GSSI Sports Science News - Published November 2003
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There is little doubt that carbohydrates play a key role in
athletic performance. Carbohydrate, in the form of glucose,
fuels muscles, the brain and nerves before, during and after
exercise. Depending on the food source, carbohydrate ingestion
can affect the body in different ways, creating responses that
can either hinder or help athletic performance.

We asked Melinda M. Manore, Ph.D., R.D, FACSM, the chair of the
Department of Nutrition and Food Management at Oregon State
University, to examine the topic of the glycemic response to
carbohydrate ingestion and its effect on athletic performance.

We hope you find this information useful.

Bob Murray, Ph.D., FACSM
Director, Gatorade Sports Science Institute

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With all the hype today about protein being the most vital
nutrient for athletes (not true, by the way), many athletes
are beginning to look at carbohydrates differently. The truth
is, carbohydrates play an essential role in the diet because
they are a key source of energy and provide the glucose
necessary to replace the glycogen lost during training
and competition.

In fact, carbohydrates eaten before and during exercise,
primarily in the form of sport drinks, bars and gels, help
maintain blood glucose levels and prevent premature fatigue
and decreased performance. Carbohydrates are important after
exercise as well, as they replenish muscle and liver glycogen,
restoring the athlete?s capacity for intense
training and competition.

Glycemic Response to Complex and Simple Carbohydrates
_______________________________________________________

Researchers used to think that:

* Complex carbohydrates (breads, cereals, vegetables and foods
high in starch) were digested slowly and caused little change
in blood glucose levels.

* Simple carbohydrates (fruit juice and high-sugar foods and
beverages) caused blood glucose levels to rapidly rise and
then drop precipitously.

However, current research shows that the glycemic response--the
increase in blood glucose levels after a food or combination of
foods are consumed--can vary greatly. In fact, some complex
carbohydrates can be digested, absorbed and utilized as quickly
as simple sugars, meaning that they have similar
glycemic responses.

Because of this new understanding, there is confusion about
which carbohydrates should be eaten to achieve the maximum
performance benefit. In an attempt to clarify the issue, the
scientific terms "glycemic index" and "glycemic load," once
heard only in the laboratory, have become common vernacular.

Glycemic Index and Glycemic Load
_________________________________

Carbohydrate foods can now be classified as producing either
a high, moderate or low glycemic response. The glycemic response
of a food is a measure of the food's ability to raise blood
glucose (blood sugar).

Foods that produce a high-glycemic response are expected to
produce a greater increase in muscle glycogen when compared
to foods producing a low-glycemic response due to the rapid
increase in blood glucose levels.

In an attempt to standardize the glycemic response of various
foods among individuals, researchers have categorized foods
using the Glycemic Index (GI). The GI gives a numeric value
for the glycemic response produced by a food, so that foods
can easily be compared. The calculation to determine the GI
of a food is given below. The GI for a particular food or
combinations of foods is determined by:

* Comparing the blood glucose response within a two-hour time
period following ingestion of 50 g of that food.

* Comparing this number to that of white bread, which has an
arbitrarily defined GI of 100, and is used as the standard
for all comparisons. Fifty grams of glucose can also be
used as a standard.

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Calculating Glycemic Index:

Glycemic Index (GI) = blood glucose response test food
(beans) / blood glucose response to reference food
(white bread) x 100

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Fortunately, charts containing the GI responses of a variety
of foods and beverages can be used to create meals and snacks
with high or low glycemic response characteristics.

Athletes can use these charts to identify the impact of
certain foods and food combinations. For example, the greater
the GI, the greater the change in blood glucose that will
occur and the greater the glycemic load that is delivered
to the body. The glycemic load is a way of expressing the
impact of the carbohydrate consumed on the body, taking
GI into account.

The GI only reflects how the blood glucose level will change
after the ingestion of a food, beverage or meal. If an athlete
eats only a small amount of a high GI food, there will only be
a small rise in blood sugar because the amount of food is low.

Thus, it is important to know the amount of carbohydrate being
consumed and the associated GI index. Remember that when foods
with various GIs are combined, the total GI of the meal will
depend on the amount of each of these foods and their
individual GI values.

Using the Glycemic Index to Improve Performance
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Foods with a high GI cause a greater change in blood
glucose and insulin, which results in greater glycogen
replacement in the muscles. This is demonstrated in a study
that shows glycogen replacement is 30 percent higher in
well-trained cyclists who are fed high versus low GI foods
for 24 hours after two hours of exhaustive exercise.(1)

Unfortunately, it is not practical to plan all meals
around the GI of foods. When the desire is to increase
muscle glycogen, especially after intense exercise, it
may be more practical to:

* Provide 50-100g (200-400 kcal) of high GI carbohydrate to
athletes immediately after glycogen-depleting exercise.

* Encourage athletes to eat high-carbohydrate foods that
are packed with vitamins and fiber, especially whole grains,
fruits and vegetables. High GI foods and high-carbohydrate
sport nutrition products can also help improve glycogen
replacement and are especially helpful during times of
intense training or competition.

