Very complex carbs (dietary fiber) also have a very complicated molecular structure, and also are resistant to most digestive enzymes produced by the human body. As a result, they cannot be broken down into glucose or other nutrients at all. This is why fiber transits the intestinal tract largely undigested. This has a knock-on effect on the speed of digestion of other carbs around them. For example, where certain starches are "protected" by indigestible fibrous wrapping, the enzymes cannot get to grips with the starch as fast as normal. Also, the presence of soluble fiber in the stomach and intestine typically creates a viscous mass of digesting-food in which carbs and enzymes take longer to mix. Result? Carb digestion slows down.

As we have seen, because the human body runs on glucose all carbs are converted into glucose in the digestive tract. The glucose then enters the bloodstream and thus contributes to a rise in "blood-glucose".

Blood Glucose Must be Kept Within Limits

A very high level of glucose in the blood is toxic, while a very low level is detrimental to bodily functions. Therefore the body has a system to regulate the amount of glucose in the bloodstream to ensure that it remains balanced within safe parameters. This glucose balancing system depends upon two mechanisms: hunger and insulin.
Low Blood Glucose Triggers Hunger

If blood-sugar levels drop, the brain causes us to feel hungry. Result? We eat food that is then converted into glucose and our blood glucose levels rise. If we don't eat and blood-glucose levels fall too low, we trigger the condition known as hypoglycemia.

If we eat a diet that contains too many high GI carbs (carbs that are rapidly converted into blood glucose) we force our body to respond by releasing equally large amounts of insulin into our bloodstream to cope with the glucose. Over time this excessively high level of insulin can cause the "insulin-receptors" in our cells to become less sensitive to insulin.

The hunger-or-insulin see-saw mechanism works well, provided that we don't eat too many high glycemic index (GI) carbs that are rapidly converted into glucose. When this happens, when a LARGE amount of glucose enters the bloodstream (called a "sugar spike"), the system responds by releasing a LARGE quantity of insulin. (It thinks we've eaten a huge amount of food.) The amount of insulin is so large that not only does it disperse the food-glucose we have just eaten, it disperses a lot more. Result? Our blood glucose falls too low. So, within a short time (about 2-3 hours) the brain tells us to feel hungry and we recommence eating. This rapid rise and fall in blood glucose, caused by excess production of insulin, is not good for our health or our eating habits.