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Method used to Determine GI

Ten (10) normal subjects are studied on multiple occasions (maximum 3 per week) in the morning after a 10-14h overnight fast. Subjects are asked to do no unusually vigorous activities on the day before the test, to drink no alcohol and not to smoke for 24h before the test. After a fasting blood sample, subjects eat a test meal and have further blood samples at 15, 30, 45, 60, 90 and 120 minutes after starting to eat. Capillary blood is obtained by finger-prick and whole blood glucose determined with an automatic analyzer using the glucose oxidase method.

Each test meal contains 50g available carbohydrate (total carbohydrate minus dietary fiber). Unavailable carbohydrates such as fructo-oligosaccharides, resistant starch and sugar alcohols are not included as available carbohydrate. The GI is valid as a method of classifying the blood glucose responses of high carbohydrate foods. To test low carbohydrate foods, meals containing smaller amounts of carbohydrate may be used, but the amount of reference food should be adjusted so that it contains the same amount of available carbohydrate as the reference food. However, the interpretation of the GI values obtained is unclear.

A drink of the subject's choice is served with each test meal. The subject can chose to have 1-2 cups of water, coffee or tea, with 30ml 2% milk per cup if desired. However, the drink chosen by the subject is the same for every test performed. Test meals are consumed within 10 minutes.

Each subject conducts one trial of each test food and 3 trials of the reference food. The reference food can be anhydrous glucose or white bread (baked from weighed ingredients so its composition is exactly known). Other reference foods could be used (eg. maize meal) provided that its GI relative to glucose is well established (and periodically monitored to ensure no changes) so that the resulting GI values can be adjusted to the glucose scale. Typically the reference food trials are done at the beginning middle and end of the series of tests, with the order of the test foods randomized between the reference foods. If large numbers of foods are being tested, a reference food trial should be done for every 5-6 test foods (to ensure no changes in subject's glucose responses with time).

Calculation of Area Under the Curve (AUC)

There are many ways to calculate the AUC, and the method used affects the GI value obtained. The correct method for GI is shown below.
Assuming that at times t0, t1, ... tn (here equalling 0, 15 ... 120 min, respectively) the blood glucose concentrations are G0, G1, ... Gn, respectively.

x=1
AUC= ΣAx
 n
Where Ax = the AUC for the xth time interval and the xth time interval is the interval between times t(x-1) and tx.
For the first time interval (ie. X=1):
if G1>G0, A1 = (G1-G0)×(t1-t0)/2 otherwise, A1 = 0
For other time intervals (ie. X>1):
if Gx>G0 and G(x-1)>G0, Ax = {[(Gx-G0)/2]+(G(x-1)-G0)/2}×(tx-t(x-1))
if Gx>G0 and G(x-1)<G0, Ax = [(Gx-G0)²/(Gx-G(x-1))]×(tx-t(x-1))/2
if Gx<G0 and G(x-1)>G0, Ax = [(G(x-1)-G0)²/(G(x-1)-Gx)]×(tx-t(x-1))/2
if Gx<G0 and G(x-1)<G0, Ax = 0

References for Method

Wolever TMS, Jenkins DJA, Vuksan V, Josse RG, Wong GS, Jenkins AL. Glycemic index of foods in individual subjects. Diabetes Care 1990;13:126-32.

Wolever TMS, Jenkins DJA, Jenkins AL, Josse RG. The glycemic index: methodology and clinical implications. American Journal of Clinical Nutrition 1991;54:846-54.

Wolever TMS. Glycemic index vs glycemic response: non-synonymous terms. Diabetes Care 1992;15:1436-37.

Wolever TMS, Bentum-Williams A. Effect of fixed versus weight-adjusted dose of oral glucose on plasma glucose, insulin and FFA responses; implications for the design of postprandial studies. Nutrition Research 1994;14:799-806.

Wolever TMS, Bolognesi C. Time of day influences relative glycaemic effect of foods. Nutrition Research 1996;16:381-84.

Wolever TMS, Chiasson JL, Hunt JA, Palmason C, Ross SA, Ryan EA. Similarity of relative glycaemic but not relative insulinaemic responses in normal, IGT and diabetic subjects. Nutrition Research 1998;18:1667-76.

Young KWH, Wolever TMS. Effect of volume and type of beverage consumed with a standard test meal on postprandial blood glucose responses. Nutrition Research 1998;18:1857-63.

Wolever TMS. Low carbohydrate does not mean low-glycaemic index! British Journal of Nutrition 2002:88:211-12.

Wolever TMS, Vorster HH, Björk I, Brand-Miller J, Brighenti F, Mann JI, Ramdath DD, Granfeldt Y, Holt S, Perry TL, Venter C, Wu, X. Determination of the glycaemic index of foods: interlaboratory study. European Journal of Clinical Nutrition 2003;57:475-82.

Campbell JE, Glowczewski T, Wolever TMS. Controlling subjects' prior diet and activities does not reduce within-subject variation of postprandial glycemic responses to foods. Nutrition Research 2003;23:621-29.

References for AUC Calculation

Wolever TMS, Jenkins DJA, Jenkins AL, Josse RG. The glycemic index: methodology and clinical implications. American Journal of Clinical Nutrition 1991;54:846-54.

Wolever TMS. Effect of blood sampling schedule and method calculating the area under the curve on validity and precision of glycaemic index values. British Journal of Nutrition 2004;91:295-300.





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