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[RC] Understanding GI - Ridecamp Guest

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This article will demonstrate why the glucose response curves we've done on 
certain nutrients, and with individual horses, are important--but only if you 
want to be competitive.

By Melinda Minore, PhD


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.



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.


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.


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.


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.






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




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