What is the glycemic response / glycemic index?

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Obligatory disclaimer: I am not a medical doctor, and the content of this website was created for informational purposes only. Such content is not intended as a substitute for medical advice, treatment or diagnosis.

Now that we have finished our series on what macronutrients are (see the individual posts on carbohydrates, proteins, lipids and alcohol for more information), the time has come to deal with some of the minor concerns that were left unanswered during that series. This post will be dedicated to answering what the glycemic index / glycemic response is.

In our post on carbohydrates, we talked about how carbohydrates come in two different groups: simple (meaning one or two saccharide molecules) and complex (containing three saccharide molecules or more). We also learned that carbohydrate digestion can be summarized as “break complex carbs down into simple carbs, then absorb them into the bloodstream”. However, would this mean that simple carbohydrates are digested quicker than complex ones? The simple answer is “yes”, but let’s stick around a little longer to learn what else this means.

Glycemic response

Since complex and simple carbohydrates are digested at different rates, they differ in how they impact blood glucose concentration. By glycemic response, we refer to the effect that food which contains carbohydrates has on blood glucose levels. For example, food that causes a rapid rise and fall in blood glucose levels has a different glycemic response than food which has a more gradual increase in blood glucose, has a lower peak level and also falls at a slower rate. It would be very nice to have some way to classify foods based on its effect on blood glucose levels, perhaps this could be done with some sort of numeric index?

Glycemic index

The glycemic index is the numerical value that represents the impact that specific food has on blood glucose levels. According to Gropper and Smith (Advanced nutrition and human metabolism), the glycemic index is determined as follows:

Glycemic index is defined as the increase in blood glucose level over the baseline level during a 2-hour period following the consumption of a defined amount of carbohydrate (usually 50 g) compared with the same amount of carbohydrate in a reference food.

However, they also tell us how the glycemic index is actually determined:

(…) the glycemic index is measured by determining the elevation of blood glucose for 2 hours following ingestion. The area under the curve after plotting the blood glucose level following ingestion of the reference food is divided by the area under the curve for the reference food times 100. If glucose is used as the reference food, it is arbitrarily assigned a glycemic index of 100. With glucose as the reference food white bread has a glycemic index of about 71. The use of white bread as the reference assigns the glycemic index of white bread of 100.

To simplify, the glycemic index tells you how much of a “sugar hit” some food will give you relative to either glucose or white bread. Unsurprisingly, some foods have a greater effect on blood glucose levels than white bread, so their glycemic index is higher than 100 when using white bread as a reference.

The way the glycemic index is measured isn’t perfect, as methodological differences (the way food is prepared or the ingredients used), and even the food’s temperature have an impact on its numerical value. There is another problem: we rarely eat a single food, but rather meals composed of several foods. In order to deal with this issue, the idea of glycemic load was introduced.

Glycemic load

The idea behind the glycemic load is that it’s both the quality (related to the glycemic index), and the quantity of carbohydrates in a meal. Fortunately, its definition is rather simple, as the glycemic load (GL) equals the glycemic index times the grams of carbohydrate in a serving of the meal. As may be expected from its definition, a higher GL means a higher blood glucose level, and a long-term diet with a high GL is associated with greater risk of type 2 diabetes and coronary heart disease. In other words, the higher blood glucose levels rise and the longer such increased blood glucose lasts, the higher is our risk for diabetes and heart disease.

So now we know what the glycemic response, the glycemic index and the glycemic load are. Next week we’ll cover the Body Mass Index (BMI).

Hope you enjoyed this week’s post!

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