Soluble Fiber in the diet and the Glycemic Index
Fiber content affects the glycemic index value of foods in various ways. The fiber coating around the food, and the cellulose in the walls of plant cells (e.g. cabbage, broccoli) both reduce the access of digestive enzymes to the starch inside. Beans and grains, (like oats) contain soluble fiber that makes the contents of the stomach and intestines more viscous during digestion, this thicker mix retards digestion of starch.
Insoluble Fiber in the diet and the Glycemic Index
The glycemic response is also effected by insoluble fiber, such as the fiber in whole wheat. If wheat grains are whole, the outer aspects of the grain act as a physical barrier to digestion. If the whole wheat grains are finely milled, digestion and the glycemic response are more rapid. Therefore, whole wheat bread (milled) has roughly the same glycemic index value as regular white bread.
Whole wheat flour glycemic response compared to regular white bread in the diet
There are definite advantages to using whole wheat flour over white flour. Whole wheat flour is richer in vitamins and minerals. Furthermore, the insoluble fiber whole wheat bread contains, increases intestinal bulk and may increase the feeling of fullness. But, despite these nutritional advantages, finely milled whole wheat flour provokes a similar glycemic response to that of regular white flour.