Table of Contents:
[ Health /
Dietary / Liquid
]
[ Carbohydrate / Fat / Protein
]
[ Vitamin / References
]
What are the carbohydrate (CHO) requirements of student
athletes?
Carbohydrate (CHO) in the form of glycogen is the body's main
fuel for high intensity activity. CHO is stored as long chains
of glucose units in the liver and in the muscles in form of
glycogen. The glycogen content of the liver is about 100 grams.
This quantity can change depending on the amount of glycogen
broken down from the supply of blood glucose and the amount
of glucose supplied to the liver after food intake. Muscle and
liver glycogen levels are relatively small and are reduced during
training and competition (Brouns, 1993; Murray and Horswill,
1997).
What are the effects of carbohydrate on performance?
Dietary carbohydrate increases the amount of CHO available to
the working muscles. When the levels of CHO are reduced exercise
intensity and length of activity decrease, and fatigue rapidly
increases. Costill (1988) recommended that athletes ingest 9-10
grams CHO/kg of body weight per day to restore and maintain
muscle glycogen levels. Student athletes should consume a diet
in which at least 60% of the total energy is supplied by carbohydrate
(Costill, 1993). Pre-exercise carbohydrate loading has been
shown to improve performance (Sherman, 1995).
Team sports: Athletes involved in team sports
rely greatly on glycogen stores. Maximizing muscle glycogen
stores provides greater energy reserve for performance resulting
in greater endurance and delayed fatigue.
Short duration high intensity sports: Athletes
such as wrestlers, sprinters and similar athletes should ingest
high carbohydrate diets during intense periods of training.
A high carbohydrate diet allows higher training intensities
(Nevill et. al., 1993).
Long duration moderate intensity sports: Athletes
who train for long hours and compete in endurance sports should
consume 65% to 70% of their energy from carbohydrate (American
College of Sports Medicine, 1991). Before long duration activities
athletes should include at least 3 days of intake which includes
800g carbohydrate per day (or 6-10 g CHO/kg/body weight/day)
and a week of tapered workouts that ends in complete rest the
day before the competition. This carbohydrate loading plan maintains
high glycogen stores in the body (Food and Nutrition Board,
1989).
How can a student athlete ensure optimal levels of carbohydrate
(CHO) before an athletic event?
Pre-exercise / competition CHO intake: The importance of dietary
intake of athletes before exercise or competition is well established.
The majority of energy released during muscle work is derived
from CHO and fat (Brouns, 1993). Depending on the level of intensity
of the exercise one of the fuels may become the major energy
deliverer. During the resting period most of the energy is derived
from fat. The possible energy supply ratio is in the order of
90% fat to 10% CHO. During more intense sport activity glucose
is mobilized from the liver and the muscle glycogen pool to
deliver energy. Fatty acid mobilization increases until a steady
metabolic state has been achieved (Newsholme and Start, 1973).
At higher exercise intensities CHO is the most important fuel
source (Brouns, 1993).
When CHO stores in muscle and liver are increased
athletes are able to perform at longer and at higher exercise
intensity. The amount of glycogen available in the muscle is
an important factor in performance. As soon as the muscle glycogen
stores have been depleted the ability to perform repeated high
intensity, contractions will be reduced (Maughan and Greenhaff,
1991).
Exercise intensity and duration: Endurance athletes
who train at a high level of energy output on a daily basis
require 65% to 75% of total calories from carbohydrates or 4.5
-6.0 g carbohydrate/kg/body wt/day to optimize performance.
Carbohydrate diets can also increase time to exhaustion with
short-term intense exercises. (Wilkinson and Liebman , 1997).
Glycogen depletion should be prevented by a high CHO diet during
training and periodic rests to allow the muscle to rebuild glycogen
stores (Superko, 1989). Tiredness associated with over training
can be caused by lowered glycogen stores (Hubinger et. al.,
1995). Foods rich in complex carbohydrates and a great variety
of grains, legumes, fruits and vegetables will allow sufficient
glucose absorption and maintenance of glycogen stores (Wilkinson
and Liebman, 1997).
Type of CHO intake: CHO intake should be light
(approximately 300 Kcal), and obtained from CHO foods that the
student athlete has tried during the training period and found
to be easily digested. CHO should have a low fiber content and
taken with a moderate amount of protein to maintain good blood
sugar levels. Pre-exercise supplementation should contain 1-5
g CHO/kg body weight. Liquid carbohydrate intakes at the lower
end of this range are better tolerated than solid meals and
higher intakes when consumed close to competition.
