In an attempt to better understand the glycaemic index, researchers from the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University (USDA HNRCA) used a simple test food, white bread, to show that a relatively high level of inter-individual (among different individuals), and intra-individual (within the same individual) variability occurs on consumption of white bread.
"Larger studies of diverse populations are needed to determine why inter-individual, and particularly intra-individual, glycemic index values are so variable. If we can identify the source of the variability, it will allow for more insight into the applications of the glycemic index as a tool for both researchers and in public health messages," stated corresponding author Alice Lichtenstein.
The interest in the GI of foods and the digestibility of carbohydrates has increased considerably in recent years. A number of studies suggest that a low GI and slowly digestible carbohydrates can contribute to the prevention of obesity and diabetes.
The glycaemic index measures how quickly certain foods release carbohydrates into the body, which then raise consumers' blood glucose levels. High GI foods, including white bread, white rice, many prepared breakfast cereals and concentrated sugar, cause blood sugar levels to rise more rapidly. Low GI foods include most vegetables, fruits, beans and unprocessed grains.
The study, published in the journal Diabetes Care, compared the glycaemic response to white bread and glucose (reference food) among 14 healthy adults (age range 20 to 70). The responses were measured after consuming 50 grams of carbohydrate from either source.
"Using glucose as the control food, previous studies indicate that white bread has a glycemic index of about 70," said Lichtenstein. "In our study the combined average was 71, virtually identical to the published value. However, quite strikingly, individual values ranged from 44 to 132. What is critical is to determine why there is such a wide range of responses among individuals."
The coefficient of variation (CV), a measure of how much change was observed between individuals and change in the same individual, was found to be significant, with the inter-individual CV calculated as 17.8 per cent, and the intra-individual variation found to be 42.8 per cent.
"These results show that perhaps using glycemic index for groups is a reasonable indicator to predict chronic disease risk, but there is still considerable uncertainty when applying glycemic index to individuals," added Lichtenstein.
Moreover, lead author of the study, Sonia Vega-Lopez, said: "Further understanding of all the sources of variability would be helpful in better defining the utility of glycemic index values."
The reasons for the wide variations could be due to how the food is consumed, with multiple simultaneous consumption of other foods as would occur if someone consumed a filled sandwich potentially resulting in a different glycemic index than if that person had eaten the white bread alone.
"Since most food is consumed as combinations during meals and snacks, there is a need to assess the significance of using glycemic index values determined on individual foods for food mixtures," said Lichtenstein. "Similarly, it is important to know whether the food consumed prior to a meal or snack alters subsequent glycemic response."
Lichtenstein confirmed that the research is ongoing, with a five-year grant from the National Institute of Diabetes and Digestive and Kidney Diseases already in the bank to further their understanding of the glycemic index and its utilities. The current study was supported by the U.S. Department of Agriculture's Agricultural Research Service.
Source: Diabetes Care Volume 30, Issue 6, Pages 1412-1417
"Interindividual Variability and Intra-Individual Reproducibility of Glycemic Index Values for Commercial White Bread"
Authors: S. Vega-Lopez, L.M. Ausman, J.L. Griffith, A.H. Lichtenstein