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Measuring the glycemic index of foods: interlaboratory study^1,2,3,4,5

¹ From the Department of Nutritional Sciences, University of Toronto and Glycemic Index Laboratories, Inc, Toronto, Ontario, Canada (TMSW); the Human Nutrition Unit, Department of Biochemistry, University of Sydney, Sydney, NSW, Australia (JCB-M, FA, and GSD); AMK Research, Inc, Gainsville, FL, USA (JA); the Department of Human Nutrition, Centre for Advanced Food Studies, Faculty of Life Sciences, University of Copenhagen, Frederiksberg, Denmark (AA and BS); the Department of Clinical Nutrition, Gothenburg University, Gothenburg, Sweden (MA and LD); the Centre for Chemistry and Chemical Engineering, Lund University, Lund, Sweden (IB and YG); the Human Nutrition Unit, Department of Public Health, University of Parma, Parma, Italy (FB and FS); the Department of Human Nutrition, University of Otago, Dunedin, New Zealand (RB and TP); the Nutrition and Dietetic Department, Hammersmith Hospital, Imperial College, London, United Kingdom (AB and GF); DiSTAM - Nutrition Unit, University of Milan, Milan, Italy (MCC and DE); Biofortis, Nantes, France (MC); the GI Foundation of South Africa, Mpumalanga, South Africa (ED); the Faculty of Health and Medical Sciences, University of Surrey, Guildford, Surrey, United Kingdom (SHampton, KLJ, and LMorgan); the Nutrition Unit, Department of Health Promotion and Chronic Disease Prevention, National Public Health Institute (KTL), Helsinki, Finland (KH and LMV); the School of Life Sciences, Oxford Brookes University, Oxford, United Kingdom (CJH and HL); the Department of Human Nutrition, Ohio State University, Columbus, OH (SHertzler and PW); the Nutrition and Health Section, Leatherhead Food International, Surrey, United Kingdom (SHull); the Department of Nutrition, Northwest University, Potchefstroom, South Africa (JJ and MP); the International Diabetes Institute, Melbourne, Victoria, Australia (NM and CR); Reading Scientific Services, Ltd, Reading, Berks, United Kingdom (VAH and JS); the College of Home Economics, University of the Philippines, Diliman, The Philippines (LP and DCDS); the Department of Exercise and Nutrition Sciences, University at Buffalo, Buffalo, NY (CP); the Department of Clinical Nutrition, German Institute of Human Nutrition (DIFE) Potsdam-Rehbruecke, Nuthetal, and the Department of Endocrinology, Diabetes and Nutrition, Charité-University-Medicine Berlin, Germany (AFHP and MOW); the Biochemistry Unit, Department of Preclinical Sciences, Faculty of Medical Sciences, The University of the West Indies, St Augustine, Trinidad and Tobago (DDR and RTR); the Program in Dietetics, School of Health Sciences, Health Campus, Universiti Sains Malaysia, Kelantan, Malaysia (SDR); Oy Foodfiles Ltd, Kuopio, Finland (ES and NT); NutriScience BV, Maastricht, Netherlands (IV and AW); and the Institute of Nutrition and Food Safety, Chinese Center for Disease Control and Prevention, Beijing, China (JZ)

ABSTRACT

Background: Many laboratories offer glycemic index (GI) services.

Objective: We assessed the performance of the method used to measure GI.

Design: The GI of cheese-puffs and fruit-leather (centrally provided) was measured in 28 laboratories (n = 311 subjects) by using the FAO/WHO method. The laboratories reported the results of their calculations and sent the raw data for recalculation centrally.

Results: Values for the incremental area under the curve (AUC) reported by 54% of the laboratories differed from central calculations. Because of this and other differences in data analysis, 19% of reported food GI values differed by >5 units from those calculated centrally. GI values in individual subjects were unrelated to age, sex, ethnicity, body mass index, or AUC but were negatively related to within-individual variation (P = 0.033) expressed as the CV of the AUC for repeated reference food tests (refCV). The between-laboratory GI values (mean ± SD) for cheese-puffs and fruit-leather were 74.3 ± 10.5 and 33.2 ± 7.2, respectively. The mean laboratory GI was related to refCV (P = 0.003) and the type of restrictions on alcohol consumption before the test (P = 0.006, r² = 0.509 for model). The within-laboratory SD of GI was related to refCV (P < 0.001), the glucose analysis method (P = 0.010), whether glucose measures were duplicated (P = 0.008), and restrictions on dinner the night before (P = 0.013, r² = 0.810 for model).

Conclusions: The between-laboratory SD of the GI values is 9. Standardized data analysis and low within-subject variation (refCV < 30%) are required for accuracy. The results suggest that common misconceptions exist about which factors do and do not need to be controlled to improve precision. Controlled studies and cost-benefit analyses are needed to optimize GI methodology. The trial was registered at clinicaltrials.gov as NCT00260858.

Key Words: Clinical trial • humans • dietary carbohydrate • glycemic index • glucose • methodology

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