12 January - 20 June 2016

Glycaemic Index may not be reliable suggests study

A study by researchers from the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University and published in the American Journal of Clinical Nutrition suggests that the glycaemic index (GI) of a food can vary by around 20% within the same individual and by more across individuals.

A study by researchers from the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University and published in the American Journal of Clinical Nutrition suggests that the glycaemic index (GI) of a food can vary by around 20% within the same individual and by more across individuals.

GI is used as a measure of how fast blood sugar rises after eating and was developed to help diabetics control blood sugar levels but is now used for many other purposes including food labelling and diets. In the current study, Matthan et al. tested 63 individuals in sessions over a period of 12 weeks. The scientists recruited males and females aged between 18 to 85 years old with a BMI of 20 to 35 and free from “chronic disease”. Participants were randomly assigned to 3 sets of food challenges and asked to fast and restrain from alcohol consumption and exercise before each session. In each challenge, each was given a glucose drink as a reference or a test food of white bread. Both contained 50g of available carbohydrate. Blood glucose and insulin levels were measured regularly for 5 hours after consumption and GI values were calculated using standard and other ‘Area under curve” (AUC) methods.

Matthan et al. found that the average GI value for white bread as calculated by the standard method was 62 ± 15. They note that a value of 62 places white bread in the ‘medium’ GI category but 15 points either way would place it in either the ‘high’ or ‘low’ category too. They found that within an individual, calculated GI values could differ by up to an average of 20% and between individuals, by up to 25%. Matthan et al. also found that repeating the reference, repeating the test food, increasing sample size, increasing monitoring length and using other AUC calculation methods did not improve the variation. When looking at insulin levels, Matthan et al. founds that insulin response to the foods accounted for 15% of the variation for an individual and 16% between individuals.

Nirupa Matthan is quoted in a press release as saying that "Glycaemic index values appear to be an unreliable indicator even under highly standardised conditions”.  Matthan added that "If someone eats the same amount of the same food three times, their blood glucose response should be similar each time, but that was not observed in our study. A food that is low glycaemic index for you one time you eat it could be high the next time, and it may have no impact on blood sugar for me”.  Alice H. Lichtenstein, senior author of the study, added that "Reports frequently tout the benefits of choosing foods with low glycaemic index and glycaemic load values. Our data suggest those values may not be reliable in terms of a daily intake".

In conclusion, Matthan et al. reiterate that their data shows there is “a substantial variability in individual responses to GI value determinations” and state that this shows that GI is therefore unlikely to provide a “good approach to guiding food choices”. They further note that current glycaemic status plays a major role in the variability of the GI values of foods in individuals and Matthan is quoted as saying that "based on our results, we feel strongly that glycaemic index is impractical for use in food labelling or in dietary guidelines at the individual level”.

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