Is the Glycemic Index Causing Carbohydrate Confusion?

Part I

The glycemic index (GI) is a very hotly debated topic in the world of nutrition. Unfortunately, most folks aren’t even aware of how it’s measured or what it means when it comes to health and disease prevention. To clarify the issue, let’s briefly review this assessment tool here in Part I and then look at some research regarding its use in Part II.

The GI is basically a measure of how carbohydrate foods affect blood glucose levels. Specifically, it represents the “area under the curve” when plotting the blood glucose elevations of a test food measured over two hours. The values of the test food, taken in 15-minute increments, are graphed and then compared to a reference food, which is most often glucose. The GI is measured on two or three separate occasions for each individual, with the “official” value representing the average obtained from 8-10 different people. Glucose is given a score of 100, with all other foods having a lower score. Foods with a GI less than 55 are considered low, while foods above 70 are considered high. Foods that fall between these values are intermediate or medium. It is important to remember that there are actually two GI lists, one using glucose and the other using white bread as the reference food. If you see numbers that differ significantly, it’s likely that you’re making comparisons using both lists.

Generally speaking, it’s thought that high glycemic carbs promote a rapid glucose spike, followed by a substantial insulin spike, which channels the sugar into the cells. The result is a corresponding drop in blood sugar, a lack of energy and increased hunger sooner rather than later. On the other hand, low glycemic carbs are thought to provide a slower glucose rise and a more sustained energy level. Despite this information, there are actually several drawbacks to using the glycemic index to classify carbohydrate foods as healthy or unhealthy. For one, there are many factors that affect the glycemic index of a particular carbohydrate. Chemical composition, physical state, fiber, fat and protein content and a host of other variables affect the glycemic index. Plus, there is great variability even among individuals consuming the same food. In other words, my glucose response to a particular test food may be quite different than yours. Lastly, we don’t commonly eat carbohydrate foods by themselves…we eat them in concert with other foods, which significantly alters the glycemic impact of the entire meal.

There are also several misconceptions that are circulating when it comes to the glycemic index. As an example, many people think pastas and rices are very high when it comes to GI, but in fact, many are low or moderate. This is because certain varieties have a higher percentage of amylose vs. amylopectin (the two components of starch). Amylose is a straight chain molecule, providing less surface area for enzymes to attack compared to amylopectin, which is a much more branched form. This branched configuration allows for plenty of surface area for enzymes to work, thus contributing to rapid breakdown and significant blood-glucose elevation. Most folks think sugars all have very high GI’s too, but in actuality, sucrose (table sugar) is moderate and fructose is very low. This is because fructose is absorbed via facilitated diffusion, which is a fairly slow process, compared to the active transport of glucose. Don’t forget, fructose makes up about half of sucrose or table sugar, which obviously contributes to sugar’s moderate glycemic impact. To further illustrate this, please note that fructose has a GI of about 23, glucose has a GI of 100 and sucrose has a GI of approximately 65, which just happens to be right in between the two individual sugars.

These are just some of the basics when it comes to the glycemic index. In Part II, we’ll address the health implications to find out how useful it is for different populations. To accomplish this, we’ll check out a few interesting studies that just may surprise you!!