The glycemic index is a value assigned to foods based on how quickly and how high those foods cause increases in blood glucose.
However, according to Healthline, for certain foods, the cooking method used can affect the glycemic index.
For example, fried foods tend to contain a high amount of fat, which can slow the absorption of sugar in the bloodstream and decrease the GI.
Meanwhile, roasting and baking can break down resistant starch — a type of starch that resists digestion and is commonly found in foods like legumes, potatoes, and oats — thus increasing the GI.
Conversely, boiling is thought to help retain more of the resistant starch and lead to a lower GI, compared with other cooking methods.
The longer you cook foods like pasta or rice, the greater the digestibility of their starch content, and thus the higher their GI. As such, it’s best to only cook these foods until they reach an al dente texture, meaning that they’re still firm when biting into them.
In addition to the cooking method used, the degree of ripeness may also affect the GI of some fruits, including bananas. This is because the amount of resistant starch decreases during the ripening process, leading to a higher GI.
For example, bananas that are fully ripened have a GI of 51, whereas under-ripe bananas have a GI of just 30. This is one of the reasons why unripe plantains are recommended for diabetic patients.