Understanding the Glycemic Index

I’ve given quite a few presentations on diabetes prevention and management in the past few weeks, so it’s no surprise that I have the Glycemic Index (GI) on my brain.  The GI is a measure of how fast a carbohydrate-containing food raises your blood sugar.  Foods are rated on a scale of 1-100 based on how fast they raise blood sugar compared to a reference food, usually white bread or pure glucose: 

<55 = low GI (good)

55-69 = moderate GI

>70 = high GI (bad)

Foods with a low GI raise blood sugar slowly, meaning they provide a steady source of energy that helps keep blood glucose levels stable.  Foods with a high score raise blood sugar quickly, and cause it to drop just as fast, leading to unstable glucose levels.  Blood sugar fluctuations can be dangerous in diabetics, and can cause issues such as increased hunger in between meals, lack of energy, and moodiness in most people.   Therefore, it’s important to choose foods with a low GI as much as possible in order to keep blood sugar stable throughout the day.  If you must reach for a high-GI food, try to pair it with a low-GI one to avoid blood sugar fluctuations.  GI values for over 100 foods can be found here.   

Several factors can affect GI, so the aforementioned GI chart is just a guideline.  Be aware of the following GI influencers: 

  • Fat and fiber content.  Foods that contain fat or fiber take longer to digest, so they can help stabilize blood sugar.  Choose foods that are made with heart-healthy unsaturated fats (rather than saturated or trans fats), and contain at least 3g of fiber per serving.
  • Ripeness and storage time.  As fruits and vegetables ripen, the starches they contain are broken down into simple sugars, thus increasing their sugar content.  Therefore, the riper the fruit or vegetable, the higher its GI.  A ripe banana has a GI of 62, which is bordering on high.  A slightly under-ripe banana, however, will keep blood sugar much more stable.
  • Processing.  There is only one ingredient listed on the carton of orange juice I drink most mornings:  oranges.  One would think that an orange and orange juice would have exactly the same GI, but processing actually increases the GI of most fruits.  When fruits are juiced, they lose their skin, seeds, and other components that provide most of the fiber they naturally contain (and many of the nutrients, as well).  When fiber is lost, there is not much left to help slow down digestion, so blood sugar – and GI – rises more rapidly in “processed” foods like fruit juice.
  • Cooking Time/Method.  Al dente is the preferred way to cook pasta in Italy, but most Americans like their pasta slightly softer.  However, just like slightly under-ripe fruits and veggies have a lower GI, so do many slightly under-cooked carbohydrate-containing foods, like pasta.  Again, this has a lot to do with starches being broken down into sugars, so cook your pasta according to the directions for “al dente” in order to give it a slightly lower GI. 

All of this being said, while GI is important, the amount and type of carbohydrate you’re eating also matter.  Be sure to choose complex carbs like fruits, veggies, and whole grains more often, and limit your intake of refined carbs like those made with white flour in order to help keep your  blood sugar levels where they need to be.



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