Why does glycemic index vary among foods?
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To understand why different carbohydrate containing foods have a different glycemic index we can start with the type of monosaccharide derived from a food. This is important because fructose and galactose do not raise blood glucose to the same extent that glucose does. For instance, the digestible carbohydrate in breads and potatoes is starch, which is made up of glucose. Meanwhile, milk and milk products contain lactose which is made up of glucose and galactose. Based on the difference in glucose content between starch and milk products, it is predictable that milk would have a lower glycemic index than bread. |
Ripened fruits contain mostly fructose and glucose as well as some sucrose. For example a medium apple contains about 8 grams of fructose and 3 grams of both glucose and sucrose. Meanwhile a medium banana contains between 5 to 6 grams of both fructose and glucose and 2 grams of sucrose. One tablespoon of honey contains 8 grams of fructose and 7 grams of glucose and less than one gram of sucrose, galactose and maltose combined. So even though fruits and honey are very sweet, they will have a moderate glycemic index and load (see Glycemic Index and Load Tables).
In addition to monosaccharide type, the presence of protein, fiber and fat as well as the processing of a food can influence its glycemic index. Fiber and fat seem to be able to slow the digestion process and thus can lower glycemic index. Certain types of fiber, often referred to as viscous fibers, can thicken the digestive contents in the stomach and small intestine, sort of like thickening up gravy with starch. This slows the digestion of carbohydrate and absorption of monosaccharides, which in turn reduces the rise in glucose.
Some amino acids in protein can increase the level of insulin released in response to carbohydrate and thus decrease glycemic index. Meanwhile, pasta has a lower glycemic index than what might be expected of such as high starch food. That’s because starch molecules become trapped within gluten protein networks within the dough. Thus, wheat based pastas have a relatively lower glycemic index value than expected and relatively lower than pastas made from other grains (e.g. rice or corn) which don’t contain gluten.
How is glycemic index determined?
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Glycemic index is determined in a research lab. Basically fasting people are fed 50 grams of either pure glucose or enough white bread to provide 50 grams of digestible (non-fiber) carbohydrate is fed to people and blood glucose is measured over the next two hours. On a different day, the same people would be provided a food in an amount to allow for 50 grams of digestible carbohydrate and again blood glucose is measured over the next two hours. If a food raises blood glucose to 50% of the rise cause by glucose then the glycemic index is 50. |
Because of the difference between white bread and pure glucose, glycemic indexes determined for foods using these different standards can vary. The glycemic index scale when using pure glucose is 0 to 100 and is more common because it is a little easier for the public to use. Meanwhile, when white bread is used as the standard for determining glycemic index several foods, such as a baked potato, rice cakes, jelly beans and Cheerios have a value greater than 100. When this book discusses the glycemic index of foods we will use glucose as the standard as per the values of the Human Nutrition Unit at the University of Sydney (www.glycemicindex.com).
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