title: Calculating meal glycemic index by using measured and published food values compared with directly measured meal glycemic index.
abstract: BACKGROUND
  Glycemic index (GI) testing is normally based on individual foods, whereas GIs for meals or diets are based on a formula using a weighted sum of the constituents. The accuracy with which the formula can predict a meal or diet GI is questionable.
  
  
  OBJECTIVE
  Our objective was to compare the GI of meals, obtained by using the formula and by using both measured food GI and published values, with directly measured meal GIs.
  
  
  METHODS
  The GIs of 7 foods were tested in 30 healthy people. The foods were combined into 3 meals, each of which provided 50 g available carbohydrate, including a staple (potato, rice, or spaghetti), vegetables, sauce, and pan-fried chicken.
  
  
  RESULTS
  The mean (95% CI) meal GIs determined from individual food GI values and by direct measurement were as follows: potato meal [predicted, 63 (56, 70); measured, 53 (46, 62)], rice meal [predicted, 51 (45, 56); measured, 38 (33, 45)], and spaghetti meal [predicted, 54 (49, 60); measured, 38 (33, 44)]. The predicted meal GIs were all higher than the measured GIs (P < 0.001). The extent of the overestimation depended on the particular food, ie, 12, 15, and 19 GI units (or 22%, 40%, and 50%) for the potato, rice, and spaghetti meals, respectively.
  
  
  CONCLUSIONS
  The formula overestimated the GI of the meals by between 22% and 50%. The use of published food values also overestimated the measured meal GIs. Investigators using the formula to calculate a meal or diet GI should be aware of limitations in the method. This trial is registered with the Australian and New Zealand Clinical Trials Registry as ACTRN12611000210976.
authors.affiliation: Department of Human Nutrition, University of Otago, Dunedin, New Zealand.
authors.forename: Hayley
authors.initials: H
authors.lastname: Dodd
authors.forename: Sheila
authors.initials: S
authors.lastname: Williams
authors.forename: Rachel
authors.initials: R
authors.lastname: Brown
authors.forename: Bernard
authors.initials: B
authors.lastname: Venn
mesh_terms: D000328:Adult; D000465:Algorithms; D001786:Blood Glucose / Q000032:analysis; D004032:Diet / Q000009:adverse effects; D005260:Female; D005502:Food / Q000009:adverse effects; D038321:Glycemic Index; D006801:Humans; D007700:Kinetics; D008297:Male; D019518:Postprandial Period; D015203:Reproducibility of Results
chemical_list: D001786:Blood Glucose