Some scientists believe that whole grains, containing high amounts of insoluble fiber, protect against several forms of cancer. In an analysis of the data from many studies, people who eat relatively high amounts of whole grains were reported to have low risks of lymphomas and cancers of the pancreas, stomach, colon, rectum, breast, uterus, mouth, throat, liver, and thyroid. Most research focusing on the relationship between cancer and fiber has focused on breast and colon cancers.

A diet high in fiber is protective against heart disease. Soluble fiber from beans, oats, psyllium seed, and fruit pectin has lowered cholesterol levels in most trials. Diets high in overall fiber have reduced triglyceride levels in several clinical trials, but have had no effect in others. Research suggests that soluble, not insoluble, fibers are helpful in lowering triglyceride levels. A high-fiber diet that features both soluble and insoluble fibers also seems to reduce the risk of both fatal and nonfatal heart attacks.

High-fiber diets also may be beneficial for certain gastrointestinal conditions. Fiber slows the movement of food and acidic fluid from the stomach to the intestines. It may help people with duodenal ulcers by reducing the exposure of the small intestine to stomach acids. Diverticular disease has become increasingly common and is thought to be due primarily to the consumption of a low-fiber diet; diets high in fiber have consequently been shown to protect against diverticular disease.

Preliminary evidence suggests that high-fiber diets also may benefit people with type 2 diabetes, as well people with chronic pancreatitis. More research is needed to confirm these relationships.

In addition to reducing the risk of certain chronic diseases, high-fiber diets may have other benefits. Fiber fills the stomach, thereby reducing appetite. Recent research has suggested that high-fiber diets may protect against obesity. Increased intake of fiber promotes digestive health and reduces constipation. Additionally, high fiber foods are generally rich in vitamins, minerals, and phytonutrients that are important for overall health.

Although people can be allergic to certain high-fiber foods (most commonly wheat), high-fiber diets are more likely to improve health than cause any health problems. Beans, a good source of soluble fiber, contain special sugars that are often poorly digested, often leading to flatulence (gas). Cooking beans with kombu (a type of seaweed), epazote (a Mexican herb), or using an enzyme supplement called alpha-galactosidase can help reduce this problem by improving digestion of these sugars. The following cooking method can help too: When cooking dried beans, be sure to soak them overnight, then drain and rinse the beans. Cover with water again, bring to a boil, and skim the foam off the top of the water before reducing the heat to a simmer to finish cooking.

It is important to drink adequate fluids, especially water, when increasing fiber intake to minimize the possibility of constipation.

Fiber reduces the absorption of most minerals. To minimize this effect, multimineral supplements should not be taken at the same time as a high-fiber meal.