Recall that refined white flour has had the bran and fiber removed so all that is left is the starchy flour. White bread is made from bleached, refined flour. When you eat a piece of white bread, the carbohydrates in the flour are easily broken down into glucose, which is quickly absorbed through the wall of the stomach and the first part of the small intestine. As the glucose enters the bloodstream, the body senses the additional glucose and the pancreas releases insulin.
Insulin is a glucose escort. Without it, glucose cannot enter a cell because it cannot get through the cell wall. Because the digestion of white bread is so fast, there is a rapid increase in the amount of glucose that enters the bloodstream. This prompts a rapid and large release of insulin. So much insulin is released that the amount of glucose in the blood drops dramatically.
The cells are happy because they get lots of glucose for energy, but the bloodstream is left with too little glucose, leaving you in a state of hypoglycemia, or low blood sugar. Although hypoglycemia sounds alarming, it's actually quite normal—it happens to all of us every time we feel hungry. Low blood sugar is one of the reasons we sense hunger. Different foods are absorbed at different rates; white bread is absorbed rather quickly. In fact, the rate at which the body digests white bread is the measuring stick by which the absorption rates of all other foods are determined. The rate at which white bread is digested and absorbed into the bloodstream is called white bread's glycemic index. The value for the glycemic index of white bread is 100.
Instant rice is absorbed even faster than white bread. It has a glycemic index of 124 and is considered to be a high-glycemic-index food. Table sugar has a glycemic index of 83. That's right, you read correctly; table sugar has a lower glycemic index than white bread because it takes longer to enter the bloodstream. Popcorn, fruit, vegetables, whole grains, beans, and skim milk all have indexes much lower than that of white bread.
The rate at which foods are digested is dependent on many different factors. For example, a piece of fruit has a different index depending on its ripeness—ripe fruits have a lower index. If you chew your food a lot, it will have a higher index because the smaller particles are digested more quickly.A whole apple has one index, applesauce has a different index, and apple juice has a third index. Different pastas have different values depending on the type of pasta you eat. Foods that are processed or cooked more than others have a different index, so if you boil, bake, or fry something (like a potato), you will get three slightly different indices. Suppose you use white bread and make a tuna sandwich with lettuce, pickles, and mayonnaise served with fresh fruit and some chips made without trans fats. Even though the white bread has a high glycemic index, the entire meal, when eaten at one setting, actually has a low glycemic index. The way glucose is absorbed into the bloodstream is dependent on what foods are eaten at the same time.
You can see that trying to make a recommendation on which foods to eat, how to cook them, when to pick them, how to chew them, and so on and so forth, would be unrealistic. Yet that's what it would take to closely follow a low-glycemic-index diet. Until additional well-designed studies are completed, there is not enough information to make recommendations to the public. Besides, if you really think about it, learning the glycemic index of every food you eat is rather silly. If you follow a prudent diet consisting of whole grains; fruits and vegetables; low-fat dairy products; good fats; and fish, poultry, eggs, and with red meat consumed in moderation, you are already getting a diet that has a low glycemic index.
You might be wondering, why spend so much time discussing the glycemic indices of foods? Take another look at the Healthy Eating Pyramid. At the tip you will see white rice, white bread, potatoes, pasta, and sweets. These are all foods that have a high glycemic index, and they are listed at the top for that reason.
Laboratories all over the world have been carefully testing the glucose absorption properties of literally thousands of different foods. (The results of this testing can be found online at www.glycemicindex.com and in countless books that have been published on the subject.) It took a while but eventually the glycemic indices for most foods were determined. This gave scientists the ability (after surveying people about their nutrition) to calculate the glycemic index for the whole diet, an index value that represents the sum total of a person's diet. People who eat a lot of refined carbohydrates have an index higher than people who eat a more prudent diet. With index values for whole diets, researchers were finally able to determine if there was any relationship between diet, glycemic index, and many chronic diseases: it appears there is.