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The diet pendulum swings in perpetual motion with dizzying effects. Carbs are good. Carbs are bad. Eat lots of animal protein. Eat vegan. Gluten is bad. Fruits are good. With all of the information—and misinformation—swirling around us, it’s no wonder we’re confused about what to eat.

The truth is, learning what to eat is a bit like a chemistry experiment. We tend to evaluate food based on its calorie and nutrient density, but it’s also important to remember that food affects our blood sugar levels, which in turn affect our insulin levels, which in turn can affect our risk for cardiovascular disease, diabetes, and cancer, not to mention fat storage.

Choosing foods that won’t spike our blood sugar is important for our mood, energy, and weight. But how do you know which foods will send your blood sugar soaring and which ones will keep it stable? That’s where the glycemic index is a handy tool.

What is the Glycemic Index?

The glycemic index (GI) is a measure of the effects of carbohydrates on blood sugar levels. Carbohydrates that break down quickly and thus, release glucose into the bloodstream quickly, have a high GI; whereas carbohydrates that break down slowly, releasing glucose into the bloodstream gradually, have a low GI.

The glycemic index was originally designed as a tool for diabetics because a lower glycemic response typically results in a lower insulin demand; however, the index is a useful tool for everyone. Glucose is considered the reference food and has a value of 100. To put it into perspective—a baguette has a value of 95 and chickpeas have a value of 33.

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Using the Glycemic Index

The glycemic index need not become a punishing tool that forbids you from eating all of your favorite foods. On the contrary, the glycemic index provides valuable information so that you can make informed dietary choices—but you have to understand how to use it. The glycemic index is not black and white—not all high-GI foods are “bad” and not all low-GI foods are “good”. For example, a potato is a high-GI food that is low in calories and high in nutrients. Pizza, on the other hand, falls lower on the glycemic index, but is high in calories and low in nutrients. Common sense should prevail.

Choosing Foods with the Glycemic Index

The glycemic index is a supplementary tool. Your first job as a healthy eater is to choose whole foods over processed foods. After that choice, the glycemic index can be handy for tweaking choices. It should be used to guide, not dictate.

Sometimes high-GI foods are appropriate. For example, during an intense workout, a high-GI sports drink can boost energy and performance. That same drink will have a different effect on your body during an afternoon spent on the couch.

When using the glycemic index, it’s important to think about balance and glycemic load:

  • Glycemic load: Glycemic load (GL) is a measure based on the glycemic index and the portion size. In other words, the GL is based on quality and quantity. To determine the GL, divide the GI by 100 and then multiply by the grams of digestible carbs in a typical serving. For example, an apple has a GI of 32 and a GL of 4.
  • Balance: Eating a balanced meal will offset the GI score. A baked potato falls high on the glycemic index, but when consumed with steamed broccoli and baked chicken, its impact on blood sugar levels will be minimized.