at the onset of exercise, leading to
premature fatigue
– Jeukendrup and colleagues have done a series of
systemic studies in male cyclists to examine
rebound hypoglycemia
Carbohydrate Feeding
During Exercise
Fatigue During Exercise
• Individuals fatigue during moderate exercise
(60-80% of VO2 max) of long duration (>90
min) in part due to a decrease in BG and
depletion of muscle and liver glycogen
stores
• Exogenous CHO consumed during
exercise may reduce fatigue and
improve performance
Fatigue During Exercise
• Exogenous CHO feeding during exercise:
– Spares muscle glycogen and oxidation of CHO
– Spares synthesizing glycogen during low intensity
exercise
– Provides CHO which has a direct effect on the brain
Figure 2.9
CHO Feeding During Exercise
Prevents Hypoglycemia
• For some individuals, exhaustive exercise
(60-75% VO2 max for 2.5-3.5 hr) without
exogenous CHO intake can result in
hypoglycemia
– Hypoglycemia = BG < 2.5 mmol/L
– Symptoms: light-headedness, dizziness, inability to
concentrate, nausea, irritability, and fatigue
– Hypoglycemia leads to a decline in total body
glucose oxidation and eventually to exhaustion
• once BG dropped to 2.5-3.0 mmol/L, exhaustion occurred
and subjects could no longer exercise
Figure 2.10a
CHO Feeding During Exercise Improves
Performance & Reduces Fatigue
• Feeding CHO during prolonged exercise:
– Improves performance
– Lengthens the time an athlete can exercise before
becoming fatigued
• Early research by Coyle et. al (1986)
– Measured plasma glucose & muscle glycogen in 7
trained cyclists exercising at 70-75% VO2 max to
fatigue
– 2 sessions: (1) with exogenous CHO, (2) one
without
CHO Feeding During Exercise Improves
Performance & Reduces Fatigue
• CHO feeding during shorter (< 1 hr), more
intense exercise sessions:
– CHO feeding during exercise >75% of VO2 max (~1 hr)
can also improve performance
– Below and Coyle, 1995: Male cyclists consumed a CHO
drink (with 78 g CHO) during 1 hr of high-intensity
exercise (80-90% VO2 max) increased their mean
exercise intensity by 6.3% compared to water only
– Davis et. al, 1997: no differences between genders
– Unclear of the benefits of CHO feeding during exercise
if exercise is <1 hr and is of high intensity
CHO Feeding During Exercise Improves
Performance & Reduces Fatigue
• Recreational marathon running:
– Real-world example of CHO feeding during exercise
– Utter et. al, 2002: CHO feeding during the marathon
reduced marathon running times (~16 min shorter)
and significantly decreased rate of perceived
exertion during the last 10 km of the race compared
to a placebo drink
Timing & Rate of Carbohydrate
Feeding During Exercise
• CHO ingestion should generally begin early in
an exercise event to ensure that adequate
CHO is available during the later stages of
exercise
– Coggan and Coyle, 1987: the latest an individual
can consume CHO and still prevent fatigue is 30 min
before the onset of fatigue
Timing & Rate of Carbohydrate
Feeding During Exercise
• Current research studies typically provide CHO
solutions (5-8%) at regular intervals, usually
every 15-30 min
– Most studies feed between 40 and 75 g of CHO per
hour and observe performance benefits
– This provides ~ 1 g CHO per minute
– Any sport drink containing at least 6% to 8% CHO
would provide 60 to 80 g of CHO per litre
Type of Carbohydrate
• What type of CHO should be consumed
during exercise?
• Does one type of CHO absorb more quickly
than another?
Type of Carbohydrate
• CHO types:
– All simple sugars (i.e. glucose, fructose, sucrose,
and maltodextrin) are absorbed rapidly from the gut
– These CHO sources are equally effective in
maintaining BG levels during exercise
– Glucose can be used to maintain BG levels
immediately, while fructose must 1st be converted to
glucose in the liver
– Using a combination of sugars increases the ability
of various transport mechanisms to be utilized in the
gut, thus increasing absorption and subsequent
oxidation of these sugars
Type of Carbohydrate in Sports Drinks
• Sport drinks
– Use a combination of sugars
– Absorbed, transported, and oxidized more quickly
during exercise than a single CHO source
– Maltodextrins: frequently added to drinks/gels
because they are less sweet than glucose or
sucrose, permitting a higher CHO concentration
without making the product unbearably sweet
– Sport drinks on the market:
• Combo of glucose, sucrose, fructose, maltodextrin
• 6-8% CHO plus sodium are generally well-absorbed
Type of Carbohydrate
• Fructose:
– Fructose (usually in the form of HFCS) is absorbed
more slowly from the gut than glucose
– Absorbed through facilitated diffusion vs. active
transport
– Large doses of fructose can overload the absorption
capabilities of the gut and cause GI distress (i.e.
