oa South African Journal of Clinical Nutrition - More evidence for capillary sampling in the determination of glycaemic index



The glycaemic index (GI) can be determined using capillary blood or venous plasma sampling. The aim of this study was to compare the GIs of three South African oat products determined using both capillary blood and venous plasma.

Twenty healthy, non-smoking male students (mean age 21.0 years, standard deviation (SD) 1.28 years, mean body mass index (BMI) 24.55, SD 2.62) volunteered to participate. Each subject received a standard pretest meal the evening before test days. The reference food used was glucose, and the test meals were three oat porridges, namely Jungle oats (66 g dry weight), Bokomo oats (72.8 g) and Oatso Easy (101 g) in the form they are normally consumed, namely with milk (150 ml) and sugar (20 g), or water only in the case of Oatso Easy, an instant porridge. Each meal contained 50 g glycaemic carbohydrate as determined by the Englyst method. Capillary blood glucose and venous blood samples were taken simultaneously at 0, 15, 30, 45, 60, 90 and 120 minutes and areas under the curve (AUC) and GI were calculated.
The mean fasting glucose value obtained from venous plasma before all tests (N = 80) was significantly lower (4.5 mmol/l, SD 0.7, p < 0.002) than the mean capillary blood glucose concentration (4.8 mmol/l, SD 0.04). The mean incremental AUCs obtained from venous plasma after all test meals and the reference (oral glucose) differed significantly (p < 0.01) from the AUCs obtained from capillary blood. The SDs for GIs calculated from venous plasma samples were at least 3 times larger than the SDs for GIs calculated from capillary samples. No significant differences were found between the GIs of the three oat products.
The present data are consistent with previous results showing that glycaemic responses measured in venous plasma are lower and more variable than those in simultaneously obtained capillary blood. The most precise and accurate determination of GI may be achieved using capillary rather than venous blood sampling, supporting the recommendation of the working group on the standardisation of GI testing in South Africa.


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