Glycemic Index (GI) is a measurement carried out on carbohydrate-containing foods and their impact on our blood sugar. GI is a relatively new way of analyzing foods. Previously, most meal plans designed to improve blood sugar analyzed the total amount of carbohydrates (including sugars and starches) in the foods themselves. GI goes beyond this approach, looking at the impact of foods on our actual blood sugar. In other words, instead of counting the total amount of carbohydrates in foods in their unconsumed state, GI measures the actual impact of these foods on our blood sugar. We rank our WHFoods as being very low, low, medium, or high in their GI value. You can find a GI value for each individual food in its food profile. Just go to any food profile and take a look at the DRI%/DV% bar chart that is located towards the beginning of the article. At the top right-hand side of the chart you will find the food's GI value.
Why Is GI Important?
Over the past 15 years, low-GI diets have been associated with decreased risk of cardiovascular disease, type 2 diabetes, metabolic syndrome, stroke, depression, chronic kidney disease, formation of gall stones, neural tube defects, formation of uterine fibroids, and cancers of the breast, colon, prostate, and pancreas. Taking advantage of these potential health benefits can be as simple as sticking with whole, natural foods that are either low or very low in their GI value. We've ranking all of our WHFoods as being either high GI, medium GI, low GI, or very low GI to make it easier for you to focus on foods whose GI values fall into the low or very low range (we've also done this for the herbs and spices we include on the website).
How Is GI Measured?
Typically, a food is consumed in whatever serving size will provide 50 grams (about 1.8 ounces) of available carbohydrates. Available carbohydrates (or avCHOs) are carbohydrates that get readily digested, absorbed, and metabolized by our body. These carbohydrates have a much greater impact on our blood sugar level than carbohydrates in general because carbohydrates in general include substances that aren't readily digested, absorbed, and metabolized. Insoluble fibers, for example, are carbohydrates that do not have an immediate impact on our blood sugar level because they cannot be readily digested. As a very general way of estimating available carbohydrates in a serving of food, researchers take the total amount of carbohydrates and subtract out the total amount of fiber. Available carbohydrates are what's left.
After 50 grams of available carbohydrates have been consumed, blood sugar levels are measured over a period of 2 hours. The results are plotted on a graph and summarized in what is called glucose AUC, or "area under the curve." Glucose AUC shows the immediate impact of the food on our blood sugar.
Measuring GI also requires a second step. In this second step, 50 grams of available carbohydrate are consumed, but this time the food involved is one of two reference foods: either white bread or pure sugar (pure glucose). Once again, blood sugar levels are measured over a period of 2 hours, and the glucose AUC is calculated. At this point, it is possible to compare the two results. The impact of the first food on our blood sugar is compared to the impact of either white bread or glucose itself. When these two results are compared, the impact of the white bread or glucose is arbitrarily given a value of 100 to make the comparison easier. As an example, let's say that researchers are trying to establish a GI for green peas and they decide to compare the impact of green peas on blood sugar to the impact of white bread. And in this example, let's say that a person consumes a starchy vegetable like green peas, and the glucose AUC (area under the curve) is 48% as large the glucose AUC when white bread is consumed. In this case, the GI for green peas would be established at 48% of 100, or 48. (In fact, this is precisely the GI that we use for green peas at WHFoods.)
Most healthcare organizations use a "high," medium" and "low" rating system for GI. Using this system, foods get classified in the following way:
|Low GI||Medium GI||High GI|
|0-55||56-69||70 or greater|
We like this rating system. It's also the one we used as the foundation for our own rating system at WHFoods. We kept each of these same categories (low, medium, and high). But we used the concept of avCHO (available carbohydrate) to establish a fourth category called "very low" as well as to rank some of our WHFoods as "low" even though they did not have a published GI value. Since over one-third of our WHFoods are vegetables that often contain very small amounts of avCHO, we thought that the addition of a "very low GI" category would be helpful for sorting out our vegetables in terms of their GI value. To qualify as "very low" our foods needed to have an avCHO of less than 5 grams. (We calculated avCHO by setting a standardized food serving size of 100 grams, recording the food's total carbohydrate content in grams, and subtracting out its total fiber in grams.) To qualify as "low" our foods needed to have an avCHO of 5 grams or more but less than 12 grams. Finally, we ended up classifying a few of our foods as low-GI even when they did not meet our avCHO criterion. While researching the GI values for our foods, we came across research evidence for some particular foods that showed those foods to have a beneficial impact on blood sugar. When the preponderance of research studies for any food showed a beneficial impact on blood sugar, we classified that food as low-GI even if it did not meet our avCHO criterion. Here is a summary chart showing our WHFoods rating system criteria.
|Very Low GI||Low GI||Medium GI||High GI|
|Criteria for Classification||No established GI value and an available carbohydrate (avCHO) of less than 5 grams OR an established GI of less than 20 and an avCHO of less than 7 grams||Either an established GI value of 55 or less OR an available carbohydrate (avCHO) value greater or more than 5 grams but less than or equal to 12 grams OR a beneficial impact on blood sugar in a preponderance of research studies||An established GI value of 56-69||An established GI value of 70 or greater|
One of the most interesting aspects of GI involves its relationship to the unique features of carbohydrates. Carbohydrates are definitely not the same with respect to their immediate impact on our blood sugar. For example, non-whole grain breads and pasta noodles both contain similar amounts of starch, and their starches are similarly composed of long chains of the simple sugar, glucose. But the 3-dimensional structure of bread allows more of the starch to be exposed to enzymes in our saliva and in our digestive tract. This greater exposure to enzymes allows more of the starch to be broken down into sugar and gives non-whole grain breads a generally higher GI value than non-whole grain pastas. Similarly, two basic types of starch found in many foods - amylose and amylopectin - also influence their GI values, even if the foods have identical amounts of total starch.
