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What is the Glycemic Index?

Glycemic index is a relative ranking of the impact a specific carbohydrate-containing food has on your blood sugar. As a tool, it may help you to stabilize your blood sugar. If you don’t have diabetes or prediabetes, you may be wondering if this applies to you. Our answer: it certainly can! Eating foods that have a lower impact on blood sugar can help reduce the risk for heart disease, slow the progression of diabetes, improve sleep quality, result in weight loss, and lead to an overall healthier diet. 

So how exactly does the glycemic index work? Each carbohydrate-containing food has a glycemic index, or GI, which ranks that food on how fast it raises your blood sugar. Foods’ GIs are measured and determined against a benchmark food like white bread or glucose. Foods with a high GI are rapidly digested and absorbed; these carbohydrates are converted to glucose very quickly. They may include refined carbohydrates like white bread and white rice, as well as sugary beverages like soda or juice. On the other hand, foods with a low GI take longer to digest; our bodies release their absorbed carbohydrates into the bloodstream over a longer period of time. Examples of low GI foods include high-fiber grains like quinoa or brown rice as well as legumes, beans, non-starchy vegetables, certain fruits like berries, apples, and peaches, nuts and other fats, and high-protein foods like fish, meat, dairy, soy, and eggs.

The American Diabetes Association classifies carbohydrate-containing foods as high, medium, or low GI as follows:

  • High: 70 or more
  • Medium: 56-69
  • Low: 55 or less

What might cause our body to digest one food more or less rapidly than another? Generally, whole foods that are higher in fiber, fat or acid will take longer to digest. The type of sugar and starch can also play a role. For instance, fructose and lactose are lower GI sugars, landing at 19 and 46, respectively. Honey, maple syrup, and sucrose (table sugar) range from 50-65. On the other hand, maltose exceeds glucose (100) at 105 with a very high GI. Starches are composed of two molecules: amylose and amylopectin. While amylopectin is easy to digest, the structure of amylose takes longer to digest. As a result, starches with more amylose than amylopectin will have a lower GI than those with a higher ratio of amylopectin. Consider quinoa, which is higher in amylose: it has a GI of 53; compare this to short grain white rice, which is higher in amylopectin, and has a GI of 76.

Overall, foods that are highly processed, more ripe, or cooked for longer will be easier to digest. With that in mind, you can pair higher GI foods with fat and fiber to help slow their digestion and impact on your blood sugar. You can also enjoy higher GI foods in smaller amounts. Ideally, a food’s GI should provide you with context on how a food may impact your blood sugar. 

But proceed with caution: not all low glycemic index foods are healthy, and not all high glycemic foods are unhealthy. Consider ice cream, for instance: because ice cream contains a lot of fat in addition to sugar, it will take longer to digest and thus have a reduced impact on your blood sugar, giving it a lower GI of 51. On the flip side, a boiled potato may have a high GI of 78, but the fact that it is high in vitamin B6, vitamin C, potassium, and manganese should not be overlooked. 

This leads us to the other glycemic term you should be familiar with: glycemic load (or GL). This measurement takes into account both the glycemic index of a food, as well as the amount of the food (in grams of carbohydrates) consumed. Think about how carrots have a high glycemic index of 71, yet their glycemic load is low (7.5) since 1 medium carrot only contains ~10g of carbohydrates. Similarly to the GI, the GL is classified below:

  • High: 20 or more
  • Medium: 11-19
  • Low: 10 or less

Let’s look at one more example: watermelon. Watermelon’s structure is mostly fructose, glucose, sucrose, and a little bit of fiber, giving it a high glycemic index of ~75. Based on what we have reviewed so far, it makes sense that, being mostly sugar, it would raise your blood sugar quickly. However, since watermelon is also saturated with water (hence its name), it only has a glycemic load of 4! A full 1 1/4 cup of diced watermelon contains roughly 15 grams of carbohydrates — a reasonable amount for a snack. Remember that watermelons are full of vitamins C and A and the antioxidant lycopene. High GI does not necessarily equal unhealthy.  

Keeping the glycemic load in consideration, eating a diet packed with low glycemic index foods can offer many health benefits. Studies have found that a low GI diet is correlated with reduced total and LDL cholesterol levels, which may lead to a reduced risk of heart disease and stroke. Compared with those who consume a low GI diet, studies also showed that participants eating a high GI diet were more likely to develop cancers like endometrial, colorectal, and breast cancer, and were 25% more likely to develop heart disease. While research on long-term effectiveness of low GI diets is still needed, there is evidence that a low GI diet can lead to short term weight loss. One study found that healthy adults lost 1.5-4.2 pounds over 5-10 weeks. And while some theorize that a low GI diet can lead to better appetite control, further studies should investigate this relationship.

Ultimately, the glycemic index can empower you to think through your carbohydrate portions and food pairings, but it should always be used in the context of the bigger picture. As with most things in life, it is not black or white. Regardless, being mindful of how your diet may impact your blood sugar is a great step towards better health. 

To learn more about glycemic index as a nutrition tool and for the glycemic indices of some common foods, we recommend the following resources: 

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