The Glycemic Index (GI) measures how quickly foods raise blood sugar levels, typically scored between 1 and 100. Foods with a low GI raise blood sugar more slowly. But how is it calculated? Essentially, a food containing 50 grams of digestible carbohydrates is compared to how much glucose raises blood sugar in a reference test. Pretty interesting, right?
What is the glycemic index and how is it calculated?
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I still remember my attempt to better understand the glycemic index (GI) myself—it was a year ago when I started diving deeper into nutrition. Back then, I decided to replace my morning oatmeal with something lower in GI because I’d often get hungry again just an hour later. So, I tested different breakfast alternatives for a week: whole-grain bread with peanut butter, quinoa porridge, and even a day of just nuts and cheese.
But the real eye-opener was the self-experiment: I got a blood glucose monitor and checked my levels before eating, then again at 30, 60, and 120 minutes after. The comparison between classic white toast (GI ~70) and whole-grain bread (GI ~50) was especially interesting. The white toast spiked my blood sugar sharply within 30 minutes, only to crash just as fast—leaving me with that familiar "crash" feeling. Whole-grain bread, on the other hand, showed a much steadier rise and kept me full longer. Since then, I’ve paid more attention to the combination of GI and fiber content—not just at breakfast, but in all my meals.
A good comparison would be the **Glycemic Load (GL)**, which takes into account not just the insulin response but also the portion size. While the GI alone only indicates the speed at which blood sugar rises, GL combines the amount of available carbohydrates per serving, meaning: GI × grams of carbohydrates ÷ 100. For example, watermelon has a high GI (76), but due to its low carbohydrate content (around 7 g/100 g), its GL per serving is low (GL = ~6). The same goes for whole grains: they have lower GI values than white bread, but because of higher fiber content, their GL often has a less pronounced effect—a benefit for longer-lasting satiety.
Wouldn't it be more accurate if people considered the amount of dietary fiber when calculating the glycemic index? Since fiber-rich foods stay in the stomach longer and slow down the digestive process, they naturally cause blood sugar to rise more slowly. For example, whole wheat bread and white bread have different fiber contents, so their GI values differ, but this isn't factored in, right? I think fiber should also be included as a separate coefficient in GI calculations; otherwise, some foods being marketed as "low GI" can be misleading.
The Glycemic Index (GI) was developed in 1981 by Dr. David Jenkins and his team at the University of Toronto to standardize glycemic response. When calculating GI values, the average blood glucose increase caused by foods containing 50 grams of digestible carbohydrates is compared to the response elicited by pure glucose (GI = 100) over a 2-hour period. It's important to note that the amount of digestible carbohydrates is the criterion—fibers and indigestible parts are excluded.
To correctly interpret GI values, the classification system should also be considered: <55 is low, 56-69 is medium, and ≥70 is high GI. For example, whole wheat bread has a GI of ~51, while white bread has a GI of ~75. Key factors affecting a food's GI include starch structure (amylose/amylopectin ratio), fiber content, cooking time, and even the acidity of the food—adding lemon juice, for instance, can lower a meal's GI.