Interindividual Variability and Intra- Individual Reproducibility of Glycemic Index Values for Commercial White Bread SONIA VEGA-L´ OPEZ, PHD 1 LYNNE M. AUSMAN, DSC 1 JOHN L. GRIFFITH, PHD 2 ALICE H. LICHTENSTEIN, DSC 1 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, adminis- tered 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. Diabetes Care 30:1412–1417, 2007 I t has been recognized for the past 3 decades that carbohydrate-containing foods elicit widely different postpran- dial blood glucose responses (1,2). A sys- tem to classify foods on the basis of glycemic response, termed glycemic in- dex, emerged to capture this information (3). The classification system has been en- dorsed for use as a tool to guide food choices to reduce chronic disease risk by some individuals and organizations (4 – 6) but not by others (7–9). Reluctance to universally recommend the system for use in formulating dietary guidance stems from a number of issues, some relating to uncertainties in reproducibility among people and variability in the composition and preparation of individual foods. The glycemic index value of a given food is determined by monitoring the incre- mental area under the curve (AUC) for blood glucose response over a 2-h period after feeding a 50-g carbohydrate portion of both a test food and standard food (50 g glucose or a 50-g carbohydrate portion of white bread) and expressing the data on a percentage of the test food relative to the standard food (6,10). By definition, any area below the fasting glucose concentra- tion is not included in the AUC value. If white bread is used as the standard food, the glycemic index value for the test food is fre- quently corrected to glucose using a factor of 1.4 (6,10). An extensive compilation of glycemic index and glycemic load values derived from different laboratories world- wide is currently available (10,11). Implicit in the recommendation to for- malize glycemic index as a dietary guidance tool is the assumption that the glycemic re- sponse an individual has to a given food is similar among individuals regardless of metabolic and physiological factors. Guide- lines for the determination of glycemic in- dex values recommend that it be determined in six or more subjects and the values averaged (6). It is additionally rec- ommended that the glycemic index value for more than one food “. . . be determined in one series of tests, for example, each sub- ject might test four foods once each and the standard food three times for a total of seven tests in random order on separate days.” It is further recommended that subjects should be studied on separate days in the morning after a 10 –12 h overnight fast and a stan- dard drink of water, tea, or coffee be given with each test meal. There are limited data that have ad- dressed the issue of reproducibility. There- fore, the current study was carried out to assess the interindividual variability (among individuals) and intra-individual reproduc- ibility (within the same individual, when re- peatedly measured) of glycemic index values for white bread, using glucose as the standard food, determined under con- trolled conditions. RESEARCH DESIGN AND METHODS — A total of 25 adults (aged 20 –70 years) were recruited from ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● From the 1 Cardiovascular Nutrition Laboratory, Jean Mayer U.S. Department of Agriculture Human Nutri- tion Research Center on Aging, Tufts University, Boston, Massachusetts; and the 2 Biostatistics Research Center, Tufts New England Medical Center, Boston, Massachusetts. Address correspondence and reprint requests to Alice H. Lichtenstein, DSC, Cardiovascular Nutrition Laboratory, J.M. USDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington St., Boston, MA 02111. E-mail: alice.lichtenstein@tufts.edu. Received for publication 28 July 2006 and accepted in revised form 13 March 2007. Published ahead of print at http://care.diabetesjournals.org on 23 March 2007. DOI: 10.2337/dc06-1598. Additional information for this article can be found in an online appendix at http://dx.doi.org/10.2337/ dc06-1598. This article is based on work supported by the U.S. Department of Agriculture under agreement no. 58-1950-4-401. Any opinions, findings, conclusions, or recommendations expressed in this article are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture. Abbreviations: AUC, area under the curve; GRR, glycemic response ratio. A table elsewhere in this issue shows conventional and Syste `me International (SI) units and conversion factors for many substances. © 2007 by the American Diabetes Association. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Clinical Care/Education/Nutrition O R I G I N A L A R T I C L E 1412 DIABETES CARE, VOLUME 30, NUMBER 6, JUNE 2007 Downloaded from http://diabetesjournals.org/care/article-pdf/30/6/1412/597162/zdc00607001412.pdf by guest on 25 June 2022