With respect to their GI, foods are also differently impacted by cooking. Many legumes, for example, have cell structures that are fairly resistant to disruption and help prevent breakdown of the starches inside their cells. For this reason, legumes tend to have lower-than-expected GI values, provided that they have not been overcooked. Before they have been ground into flour, whole grains also tend to have lower GI values due to the sturdiness of their cell structures. But after being ground into flour, their starches become more susceptible to breakdown and their GI value tends to increase. Of course, these descriptions are generalizations and can be different for specific legumes, specific grains, and specific flours. Still, they reflect a general pattern and principle: namely, that for a carbohydrate-containing food, the more its natural integrity becomes disrupted by processing or overcooking, the more its GI value is likely to be increased. Minimal disruption of whole foods from their natural, unprocessed state is one of our key principles at WHFoods, and it is a principle 100% aligned with promotion of lower GI values.
|Food Group||Very Low GI||Low GI||Medium GI||High GI|
|World's Healthiest Foods|
|bell peppers||green peas||sweet potatoes|
|Brussels sprouts||winter squash|
|Romaine and other lettuce|
|plums & prunes|
|Nuts & Seeds||flaxseeds||almonds|
|Beans & Legumes||soybeans||black beans|
|cow's milk, grass-fed|
|World's Healthiest Spices and Herbs||black pepper|
|cilantro & coriander seeds|
Much of the pioneering work on glycemic index was carried out by Professor Jennie Brand-Miller,PhD, Personal Chair in Human Nutrition in the Human Nutrition Unit, School of Molecular and Microbial Biosciences at the University of Sydney in Sydney, Australia. The glycemic index website at the University of Sydney provides extensive information about her work as well as a searchable database for GI values.
Another resource we like is the website established by medical writer David Mendosa and devoted to issues involving glycemic index, glycemic load, and diabetes management. The Mendosa site also provides comprehensive lists of foods and their GI values.
For Established Glycemic Index values, We Used the Following Databases and Publications
Atkinson FS, Foster-Powell K, Brand-Miller JC. International Tables of Glycemic Index and Glycemic Load Values: 2008. Diabetes Care 2008; 31(12).
Foster-Powell K, Holt HA, and Brand-Miller JC. International table of glycemic index and glycemic load values: 2002. Am J Clin Nutr2002;76:5—56.
Human Nutrition Unit, School of Molecular Biosciences, University of Sydney, Sydney, Australia. (2013). GI Foods Advanced Search Database. Online at http://www.glycemicindex.com/foodSearch.php.
National Cancer Institute (NCI). DHQ Nutrient Database. Applied Research: Cancer Control and Populations Sciences. National Institutes of Health, Bethesda, MD. Available online at: http://appliedresearch.cancer.gov/DHQ/database/
In Addition We Used the Following Non-Database References
Castro-Quezada I, Sanchez-Villegas A, Diaz-Gonzalez V, et al. Relationship between dietary glycemic index, dietary glycemic load and major cardiovascular events in the PREDIMED study. European Geriatric Medicine, Volume 4, Supplement 1, September 2013, Pages S128-S129.
Frost G and Dornhorst A. Glycemic Index. Encyclopedia of Human Nutrition (Third Edition), 2013, Pages 393-398.
Kumar SB and Prabhansankar P. Low glycemic index ingredients and modified starches in wheat based food processing: A review Review. Trends in Food Science & Technology, Volume 35, Issue 1, January 2014, Pages 32-41.
Lin CS, Kimokoti RW, Brown LS, et al. Methodology for Adding Glycemic Index to the National Health and Nutrition Examination Survey Nutrient Database. Journal of the Academy of Nutrition and Dietetics, Volume 112, Issue 11, November 2012, Pages 1843-1851.
Ma XY, Liu JP, and Song ZY. Glycemic load, glycemic index and risk of cardiovascular diseases: Meta-analyses of prospective studies. Atherosclerosis, Volume 223, Issue 2, August 2012, Pages 491-496.
O'Reilly J, Wong SH, and Chen Y. Glycaemic index, glycaemic load and exercise performance. Sports Med. 2010 Jan 1;40(1):27-39.
Wolever TM. Is glycaemic index (GI) a valid measure of carbohydrate quality? Eur J Clin Nutr. 2013 May;67(5):522-31. doi: 10.1038/ejcn.2013.27. Epub 2013 Feb 13.