Evaluation of the methodology for determining the glycaemic index of foods with special reference to blood sampling
Van Heerden, Yvonne
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Different types of carbohydrates from different food sources affect blood glucose differently. This physiological effect of carbohydrate containing foods has been quantified and expressed as the glycaemic index (GI) of the food. The GI is defined as the ratio of the incremental area under the blood glucose response curve for a test food containing 509 available carbohydrate to the corresponding area after an equal carbohydrate portion of a standard food is taken by the same subject. The GI of a food can, therefore, be used to guide consumers in choosing a particular food with a predicted known effect on blood glucose levels and homeostasis. Numerous methodological factors may influence the interpretation of glycaemic response data. One of the major problems regarding labelling foods with GI values is the lack of standardised methodology amongst different researchers in determining the GI. Furthermore, clear directions are needed regarding standardised methodology in accredited laboratories, including clarity on issues such as the reference (standard), total (available) carbohydrate of the test food, number and characteristics of experimental subjects, capillary versus venous blood samples, analytical method for determination of blood glucose value and the method for calculation of the area under the glucose curve. A food company commissioned an independent assessment of the GIs of Jungle Oats, Bokomo Oats and Oatso-Easy using methods complying with the most recent internationally accepted methodology and carried out under strictly standardised conditions. Thus, the area under the curve (AUC) and GI of Jungle Oats, Bokomo Oats and Oatso-Easy was determined using both capillary whole blood and venous plasma. Another objective of the study was to determine if there were significant differences between the GI of the three oats porridges. Methods Twenty healthy, non-smoking fasting male students, aged 21-27 years, each consumed 509 available carbohydrate from Jungle Oats, Bokomo Oats, Oatso-Easy and the standard food (glucose) on four different occasions. Finger-prick capillary blood and venous whole blood were collected simultaneously before and at 15 and 30 minute intervals for the first and second hour after ingestion respectively. The capillary whole blood glucose values were determined by using SureStep test strips and SureStep glucometres (Lifescan) and the venous plasma glucose was determined with an enzymatic colorimetric method. The AUC and the GI for the three different oats porridges, taken at four different occasions randomly by the same subjects was calculated using one glucose response as standard. Results Statistically significant differences (p<0.05) were found between the AUCs of the three different oats porridges for capillary blood and venous plasma. However, no statistically significant differences (p>0.05) were found between the mean GIs of the three different oats porridges both for capillary blood and venous plasma (77.1, 67.7 and 78.0 for Oatso-Easy, Jungle Oats and Bokomo Oats, respectively using capillary sampling and 112.4, 112.4 and 113.8 respectively, using venous sampling). The 95% confidence interval (CI) and standard deviation (SD) of the mean capillary blood glucose were notably smaller than those of the venous plasma. Conclusions It can be concluded from the study that the three different oats porridges fell between the intermediate to high categories and that glycaemic responses measured in venous plasma are lower and more variable than those simultaneously obtained in capillary blood. Recommendations It is recommended that the methodological guidelines determined by the GI Task Force should be followed. Capillary blood glucose samples are preferred to determine the GI. The last recommendation is that in using the GI to choose carbohydrate foods, patients and consumers should be made aware of the fact that physiological responses to a food may vary between individuals. For example, when advising on the GI, it should be mentioned that the GI of a particular food is & low, medium or high, but that exceptions can be expected and that these exceptions are normal. Therefore, the label indicating the GI of foods, food products and beverages should be accompanied by clear instructions.
- Health Sciences