Why Does Glycemic Index Vary Among Foods?

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 1 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 Table 4.4).

Glycemic load is a glycemic index adjusted for a standard serving size.
 

In addition to monosaccharide type, 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 a 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 (for example, rice or corn) which don’t contain gluten.