Conversely, consuming moderate and low GI foods may also play
a role in sport because these foods slowly allow glucose to
enter the bloodstream. For example, it has been shown that
moderate GI foods fed before endurance exercise actually help
prevent the fall in blood glucose observed during 90 minutes
of exercise compared to higher GI foods.(2) Thus, foods with
lower GI scores might work in the following situations:

* Athletes who want to minimize changes in blood glucose
should select more medium to low GI types of foods (beans,
legumes, whole grains, fruits or vegetables). Moderate and
low GI foods are good choices for mealtime when rapid
carbohydrate replacement is not a critical issue.

* Athletes who are doing endurance exercise may want to consume
a moderate to low GI meal before exercise to promote sustained
carbohydrate availability during exercise.

The scientific thinking about and practical applications
for glycemic response are still evolving. For example, a
low-glycemic response could result from slower entry of the
ingested carbohydrate into the bloodstream or as a result of
rapid removal from the blood into the muscles. If the latter
is true, then some low glycemic foods may actually be
preferable for speeding recovery. Future research will
undoubtedly help refine the practical recommendations
for athletes.

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References:

(1) Burke LM, Collier GR, Hargreaves M. Muscle glycogen storage
after prolonged exercise: effect of the glycemic index of
carbohydrate feeding. J Appl Physiol. 1993;75:1019-1023.

(2) Kirwan JP, O'Gorman D, Evans WJ. A moderate glycemic meal
before endurance exercise can enhance performance. J Appl
Physiol. 1998;84(1):53-59.

Melinda M. Manore, Ph.D., R.D., FACSM, is the chair and a
professor in the Department of Nutrition and Food Management
at Oregon State University in Corvallis, OR.

Table 1. High Glycemic Index Foods (GI>85)*

Angel Food Cake
Doughnut
Hard Candy
Bagel, White
Cornflakes
TOTAL(R) Cereal
Raisin Bran Cereal
Shredded Wheat
Raisins
GRAPE-NUTS(R) Cornmeal
Couscous
Corn Chips
Croissant
Maltose
Glucose
Sucrose
Barley Flour Bread
CHEERIOS(R)
CREAM OF WHEAT(R)
Millet
Soda Crackers
Watermelon
Pancakes
Honey/Syrups
English Muffins
Waffles
White Bread
Corn Bran Cereal
CRISPIX(R) Cereal
RICE KRISPIES(R) Cereal
Ice Cream
Molasses
Baked/Mashed Potatoes
Pretzels
Sport Drinks
POP-TARTS(R)
SPECIAL K(R) Cereal
Rye Flour Bread
CORN CHEX(R) Cereal

Table 2. Moderate Glycemic Index Foods (GI=60-85)*

Sponge Cake
Corn Tortilla
Brown Rice
Green Peas
Cracked Barley
White Rice (long-grain)
Sweet Potato
Brown Rice
Mango
Pastry
Pita Bread, White
MULTI-BRAN CHEX(R) Cereal
Buckwheat
Orange/Grapefruit Juice
Oatmeal, Cooked
Basmati Rice
Kiwifruit
SNICKERS(R) Bar
POWERBAR(R) Chocolate
Oat Bran Cereal
Bulgur
Banana
7-Grain Bread
Ice Cream, Low Fat
Grapes
Durum Spaghetti
Oat Bran Bread
Linguine
Sweet Corn
100% Whole Wheat Bread

Table 3. Low Glycemic Index Foods (GI <60)*

Barley Kernel Bread
Milk (whole/skim)
9-Grain Bread
Plums
ALL-BRAN(R) Cereal
Barley, Boiled
Yogurt (all types)
Grapefruit/Oranges
Beans (all types)
Peanuts/Cashews
Rice Bran
Apple (whole/juice)
Peaches (fresh)
Lentils
Tomato Soup/Juice
IRONMANTM(TM) Bar, Chocolate
Apricots (dried)
Pears (fresh)
Brown Rice
Chickpeas/Hummus

*White bread was used as the reference food (GI = 100).

Adapted from these References:
Foster-Powell K, Holt SHA, Brand-Miller JC. International
table of glycemic index and glycemic load values: 2002.
Am J Clin Nutr. 2002;76:5-56.

Trademark Acknowledgments:
CHEERIOS(R) and TOTAL(R) are registered trademarks of
General Mills, Inc.
CREAM OF WHEAT(R) and GRAPE-NUTS(R) are registered trademarks
of Kraft Foods Holdings, Inc.
CRISPIX(R), RICE KRISPIES(R), POP-TARTS(R), SPECIAL K(R) and
ALL-BRAN(R) are registered trademarks of Kellogg Company.
CORN CHEX(R) and MULTI-BRAN CHEX(R) are registered trademarks
of Gardetto's Bakery, Inc.
SNICKERS(R) is a registered trademark of Mars, Incorporated.
POWERBAR(R) is a registered trademark of Societe Des Produits
Nestle S.A: 2002.
IRONMAN(TM) is a trademark of World Triathlon Corporation.