cramping, diarrhea)
– Once absorbed, fructose is transported to the liver,
where it is converted into glucose
Figure 2.13a
Type of Carbohydrate
• Fructose:
– Feeding of fructose in combination with other sugars
increases oxidation of CHO
– Not great for the replacement of muscle glycogen
Solid vs. Liquid Carbohydrates
• Glycemic response of solid vs. liquid CHO:
– Solid CHO (e.g. energy or sport bars, whole fruit)
– Liquid CHO (e.g. sport drink, blended fruit)
– Murdoch et. al, 1993; Jeukendrup, 2004: similar
amounts of CHO (solid vs. liquid)
• Found no significant difference in BG levels during exercise
• Produced similar BG and insulin responses
• Form of CHO consumed during exercise is a matter of
availability and personal preference
Practical Guidelines for
Carbohydrate Intake
During Exercise
Practical Guidelines for
Carbohydrate Intake During Exercise
• Test during training: Athletes should use the
CHO supplement during training that they will
use during competition.
• Ingest CHO early: Athletes should ingest CHO
early in an exercise session to prevent the
decrease in BG often seen during endurance
events.
• Sports drinks: Should have a concentration of
6-8% CHO (60-80 g per 1 L) and contain Na.
Practical Guidelines for
Carbohydrate Intake During Exercise
• Drink enough fluid: Athletes should drink
enough fluid to provide 40 and 75 g of CHO per
hour. Long duration exercise events or
extreme temperatures may require higher fluid
and CHO intakes.
• CHO intake based on BW: Determine CHO
intake during exercise based on 1-1.2 g
CHO/kg/min.
Carbohydrate Feeding
Post-Exercise & During
Training Periods
Carbohydrate Post-Exercise
• Post-exercise feeding:
– Need to replenish muscle glycogen and refuel the
body for the next exercise event
– Should provide the energy and nutrients to repair
and strengthen muscle tissue that may have been
damaged during exercise
– Should provide fluids to rehydrate the body
Glycogen Synthesis Post-Exercise
• Glycogen depletion:
– Can occur after 2-3 hours of continuous exercise
performed at 60-80% of VO2 max, or after highintensity exercise (90-130% of VO2 max) that occurs
intermittently over a shorter time (15-60 min)
– After exercise, the majority of glucose for glycogen
synthesis comes from oral glucose ingestion
• Rate of muscle glycogen replacement:
– Post-exercise  ranges from 20-50 mmol/kg of dry
muscle per hour when a CHO supplement is
provided post-exercise
Glycogen Synthesis Post-Exercise
• Factors that determine rate of glycogen
synthesis:
– Degree of muscle glycogen depletion
– Degree of insulin activation of glycogen synthase
– CHO content of the post-exercise diet
• Rapid vs. slow glycogen resynthesis:
– Rapid phase: ~30-60 min post-exercise
– Slow phase: lasts for several hours
Glycogen Synthesis Post-Exercise
• Protein & CHO:
– Combining some protein or
aa’s with CHO post-exercise
can lead to higher muscle
glycogen synthesis versus
the same amount of CHO
without the additional protein
Figure 2.17a
High-CHO Diets During Training
Improve Performance & Power Output
• Replacement of glycogen after exercise is
important, esp. during periods of high
training or endurance exercise
– Do higher levels of glycogen always translate into
increased exercise performance?
– Simonsen, 1991: high-CHO (10 g/kg BW, 70% of kcal)
vs. moderate-CHO (5 g/kg BW, 42% of kcal)
• Male & female rowers randomly assigned 2 different diets
• 4 weeks of intense twice-a-day rowing exercise
• Mean power output in the TTs increased by 10.7% in the
high-CHO group, but only by 1.6% in the moderate-CHO
group after 4 weeks of intense training
High-CHO Diets During Training
Improve Performance & Power Output
• Not all studies show improved exercise
performance with increased dietary CHO
and improved glycogen stores
– # of factors affect performance, and level of stored
glycogen is just one factor
– When muscle glycogen levels are adequate,
increasing levels above normal may not increase
exercise performance unless athletes are performing
exercise that is strenuous enough to deplete muscle
glycogen
Type & Amount of Carbohydrate
• Glucose, sucrose, maltodextrins, and starch
all appear to replace muscle glycogen equally
well
– Fructose did not replace muscle glycogen nearly as
well as the other sugars
• 4 large meals (“gorging”) vs. 16 frequent small
meals (“nibbling”) post-exercise
– 10 g CHO / kg BW
– No statistically significant difference between the
groups in muscle glycogen storage over a 24 hr period
Solid vs. Liquid CHO Post-Exercise
• Solid vs. liquid CHO post-exercise:
– When solid and liquid CHOs are fed at the same
rate, muscle glycogen synthesis rates appear to be
similar
– If quick glycogen replacement is needed, then the
post-exercise CHO fed, regardless of the form,
should have a high glycemic index and be adequate
in amount (~ 1-1.2 g of CHO per kg BW / hour)
Glycogen Replacement Using
High Glycemic Index Foods
• Hypothesis: Does feeding high GI foods postexercise produce a greater increase in muscle
glycogen storage than low GI foods, even if CHO
content is held constant?
