The glycemic index (GI) is basically a measure of how quickly a food causes our blood sugar levels to rise. GI ranks food on a scale of 0 to 100. Foods with a high GI are digested and absorbed rapidly, producing a fast blood glucose response. These foods that rank high on the GI scale are often higher in processed carbohydrates and sugars. Pretzels, for example, have a glycemic index of 83.
On the contrary, foods with a low GI are digested and absorbed at a slower rate, and, thus, cause a slower rise in blood sugar levels. These are typically rich in fiber, protein and/or fat. Examples of these include non-starchy vegetables, apples (Gi = 28), Greek-style yogurt (GI = 11), and peanuts (GI = 7). It is important to remember that a low GI does not guarantee a food is high in nutrients, so still consume foods from all major food groups.
A limitation of using GI when selecting foods is that a food's GI ranking only applies when a food is consumed on an empty stomach without any other type of food. This is not usually how we eat! Perhaps a plain white potato has a high GI, however, add a lean steak or a piece of salmon, a side of broccoli and a salad with vinaigrette, and the protein, fiber and fat all will serve to lower the GI of the meal.
Another flaw of the system is that the GI does not account for how much we're actually consuming. The GI value of a food is determined by giving people a serving of the food that contains 50 grams of carbohydrate minus the fiber, then measuring the effect on their blood glucose levels over the subsequent two hours.
A serving of 50 grams of carbohydrate in one sitting is reasonable for a food such as rice, which has 53 grams of carbs per cup. But for beets, a GI ranking of 64 is a little misleading. Since beets have just 13 grams of carbs per cup, we would need to consume nearly 4 cups of beets in order to cause that spike in blood sugar levels. A reasonable alternative to glycemic load (GL) is a formula that adjusts for potentially misleading GI by combining portion size and GI into one number. The carbohydrate content of the actual serving is multiplied by the food's GI, then that number is divided by 100. So for a cup of beets, the GL would be: 13 times 64 = 832 divided by 100 = a GL of 8.3. As a reference point, a GL higher than 20 is considered high, between 11 and 19 is considered moderate, and 10 or less is considered low. So, although the glycemic index isn't a perfect system, it can be a useful tool to identify lower-glycemic foods that often are more nutrient-dense, as well as what foods are higher in refined carbohydrates.