The glycemic index (GI) was developed in the 1980s^[1] and was used to rank carbohydrates on a scale of 0 to 100 based on their ability to raise blood sugar after consumption. To determine this ranking, researchers fed fasted participants a serving of food containing 50 grams of carbohydrates. The greater and more prolonged the glucose response, the higher the GI rating. A high GI food is typically characterized by rapid digestion and absorption into the bloodstream. A GI over 70 is considered high, 56 to 70 is moderate, and 55 and below is low. Pure glucose has a GI of 100. As an example, a plain white baguette might have a GI as high as 95, while the GI of whole-grain bread can be 50–60, depending on the recipe.^[2]

What’s the difference between glycemic index and glycemic load?

The GI does not take into account the quantity of a food consumed in a real-life, free-living setting. The use of glycemic load (GL) was designed to correct this issue.

The GI ranks foods according to the potential of 50 grams of carbohydrates from that food to raise blood glucose, compared to 50 grams of pure glucose. The glycemic load of a serving of food is the actual amount of available carbohydrate in that serving (in grams) multiplied by the GI score and then divided by 100.

glycemic index ✕ grams of carbohydrates in the serving / 100 = GL

A high-GL serving of food has a GL of more than 20, the moderate GL range is 11–19, and the low GL range is 1–10.

Some foods can have a low GL for a typical serving, despite having a high GI, if the available carbs don’t make up much of the food. For example, watermelon has a very high GI (72), but as it’s mostly water, a typical serving could have a GL of 4. A table of calculated GIs and GLs can be found here.^[2]