People with diverticulosis are usually told to avoid seeds and nuts, yet no scientific data support this recommendation (Nutrition in Clinical Practice, March 2011). Diverticulosis means that the colon has multiple outpouchings. The theory is that small food particles such as seeds might get caught in the outpouchings, but no one has shown that this actually happens. Furthermore, avoiding insoluble fiber found in all parts of plants is likely to increase risk for diverticulosis.
When solid food reaches the stomach, the pyloric sphincter closes and food is allowed to pass into the intestines only after it is converted to a liquid soup. It remains a liquid soup until it reaches the colon where fluid is rapidly absorbed to form solid material. Fiber is found in all foods from plants. It is composed of sugar molecules bound together so tightly that humans lack the enzymes necessary to separate out individual sugar molecules. Only single sugar molecules can be absorbed into the bloodstream. Thus fiber cannot be absorbed so it passes to the colon in the liquid soup.
The fiber holds water in the colon and helps keep the stool from becoming so hard that it blocks the passage of gas formed in the colon. Pressure increases behind the hard stool, swelling the colon to cause outpouchings called diverticula. Seeds, nuts and other plant materials help to keep stool soft and prevent diverticula from forming. On the other hand, refined flour that has had the fiber removed cannot hold much water, so it causes hard stool that is more likely to obstruct the passage of gas and increase the chance that outpouchings will form in the colon.