Glycemic Index And Load Assess Food's Effect On Blood Glucose
What is glycemic index?
As expected the level of circulating glucose increases after eating a carbohydrate-containing meal. But to what level and will different foods having the same amount of carbohydrate result in the same increase in blood glucose? This kind of information surely would be of interest to many people such as those managing their blood glucose levels (e.g. diabetes).
Simply put, the level of glucose circulating in the blood increases after eating or drinking a carbohydrate containing food or beverage and then is reduced back toward the normal fasting level. This response is often referred to as a glucose tolerance curve and it can be used to assess how well a person’s body is able to take glucose out of the blood and use it for energy and to build stores. Since different foods will produce a different glucose tolerance curve patterns, scientists developed the Glycemic Index. So basically, glycemic index is a measure of the power of carbohydrate containing foods to raise blood glucose levels after being eaten or drank. In addition to people managing their blood glucose levels, glycemic index has become popular for many people trying to lose weight. See Glycemic Index/Load Table.
For a long time, it was assumed that because starch was more structurally complex than simpler sugars, starchy foods would be digested more slowly and therefore absorbed more slowly and evenly after a meal. On the other hand, foods containing simpler sugars (e.g., soda, candy) would be digested and absorbed more rapidly, leading to a faster and greater rise in blood glucose. However, the relationship between different foods and blood glucose turned out to be more complex, which is why the determination of glycemic index for individual foods has been helpful.
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 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.
In addition to monosaccharide type, protein, fiber and fat as well 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?
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 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. As a quick note, the 50 grams does not include fibers and oligosaccharides that are not readily digested in the upper digestive tract.
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).
Over the past few years, it has become more common to generate a glycemic index based on 25 grams instead of 50 grams for foods with lower amounts of digestible carbohydrate. This would include vegetables and other foods high in fiber and poorly digested carbohydrates but low in sugars and starches as well as typically consumed in amounts the remain within 25 grams of digestible carbohydrates.
What is glycemic load?
While the concept of glycemic index is pretty straight forward, it is not always easy to apply to how people eat. One issue with glycemic index is that the amount of food used to determine its glycemic index is not typically the amount of food consumed. A good example is boiled carrots which will have a glycemic index of about 90. Since one cup serving of carrots only has about 4 grams of available carbohydrate, rarely would a person eat enough carrots to achieve the level used to determine its glycemic index, which would be about 12 times that amount. That’s why researchers developed a second glycemic measure more appropriate for the “real world”, called glycemic load.
Glycemic load is the glycemic index divided the number of grams of digestible carbohydrate in a standard serving. A food’s glycemic load is derived by taking the glycemic index and then multiplying it by the amount of digestible carbohydrate and then dividing by one hundred. For instance, carrots have a glycemic index of 90, which multiplied by 4 (grams of digestible carbohydrate) and divided by 100 gives you a glycemic load of roughly 4.
Are glycemic index and glycemic load important to health?
Foods with lower glycemic responses are more desirable for people who are actively managing their blood glucose levels. This includes pre-diabetics and diabetics. The lower glycemic response could mean less medication necessary to keep blood glucose levels in check. Also, lower glycemic diets are often positioned as ideal to help people lose weight. Whether or not this is true remains to be conclusively determined, however lower glycemic foods are associated with better satiety (fullness) and hunger control which can be helpful to people trying to shed a few pounds. Some of this beneficial effect is associated with increased volume to digestible carbohydrate ratio as well as the presence of other nutrients that can satiety and help reduce hunger such as protein and fiber.