Chapter 3: How is the Glycemic Index in Foods Determined?

The Glycemic Index uses pure glucose (or in some cases, white bread) as its control food and rates all other carbohydrates in relation to it. The control food or standard, either the glucose or the white bread, is given a rating of “100” and all other foods are tested as to how they affect a person’s blood sugar, insulin and lipid levels compared to the standard.

Each tested food is given a number rating and defined as either “High”, “Medium” or “Low” on the Glycemic Index. Foods fall into the High Glycemic Index when they are rated at 70 or above. If the Glycemic Index for a food is at 55 or lower, it is considered to be a Low Glycemic Index food item. This means that Medium Glycemic Index foods are those that after being tested, fall into the range of 56 to 69.

The actual testing to determine a food’s Glycemic Index is very scientific and takes into consideration many test subjects who undergo multiple tests with the same food and with the control, that being glucose.

A test subject, after fasting for at least 12 hours will have their blood drawn and tested and then is given a specified amount of glucose, usually 50 grams. Their blood is drawn and the blood sugar levels are tested at several specified times throughout the rest of the testing period. This is to determine what the control level is in this individual. Many times, this exact same test of glucose is done two or three times in the same test subject in order to have a more exact result.

After all of the blood sugar levels have been determined, they are plotted on a graph, which shows the curve of how high the sugar levels rose and also how long they remained elevated. The next step is to take the same individual on another day, after another 12 hour fast and have them eat a sample of the food that will be tested. The amount in grams of carbohydrates in the test food must equal the grams of carbohydrates in the glucose control test. Depending on the item being tested, the amount of food that the test subject has to eat may be very little in the case of foods that are very dense in carbohydrates. On the other hand, the test subject may have to eat an enormous amount of a food that has very little carbohydrate in it in order to reach 50 grams of carbohydrates.

Let’s say that the test subject had to ingest pure glucose in the amount of 50 grams of carbohydrates. If the food item being tested were a banana, the test subject would eat the equivalent of 50 grams of carbohydrates in banana. Then, their blood would be drawn and tested at the same times as with the control test with the glucose. The results of their blood sugar levels would be entered on to the same graph as the glucose tests and the results would be compared.

These “banana” tests would be repeated over the next several days with the same test subject to ensure more reliable results. Now, imagine this same testing process with bananas being repeated over and over again with many different test subjects.

This is the testing that is done for every food that has any type of carbohydrate or sugar in it. Thousands of tests have been done to determine the Glycemic Index value of every one.

Let’s get back to our “banana test”. After all of the tests, both the glucose control tests and the specified food tests, have been completed the results are determined. The number for the glucose test is always set at 100 and the food that is being tested and compared to the glucose is graphed and measured up to how it affects the person’s blood sugar levels in relation to the glucose. It was found that banana affects a person’s blood sugar levels only 53% as much when compared to the levels that pure glucose affects them. Therefore, a banana is rated as 53 on the Glycemic Index, which puts it in the Medium Range.

Does that mean that bananas are bad for you and should be avoided? We will discover what the results of the Glycemic Index mean and how to easily incorporate them into your daily food choices later on in the report. First, let’s look at the myth that all carbohydrates are bad for you and that they should be avoided at all cost.

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