Carbohydrate loading: Depletion of muscle glycogen
can cause a decrease in exercise energy output. The use of CHO
loading to maximize muscle glycogen stores at the beginning
of exercise or competition could be beneficial for student athletes
who participate in continuous exercise for more than 90-120
minutes (Wilkinson and Liebman, 1997). The classic method of
CHO loading recommended by Bergstrom et. al., (1967), includes
glycogen depletion from a long low intensity workout, followed
by loading. The student athlete should eat a CHO rich diet (pasta,
potatoes, bread other grains or starchy vegetables) in which
90% of the total k/cal are from CHO, for 2-3 days to allow the
muscles to become saturated. This involves CHO intakes of 500
to 600 g per meal. The classic method of loading occurs when
the student athlete depletes the muscle reserves of glycogen
by engaging in a strenuous workout (endurance runners, a 2-3
hour steady run) and then eats a very restricted, low CHO diet
for 3 days, followed by a CHO loading phase of 2-3 days in which
a very high CHO diet, in which more than 90% of total k/cal
are consumed from CHO.
For many athletes a low CHO diet for 3 days may
cause hypoglycemia, irritability, and extreme chronic fatigue.
A modified version involves "tapering down" of exercise
during the 6 day prior to the event. Daily CHO intake is slowly
increased from an initial level of approximately 350 to 550g
or 70% of total k/calories during the last 72 hours preceding
competition (Hoffman et. al., 1991). This method will increase
muscle glycogen stores 20-40% above normal (Coyle, 1995). Although
CHO loading can increase high intensity exercise time and duration
it will not be effective until at least after the first hour
of texercise has been completed for example, at 8-10 miles in
the marathon (Coleman, 1991). CHO loading may create an initial
feeling before an event of heaviness or stiffness in the muscles
that have been depleted earlier of glycogen, and in some individuals
may contribute to muscle cramping and premature fatigue (McArdle
et. al., 1991).
Timing of CHO intake. Pre exercise/competition:
Ingestion of a CHO snack or beverage 15 minutes to 1 hour before
exercise can lead to hypoglycemia during exercise (Costill et.
al., 1977; Koivisto et. al., 1981). Consumption of a glycogen
replacement drink during long periods of exercise has been found
to improve work performance. In general, a pre-exercise/competition
meal should be consumed at least 2-3 hours before the session
to allow adequate time for complete gastric emptying and minimize
gastrointestinal discomfort (Wilkinson and Liebman, 1997).
CHO intake during exercise: According to Coggan
et. al., (1991) CHO intake during exercise should be sufficient
to provide a minimum of 45-60 g of total CHO to sustain high
performance energy levels. Blood glucose concentrations are
maintained during moderate / intense exercise by supplying glucose
at a rate of 45g/hour (Coggan et. al., 1987: Murray et. al.,
1989). CHO supplementation during prolonged endurance exercise
or at least 30 minutes before the onset of fatigue are effective
in delaying fatigue (Coyle, 1992).
Post exercise CHO intake. The type of CHO taken
after exercise can influence the rate of muscle glycogen re-synthesis.
This effect is mediated by glycemic and insulinemic responses
to different CHOs (Febbraio et. al., 1994). Fructose intake
is associated with lower blood glucose and insulin levels compared
to sucrose (Febbraio et. al., 1994). In order to maximize the
rate of muscle glycogen synthesis it is recommended that student
athletes check the glycemic index (GI) of different CHO foods
(Wilkinson and Liebman, 1997). CHO diets with a high GI that
are eaten within a 24 hour period after prolonged exercise allow
for greater glycogen synthesis compared to the foods with low
GI CHOs (Burke et. al., 1988). The rate of glycogen re-synthesis
is more rapid in the first 2 hour. following exercise. Increasing
CHO consumption from 188g to 648 g/day will result in greater
muscle glycogen re-synthesis during the 24 hour post-exercise
period (Costill et. al., 1981). According to Wilkinson and Liebman
(1997), eating fructose post exercise compared to glucose induces
more liver glycogen synthesis but less muscle glycogen synthesis.
High CHO foods and beverages such as fruits or beverages or
commercially available CHO drinks with a high GI are good choices
for promoting post-exercise glycogen re-synthesis (Wilkinson
and Liebman, 1997).
