Sports
Nutrition Update -- Abstracts from the American College of Sports Medicine
43rd Annual Meeting
G.
Douglas Andersen, DC, DACBSP, CCN
Volume
15, number 3, 1/27/97, page 32
Carbohydrate and exercise performance.
As there was no subject index published for the 43rd meeting of the American
College of Sports Medicine supplement to their journal, Medicine and Science
in Sports and Exercise (supplement to volume 28, number 5, May 1996),
I reviewed all 1,256 abstracts to find the most interesting studies on
sports nutrition. Abstracts are studies that have been performed, but
have not yet gone through peer-review and been published in a refereed
journal.
Part I: Carbohydrates and Exercise
Last year was the year of carbohydrate bashing. Carbohydrates got the
blame for our increased obesity, and the decrease in athletic performance.
The 10 studies below from different researchers across the United States
and around the world will make an interesting comparison to what you heard
and read in the popular press last year.
Trained athletes performed 19 minutes of high intensity running after
consuming 50 gm (200 calories) of liquid carbohydrate, solid carbohydrate,
or placebo beverage. Time to exhaustion and blood glucose levels were
higher in both carbohydrate trials compared to the placebo trial. The
authors concluded that an intake of 50 gm (200 calories) of liquid or
solid carbohydrate five minutes prior to vigorous anaerobic exercise both
resulted in similar performance improvements as compared to placebo.
With a wealth of literature behind it, almost everyone agrees that carbohydrate
feeding will improve performance in endurance events of moderate intensities
over two hours. This study looked at higher intensity exercise of shorter
duration (less than one hour). In this study cyclists were given either
a 7.6% carbohydrate solution during an exercise timed trial of approximately
one hour, or a placebo. Performance in the group consuming the carbohydrates
during high intensity exercise was significantly increased.
In this very interesting abstract, male runners were given diets with
the same amount of calories, but different percentages of macronutrients.
The percentages were as follows: the normal diet of 16% protein, 61% carbohydrate,
24% fat; the fat diet of 14% protein, 53% carbohydrate, 33% fat; and the
carbohydrate diet of 13% protein, 72% carbohydrate, 15% fat. Each diet
was consumed for seven days and then endurance performance to exhaustion
was tested on a treadmill for 42 minutes at approximately 73% VO2 peak
with a 3% grade increase every two minutes until exhaustion. Under these
testing conditions, dietary manipulation of carbohydrates and fats had
no effect (positive or negative) on performance.
This study compared runners who had a carbohydrate-rich meal three hours
before exercise and then consumed a 6.9% carbohydrate electrolyte solution
during exercise with another group who, after an overnight fast, just
consumed a 6.9% carbohydrate electrolyte solution. The authors concluded
that the combination of both a pre-exercise meal and a carbohydrate electrolyte
solution improved endurance running capacity to a greater extent than
ingestion of a carbohydrate electrolyte solution alone.
In this double-blind placebo-controlled study of 11 endurance-trained
athletes, consumption of a carbohydrate electrolyte beverage significantly
reduced their times in a six mile run. There were no differences in the
time to complete the first three miles, but subjects who consumed a beverage
with carbohydrates and electrolytes, as opposed to flavored placebo, ran
miles four, five, and six significantly faster.
Subjects lifted weights and then consumed a carbohydrate supplement approximately
one-half gram per pound of body weight or placebo after weight training.
Carbohydrate supplementation increased plasma insulin and blood glucose.
Urinary 3-methylhistidine was significantly lower in the group that consumed
carbohydrate after exercise. Urinary urea nitrogen was also lower in the
group that consumed carbohydrates. Muscle protein synthetic rate was elevated
in the group that consumed the carbohydrate load following exercise. The
authors concluded that the consumption of a carbohydrate at the rate of
one-half gram per pound of body weight immediately following a resistance
training bout can significantly decrease myofibrillar protein breakdown,
and may increase muscle protein synthetic rate, thus resulting in a more
positive protein balance.
Running performances in high temperatures were compared in 10 well-trained
male athletes. They ran 15 miles on a treadmill with the first 12 miles
at 70% V02 maximum; the last three miles were a timed trial. Subjects
consumed equal amounts of carbohydrate electrolyte drinks that consisted
of either six, eight, or ten percent carbohydrate. Five percent of the
carbohydrate was in the form of high fructose corn syrup and the balance
consisted of maltodextrin. Electrolyte profiles of sodium, potassium,
and chloride were identical. The results of the three-mile timed trial
showed that the group consuming the six percent solution had an average
time of 41.3 seconds faster than when they consumed a 10 percent solution.
