The glycemic index (GI) shows how much a given carbohydrate will raise blood glucose levels on a 0-100 scale. Foods that are ≥70 are called high-GI foods. These include glucose, white bread, high-fructose corn syrup, corn flakes, puffed rice, mashed potatoes, bagels, and waffles. Those in the range of 56–69 are medium-GI foods such as basmati rice, couscous, raisins, and cranberry juice. Those in the low-GI range of ≤55 raise the blood glucose levels the least and include legumes, fruits, starchy vegetables, and whole grains.
There are many factors that can affect the GI levels of these foods: ripeness and storage time (the riper the fruit or vegetable the higher GI level), processing (juice has a higher GI than whole fruit; mashed potato higher than baked), cooking method (how long food is cooked), and variety (brown rice vs. white rice). Think of an apple: If you drink apple juice, your blood glucose level goes up quite quickly; if you consume applesauce, your blood glucose level goes up a little more slowly; and if you eat an apple, intact fiber and all, your blood glucose goes up least of all.
The GI is not necessary to count carbohydrates, but the concept that some carbs increase blood glucose levels more than others is good to know. You will learn, by trial and error, what foods make your blood glucose level go up the most and which the least.