Okay, so you're eating enough food, and you’re going high-fiber, so it's easy to fill up with relatively little in the way of calories. But there is a third point you need to know about, particularly if you've been battling a weight problem. Within the world of high-fiber foods, some do a better job of keeping your blood sugar steady than others. Let's go back to our breakfast table for a minute and compare a bowl of oatmeal, on the one hand, with a wheat cereal on the other. Yes, they're both better than bacon and eggs - by a long shot. And they both have about the same amount of fiber. But these two cereals have different effects on your body, as researchers have demonstrated. In one four-week study, researchers gave men whole wheat cereal (Weetabix) for breakfast, along with whole wheat bread. Then, for another four weeks, they switched to a muesli made of oats, apple, and a bit of fruit sugar (fructose), along with pumpernickel bread.
The researchers then checked the men's blood sugars and there was a surprisingly big difference between the two breakfasts. The oat based breakfast stabilized blood sugar much more effectively than did the wheat-based breakfast. And a more stable blood sugar means that your appetite stays under control. Now, wheat or corn cereals taste great and have no cholesterol and little fat. You really cannot fault them. But they don't have oats' power to stabilize blood sugars, and that's what counts when it comes to keeping your hunger in check.
Scientists rate how quickly foods release their natural sugars into the bloodstream using a number called the glycemic index, or GI. Foods with a low glycemic index release their natural sugars slowly over a long period of time. That's handy; it means that hunger will not return too soon. When you eat a typical low-GI food, it acts as a constant source of energy, providing you natural sugars, minute by minute, on an ongoing basis. It will not let your blood sugar climb too high, and if your blood sugar doesn't zoom up to a peak it will not be able to crash. High-GI foods are just the opposite. They release their sugars quickly, prompting a return of appetite and more snacking later in the day. One caveat: The GI value of foods is a matter of continuing scientific study, and it clearly matters more for some people than others. If you have been slim all your life and have no trouble with your weight, GI values are not especially important for you. Almost certainly, your body handles sugars very efficiently and never lets your blood sugar get too far out of line. You'll want to pay more attention to fiber content. If, on the other hand, you've struggled with your weight for some time, those added pounds have probably made your blood sugar harder to control, because extra weight makes your body tissues more resistant to insulin, the hormone that controls your blood sugar. This is especially true if diabetes runs in your family. You'll want to be sure to go high-fiber and low-GI with the foods you eat. Among low-GI foods, beans again win out and green vegetables score very well, too. Most fruits have low GIs, but there are a few notable exceptions, as we'll see. Grains vary dramatically—some are low, others are high, and we'll soon learn which are which. Candy, honey, and white bread are examples of high-GI foods.
What is it about some foods that make them release sugars slowly and steadily, while others send sugar into your bloodstream almost explosively? If you could examine any carbohydrate-rich food—a bean, a carrot, or a bit of pasta, for example—under a powerful microscope, you would find that, for some, the carbohydrate molecules are long and straight, and are stacked up in an orderly way, like a dense pile of wood. When you eat them, it takes time for digestive enzymes to break up these densely packed molecules. These slow-digesting foods will not perturb your blood sugar very much. Beans, peas, and lentils are in this low-GI category. Many types of rice are, too. In this case, fiber is not the issue. It all has to do with the arrangement of the carbohydrate molecules. On the other hand, high-GI starches are built from molecules that are branched, like piles of small twigs. Enzymes quickly break them apart, releasing all their sugars into the blood at more or less the same time. Typical wheat breads—even whole wheat bread—are in this quick release category, as are bagels. In contrast, rye and pumpernickel release their sugars much more slowly. Again, it's not fiber that does the trick. It is a question of how the carbohydrate molecules are aligned. So, to keep your blood sugar steady, don't avoid breads. Just be selective about which ones you choose. The same is true for potatoes. Baking potatoes have a high GI and release their sugars quickly, while sweet potatoes and yams have much lower GI values.