Monday, April 8, 2013

Calculate The Gi Index

How different foods affect your blood sugar

The glycemic index measures how quickly the body absorbs and metabolizes various foods. White sugar or glucose has a glycemic index of 100 and is considered the benchmark to which all other foods are compared. Knowing where a certain food sits on the glycemic index spectrum is extremely helpful for moderating insulin release, minimizing blood sugar swings and ensuring proper nutrition following arduous physical activity.


Obtain Data

1. Obtain 10 volunteers willing to participate in study. Schedule five different testing sessions with those people. Inform study participants they must engage in high-intensity exercise the day prior to their study sessions as well as fast overnight. They must also refrain from using alcohol or nicotine for the full day before their appointments.

2. Obtain blood samples from each volunteer at every session. This can be achieved by a finger-prick.

3. During the first and third sessions, administer 50 grams of a test food to each study participant. This should take 10 minutes or less. At the three other sessions, administer 50 grams of a reference food, such as white bread, instead. All meals should be served with 500 ml of water.

4. Obtain finger-prick blood samples at 15 minute intervals for each participant until one hour has passed. Then obtain two more samples at 90 minutes and 120 minutes for a total of seven samples, including the fasting blood sample.

5. Place all samples into the automatic blood analyzer according to standard protocol and record blood glucose levels.

Analyze Data

6. Average the data for each study participant from all five sessions. Plot out blood sugar levels for the two hour period on a graph, with the x-axis representing time and the y-axis representing blood sugar level.

7. Using the method of Dr. Thomas Wolever at the University of Toronto, divide the area under the plotted curve into a series of triangles and rectangles. Label the triangles and rectangles A, B, C, and so on from left to right. Ignore the area of the curve that falls below the fasting blood glucose measurement. See his paper in the references section for a visual interpretation of this technique.

8. Sum the areas of the individual triangles and rectangles using the formula: Area = [(A X t) / 2] + (A X t) + [((B-A) X t ) / 2] + (B X t) + [((C-B) X t ) / 2], and so on. A, B, and C refer to the post-meal blood glucose values minus the fasting level for each triangular or rectangular area and t refers to the time elapsed for that area. This formula sums the differences between the post-meal glucose values and the fasting glucose values.

9. Remember to only add the final area that lies above the fasting glucose level. If the entire area is above the fasting blood glucose value, follow the formula in step 3. If there is a portion of the area lying below the fasting blood glucose level, subtract out that area using the following formula: [(E^2) X T)] / [2 X (E + F)] where E refers to your final area (triangle, rectangle, etc.), T is the time interval that encompasses the final area, and F is the blood glucose level lying below the fasting measurement. Again, see Wolever's paper in the references section for a visual image of these concepts.

10. Take the area under the curve, known as the AUC, calculated for the test food and divide it by the AUC obtained for the reference food. Multiple this number by 100 to get the GI value for one individual.

11. Average the GI values for all 10 individuals to obtain the GI value for that particular test food.

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