Athletes consuming 8% versus 10% yielded a mean improvement of 37.8 seconds.
This is one of the first studies I have seen with head-to-head comparisons
of similar amounts of dilute carbohydrate beverages. In this small abstract
a six percent solution resulted in a slight performance enhancement over
an eight percent solution, and significant performance enhancement over
a 10 percent solution.
It is known that during high intensity exercise such as weightlifting,
insulin levels normally reduce and cortisol levels increase. This increase
in cortisol results in a decrease in protein synthesis and an increase
in protein degradation. In this study, young (approximately 21 years old)
and old (approximately 62 years old) weightlifters had their blood glucose,
insulin, and cortisol levels measured before, during, and after exercise
after drinking water or a carbohydrate solution. Ingestion of carbohydrate
elevated blood glucose levels suppressed cortisol concentration and increased
insulin concentration in both younger and older weightlifters. The authors
stated that higher insulin levels combined with lower cortisol levels
provides a favorable anabolic environment for weightlifters.
On three occasions, six men rode an exercise bike for 60 minutes at 45%
V02 maximum. One hour prior to their ride they were fed 0.8 gm per kilogram
body weight of glucose, fructose, or rode after an overnight fast (for
154 pound man this equates to 224 calories). Plasma insulin levels were
as follows: glucose 38+5, fructose 17+1, and fasting 8+1 uU/ml. Whole
body lipolysis was determined by measuring the rate of glycerol appearance
(RGA) in the blood (lipolysis is the breakdown of triglycerides to glycerol
and three free fatty acid molecules; once liberated, the fatty acids can
enter muscles and be are broken down in the mitochondria to form energy.
This process is called beta oxidation). After 25 minutes of exercise blood
was drawn and the RGA was as follows: glucose ingestion 3.2, fructose
ingestion 4.3, and fasting 6.9 umol/kg/min (higher blood glycerol levels
indicate greater breakdown of fats). The authors stated that even a small
increase in insulin concentration (9 uU/ml fructose versus fast) resulted
in significant suppression of lipolysis (4.3 umol/kg/min fructose versus
6.9 umol/kg/min fasting; i.e., approximately 40%). The authors concluded
that small elevations in plasma insulin prior to exercise can reduce fat
oxidation during exercise.
Comment
Until last year most sports nutrition experts recommended that adequate
carbohydrate intake will enhance athletic activity, both for long distance,
moderate intensity sports as well as shorter higher intensity activities.
Experts have also been recommending that carbohydrate ingestion is advantageous
to anaerobic athletes such as weight lifters who train to increase lean
muscle mass. These abstracts we have reviewed today reinforce these principles.
I could not find any studies in this conference that showed carbohydrates
inhibit athletic performance.
Ingestion of carbohydrates before, during, and after exercise does increase
blood glucose and insulin levels. Insulin is an anabolic hormone and will
reduce the amount of cortisol, a catabolic hormone. It also drives amino
acids and glucose into the cells. When present, insulin does limit the
amount of fat that is utilized for energy, however, insulin does not make
a person fat: too many calories do. Carbohydrate ingestion also means
calorie ingestion and if the goal of a person's exercise program is to
lose weight, more weight will be lost if fewer calories are consumed (not
exactly rocket science). In conclusion, drink water if exercise is performed
for the purpose of losing weight. Drink a dilute carbohydrate beverage
before, during, and after activity when engaged in competitive athletics
or exercising to increase endurance or build muscle.
References
1. Walton P, Rhodes E et al. Effect of pre-exercise carbohydrate ingestion
on anaerobic exercise. S30. 180.
2. Wagenmakers A, Jeukendrup A, et al. Carbohydrate feedings improve one
hour timed trial cycling performance. S37. 221.
3. Roltsch M, Slohr J, Brevard P. Role of diet in endurance performance
of male runners. S90. 536.
4. Chryssanthopoulos C, Williams C. Influence of a pre-exercise meal in
a carbohydrate electrolyte solution on endurance running capacity. S129.
765.
5. Doyle J, Elliott M. Distance running performance is improved with carbohydrate
intake. S129. 766.
6. Roy B, Tarnopolsky M et al. Effect of oral glucose supplements on muscle
protein synthesis following resistance training. S129. 769.
7. Hickey M, Mahar T. Running performance in hot, humid conditions: effect
of beverage carbohydrate content. S201. 1197.
8. Tarpenning K, Wiswell R. Modification in glucose, insulin, and cortisol
response during weight-training exercise in young and older men. S76.
449.
9. Horowitz J, Mora-Rodriguez R. Lipolytic suppression following carbohydrate
ingestion limits fat oxidation during exercise. S74. 442.
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