– Tested by Burke (1993): 5 well-trained cyclists
consumed a low GI diet for the 1st TT and a high GI
diet for the second TT
– Both diets provided 10 g CHO / kg BW, similar in kcal
– Muscle glycogen content 24 hours after recovery was
significantly greater with the high GI diet
Timing & Rate of Post-Exercise
Carbohydrate Feedings
• Timing and rate of CHO consumption after
exercise can influence the amount of glycogen
stores.
– Glycogen synthesis rates are highest immediately after
exercise when the muscle is depleted and glycogen
synthase activation is high
– Goal: get CHO into the system quickly (2 hr after
exercise)
– Athletes given a high-CHO replacement drink
immediately after exercise improved their time to
exhaustion on the following day of exercise by 11%
(Baker, 1994)
Timing & Rate of Post-Exercise
Carbohydrate Feedings
• Timing and rate of CHO consumption after
exercise can influence the amount of glycogen
stores.
– Katz, 1988: found that a 2 hour delay in feeding CHO
after exercise reduced the rate of glycogen synthesis
by 47% compared with feeding CHO immediately after
exercise
– When CHO is consumed frequently (every 15 min)
compared to less frequently (every 1-2 hours), a higher
insulin response is observed
• Insulin stimulates the uptake of glucose by the cells for
glycogen storage and stimulates glycogen synthase
Determining Overall
Carbohydrate Intake
for Individuals
Carbohydrate Recommendations
• AMDR: 45-65% of kcal from CHO
– Most athletes consume diets that contain 55-65% of
energy from CHO
• CHO recommendations based on grams of
CHO per kg BW versus % of total energy
–
–
–
–
Works better in low-kcal situations
Recreational athletes: 5-7 g CHO / kg BW
Competitive Athletes: 10 g CHO / kg BW
Minimum of 3 g CHO / kg BW
Practical Guidelines for
Feeding Carbohydrate
Post-Exercise and During
Training Periods
Practical Guidelines for Feeding
Carbohydrate Post-Exercise
• Recommendations assume the athlete is in
training or competition and thus requires
maximum glycogen replacement.
– Applicable to athletes frequently training 2x/day for a
total of 12 to 20 hours/week
– Less stringent CHO recommendations are
appropriate for recreational athletes who exercise
only 4 to 10 hours/week
Practical Guidelines for Feeding
Carbohydrate Post-Exercise
• CHO per kg BW: If exercise is to occur again
within less than 6 to 8 hours, feed approximately
1 to 1.2 g CHO / kg BW immediately after
exercise and every 30-60 min for the first 5 hours
after exercise. Combine with some dietary
protein if possible.
– Over a 2 hour period, feed ~ 5-7 g of CHO per kg BW
(moderate training); feed up to 10 g CHO per kg BW
(heavy training)
Practical Guidelines for Feeding
Carbohydrate Post-Exercise
• High GI Foods: Within 6 hours after exercise,
high GI foods or simple CHO (glucose, sucrose,
maltodextrin) provide the best glycogen
replacement.
• Sport Drinks: Provide a CHO replacement
beverage containing 40 to 80 g of CHO per
serving immediately after exercise if athletes are
eating self-selected diets, are unable to eat within
2 hours, or do not feel hungry after strenuous
exercise.
Practical Guidelines for Feeding
Carbohydrate Post-Exercise
• Individual preferences:
Recommendations must be
acceptable in relation to the
athletes’ time and money
constraints as well as their
cooking abilities.
Muscle Glycogen
Supercompensation
Carbohydrate Loading
• Muscle glycogen supercompensation
– Aka glycogen loading
– Aka carbohydrate loading
• Classical routine
– Days 1-3: athletes ate a low-CHO diet (<10% of kcal
from CHO), performed a glycogen-depleting exercise
– Days 3-6: high-CHO diet (>90% of kcal from CHO)
with little or no activity
– Day 7: muscles were supercompensated with
glycogen and water for the exercise event on day 7
Carbohydrate Loading
• Modified routine
– Days 1-3: athletes consumed a modified CHO diet
(50% of kcal from CHO, 353 g CHO per 3000 kcal),
tapered exercise protocol
– Days 4-6: high-CHO intake (70% of kcal from CHO,
542 g CHO per 3000 kcal), little to no exercise
– Day 7: muscles were supercompensated with
glycogen and water for the exercise event on day 7
– Produced similar amounts of muscle glycogen
replacement, but was easier to follow than the
classical routine