A food's glycemic index (GI) is a measure of that food's effect on your blood sugar level. The carbohydrates in food are broken down into glucose during digestion. This glucose enters the bloodstream, which causes the blood sugar level to rise. The rate at which this occurs varies greatly from one food to another. Glucose requires no digestion at all and causes the fastest increase in blood sugar level. Foods that contain only complex carbohydrates require more time to digest and therefore result in a slower increase in blood sugar. Scientists calculate the GI of particular foods by having subjects eat a specific amount of a standard food and the same amount of the test food.
Definitions
The GI of a particular food is defined as the ratio of that food's two-hour glucose response to the two-hour glucose response of a standard food. Glucose is the most common standard food used in a GI test, but white bread is used as well.
Standard Food Calculation
A human test subject consumes a specific amount of the standard food, usually 50 grams. The blood sugar level is measured for the next two hours, and the area under this curve is computed to get a summary of the measurements taken.
Test Food Calculation
After the blood sugar level returns to normal, the subject consumes an identical amount of the test food. The blood sugar curve is measured for the next two hours, and the area under this curve is computed.
GI Calculation
The area under the blood sugar curve for the test food is divided by the area under the blood sugar curve for the standard food. This ratio provides the GI of the test food for that test subject.
Final Result
This test is performed on 10 different test subjects, and the results are averaged to obtain the reference GI value for that particular food.
