Interindividual variability and intra-individual reproducibility of glycemic index values for commercial white bread

OBJECTIVE - We sought to assess the intra- and interindividual variability of glycemic index value determinations for white bread using glucose as the reference food. RESEARCH DESIGN AND METHODS - A total of 23 healthy adults (aged 20-70 years) completed up to three sets of two visits per set. Each pair of visits assessed the glycemic response to 50 g available carbohydrates from commercial white bread and glucose, administered in random order. Glycemic index values were calculated by dividing the 2-h incremental area under the serum glucose response curve after each commercial white bread challenge by the mean area under the curve (AUC) for glucose. RESULTS - The mean +/- SE ratio of the AUC after white bread intake by the AUC after glucose intake for the first set of determinations was 78 +/- 15 (n = 23; coefficient of variation [CV] 94%). When using glycemic index values calculated with the subset of participants who completed three sets of tests (n 14), glycemic index values for each of the three sets of determinations were 78 +/- 10, 60 5, and 75 +/- 10, respectively. CVs were 50, 28, and 50%, respectively. The mean glycemic index value of these three sets was 71 +/- 6, with a CV of 30%. When an ANOVA approach was applied to these data, the interindividual CV was 17.8%, and the intra-individual variation was 42.8%. CONCLUSIONS - These data suggest that in response to a challenge of white bread relative to glucose, within-individual variability is a greater contributor to overall variability than among-individual variability. Further understanding of all the sources of variability would be helpful in better defining the utility of glycemic index values.