Carbohydrates are an important
source of energy for sports that involve repeated bouts of explosive power and
for long distance events. As this defines most sports, carbohydrate intake is a
critical determinant in optimizing athletic performance. A short list of sports
that rely on carbohydrate as a predominant energy source include (basketball,
soccer, hockey, lacrosse, football, tennis squash, badminton, racquetball,
handball, middle and long distance running, swimming, rowing and cross-country
For sports of this nature
adequate carbohydrate intake before, during and after exercise is associated
with enhanced exercise performance. Thus, knowing the right amount, type and
timing of carbohydrate helps to provide and athlete with a competitive edge.
The early studies on
carbohydrate and exercise performance date back to the late 1960’s (Bergstrom
et al., 1967), which confirmed the work of Christensen and Hansen, who 30 years
earlier had demonstrated the importance of increased dietary carbohydrate for
endurance exercise performance. So, at a time when professional athletes were
still eating a pre-game steak, research was already in place to suggest that any
combination of pasta, rice, bread, vegetables, fruit, peas and beans would have
been a better choice to enhance performance.
Recent studies have proven that
an athlete’s diet should consist of 60-70% carbohydrate calories during heavy
training where carbohydrates are a predominant energy source.
The pre-exercise or pre-game
meal should consist primarily of carbohydrates and be ingested 3 to 4 hours
prior to competition. This should not include a lot of refined sugar products,
but rather carbohydrates that don’t abruptly cause a rise in blood sugar or
insulin concentrations. Good examples include oatmeal, whole wheat bread, whole
wheat pasta with tomato sauce, brown rice, most vegetables, most fruits, peas,
beans, high fiber breakfast cereals low in sugar) with low fat milk or yogurt
(non fat 1% milk fat).
The pre-exercise or pre-game
carbohydrate meal is intended to expand the liver’s carbohydrate reserves,
which become the exclusive source of blood sugar during the sport or training
session. During exercise the muscles at work extract sugar (carbohydrate) from
the bloodstream at a rate that is 30-40 times greater than under resting
conditions. Thus, to prevent liver carbohydrate depletion and a fall in blood
sugar adequate carbohydrate intake 3-4 hours prior to the event is an essential
part of optimal sports nutrition. In addition, it is possible that a portion of
these carbohydrates can contribute to reloading of the muscles’ carbohydrate
fuel tank, which is a critical factor for sports performance as we will discuss.
In one study Sherman et al.
demonstrated that ingestion of 312 grams (1,248 calories) of carbohydrate 4
hours prior to strenuous exercise resulted in a 15% improvement in exercise
performance. As a side note, no improvement was observed when either 45 grams
(180 calories) or 156 grams (624 calories) of carbohydrate was ingested.
Therefore the meal must contain a sufficient total number of carbohydrate
calories to yield a sports-enhancing effect when strenuous or prolonged exercise
In contrast to this,
carbohydrate ingestion 30 to 60 minutes prior to strenuous exercise has been
shown to impair exercise performance. This is believed to be due to the effects
of insulin which produce a rapid drop in blood sugar when combined with
exercise. Thus, it is undesirable to raise blood insulin levels just prior to
For this reason drinking or
eating carbohydrates just prior to exercise can be detrimental to performance.
The one exception to this rule is fructose sugar. Fructose sugar does not
disrupt blood sugar or insulin levels to a significant degree and its ingestion
30 minutes prior to exercise has been shown to enhance exercise performance. For
this reason I often suggest to athletes that they ingest up to 20 grams of
fructose sugar added to cold water (20-25 ounces) 30 minutes prior to their
event. This recommendation is less important for more moderate exercise
activities such as recreational walking or for submaximal exercise lasting less
than one hour. The general importance of carbohydrates for strenuous or long
distance events is that it provides a high octane type of fuel that enables
muscles to work at a higher rate of power output. During exercise if muscle
carbohydrate stores become depleted, muscle power will suffer. Hence, the
athlete slows their pace during long distance events or is unable to achieve or
maintain their optimal speed in sports that require repeated bouts of rapid
acceleration and sprints.
Prior to exercise of this nature
refueling of the liver carbohydrate and muscle carbohydrate fuel tank (glycogen
reserves) is crucial to performance capabilities.
Knowing how to replenish
carbohydrates during exercise, post exercise and during day to day meal planning
is also essential and is discussed in other articles appearing on this site
entitled “Essentials of Carbohydrate Replenishment During and After
Copyright 1998 Dr. James
Costill DL Hargreaves M.
Carbohydrate nutrition and Fatigue. Sports Medicine, 1992: 13:2:86-92.
Bergstrom J. Hermansen L,
Hultman E, Saltin B. Diet, muscle glycogen and physical performance. Acta
Physiologica Scandinavica 71: 140-150, 1967.
Costill DL, Coyle EF, Dalsky G,
Evans W et al. Effects of elevated plasma FFA and insulin on muscle glycogen
usage during exercise. Journal of Applied Physiology 43: 695-699, 1977.
Coyle EF, Coggan AR, Hemmert MK,
Ivy JL. Muscle glcogen utilization during prolonged strenuous exercise when fed
carbohydrate. Journal of Applied Physiology 61:165-172, 1986.
Coyle EF, Coggan AR, Hemmert MK,
Ivy JL. Muscle glycogen utilization during prolonged strenuous exercise when fed
carbohydrate. Journal of Applied Physiology 59: 429-433 1985.
Coyle EF, Hagberg Jm, Hurley BF,
Martin WH, Eshani AA et al. Carbohydrate feeding during prolonged strenuous
exercise can delay fatigue. Journal of Applied Physiology 55: 203-235, 1983
Hargreaves M, Costill DL, Coggan
A, Fink WJ, Nishibata I. Effect of Carbohydrate feedings on muscle glycogen
utilization and exercise performance Medicine and Science in Sports and Exercise
16: 219-222, 1984
Hargreaves M, Costill DL, Fink
WJ King DS, Fielding RA. Effect of pre-exercise carbohydrate feedings on
endurance cycling performance. Medicine and Science in Sports and Exercise 19:
Hargreaves M, Costill DL Katz A,
Fink WJ. Effect of fructose ingestion on muscle glycogen usage during exercise.
Medicine and Science in Sports and Exercise 17: 360-363, 1985.
Koivisto VA, Karvonen SL,
Nikkila EA. Carbohydrae ingestion before exercise: comparison of glucose,
fructose and sweet placebo. Journal of Applied Physiology 51: 783-787, 1981.
Sherman WM, Brodowicz G, Wright
DA, Allen WK, Simonsen J et al. Effects of 4 h pre-exercise carbohydrate
feedings on cycling performance. Medicine and Science in Sports and Exercise 21:
WM, Costill DL, Fink WJ, Miller JM. The effect of exercise and diet manipulation
on muscle glycogen and its subsequent utilization during performance.
International Journal of Sports Medicine2: 114-118,1981.