What Is the Glycemic Index?

Doughuts are high on the glycemic index

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These days nutritional labels on food contain a lot of information on calories, carbs, sodium, and fat content, among others. One measurement you may have noticed is something called the glycemic index. What is the glycemic index, and what exactly is it measuring? And more importantly, is it something you should care about?

What Is the Glycemic Index?

The glycemic index is a measurement of how quickly foods containing carbohydrates are converted by your body into glucose, which is the simplest form of sugar and what your body uses as energy.

What happens is, as the carbs are converted to glucose, the glucose enters your bloodstream. Then, your pancreas produces a hormone called insulin, which also enters your bloodstream and helps to convert that glucose into energy that your body can use.

Some carbs, like ordinary sugar (i.e. sucrose), which is abundant in soft drinks and desserts, for example, will be converted to glucose very quickly, leading to a rapid spike in blood sugar levels and requiring a relatively large influx of insulin to deal with it.

Other carbs, such as whole grains, are quite complex molecules that it takes your body quite a while to break down into glucose, which means that although they also produce an increase in your blood sugar, it happens much more gradually.  

So, in a nutshell, foods that your body quickly converts to glucose are considered to have a high value on the glycemic index, while foods that convert more slowly are considered low glycemic index foods.

Glycemic Index and Diabetes

Terms like blood sugar and insulin are probably associated in your mind with diabetes, and indeed, the glycemic index was in fact developed for the purpose of helping diabetics—whose bodies either don't produce enough insulin, or their bodies resist the insulin they do produce. Either way, the result is that their bodies can't convert the glucose into energy and the levels of glucose in their bloodstream becomes too high.

The idea is that by sticking with foods that are low on the glycemic index, diabetics would be able to avoid the sudden spike in blood glucose levels that can lead to a dangerous diabetic reaction.

In other words, unless you're a diabetic, you probably don't need to pay attention to the glycemic index.

Measuring Foods on the Glycemic Index 

To begin with, the only foods that have a glycemic value are carbohydrates. Proteins and fats would register as having a zero glycemic index. And already, you can see the problem with equating "low glycemic index" with healthy. A stick of deep-fried butter would be extremely low on the glycemic index, and yet, still be incredibly unhealthy.

But, within carbohydrates, complex carbs like whole grains, as well as fruits and vegetables—especially ones that are high in fiber—will tend to score relatively low on the glycemic index since it takes your body longer to convert them to glucose, leading to a more gradual increase in blood sugar levels. 

On the other hand, foods high in sugar (including juices and sodas), and refined wheat flour (like doughnuts and pastries), will score much higher on the glycemic index since they are quickly converted into glucose by your body.

Can a Food's Glycemic Index Change?

The glycemic index of a particular food can change based on how it's prepared and how ripe it is, as well as what other foods are eaten with it.

For instance, a raw potato has a relatively low glycemic index, but a baked potato is higher. Higher still are mashed potatoes. This has to do with the way starches change when they're heated, essentially making them easier for the body to convert to glucose.

Likewise, spaghetti can have a lower or higher glycemic index, simply as a result of whether it's prepared al dente or cooked a little bit longer.

Similarly, a newly ripe banana—mostly yellow with some slight green still visible at the ends—will have a lower glycemic index than one that has been ripe for some time and is now covered with dark brown spots. Those spots represent the starches in the banana being converted to sugar. A very ripe banana will thus have a higher glycemic index.

Finally, it's possible to change the glycemic index of a food by pairing it with a low-glycemic index food. For example, pretzels are a high glycemic index food, but cheese, which is all protein and fat, is low. Therefore, eating pretzels together with cheese will lower the overall glycemic index of the pretzels since your body has to process the combined meal as if it were a single food.