All calories are not the same. This simple concept underscores the glycemic index and its usefulness in making decisions about diet. Foods with a high glycemic index, like processed sugar, often provide much more energy than the body needs at that time. The unused calories float around the bloodstream and the body responds by storing these extra calories as fat. Another problem is that these quick calories are used up so quickly that it is easy to soon become hungry again. A much better choice, clearly, is to consume calories that do not create a spike in the bloodstream and that are burned slowly enough that it takes a long time to become hungry again. It is like the difference between taking a swallow from a fire hose and taking small, repeated, sips from a water fountain. Which sounds more appealing?
The glycemic index of foods is expressed as a number from 0-100. It reflects how quickly a food can be converted into glucose (sugar). The higher the number, the faster the food can get turned into sugar and the greater the spike of sugar in the blood stream. Pure glucose, for example, has a glycemic index of 100. Here are target ranges for the glycemic index:
- 55 or less = Low (good)
- 56- 69 = Medium
- 70 or more = High (bad)
There is a helpful webpage from Harvard University that lists the glycemic index for 100 common foods. White rice, for example, has a glycemic index of 89, whereas soy beans have a glycemic index of 15.
The glycemic index of a food, however, can be changed by preparation. For instance, added lemon juice or vinegar can lower the glycemic index of a food. Added fat or fiber can also lower the glycemic index. If allowed to over-ripen, the glycemic index of fruit, such as bananas, can increase. Finally, the glycemic indexes of pasta and other starchy foods is increased with longer cooking.
While the glycemic index is a useful starting point, there are other important considerations when choosing a food. One such consideration is the size of a typical portion. A food with a lower glycemic index may not be the best choice if more of this food is needed for a full portion. This is where the concept of glycemic load can be helpful. The glycemic load of a food reflects both the glycemic index of that food and the typical size of a portion.
- 1 to 10 = Low (good)
- 11 to 19 = Medium
- 20 or more = High (bad)
The webpage from Harvard University also has information about the glycemic loads of 100 common foods.
Another important limitation of the glycemic index is that it leaves out critical nutritional information. For example, the amount of calories (carbohydrates), fats, vitamins, minerals, salt, and fiber are not described by the glycemic index. All of these considerations are important in making decisions about diet.
The glycemic index is a great starting point in making choices about diet. Foods with a high glycemic index should generally be avoided in favor of foods with a lower glycemic index. However, it is important to consider the size of portions (reflected in glycemic load) and other nutritional content of foods to make the best choices possible.