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Fiber is not absorbed and passes through the entire digestive system largely intact. As a result, it contributes no calories to a person’s diet, which may aid in controlling weight and preventing obesity. Foods rich in fiber, such as fruits, vegetables and whole-grain foods are naturally cholesterol-free and typically low in fat, sodium (salt) and calories (energy). Fiber-rich foods take longer to chew, which may cause people to eat less. Their added bulk also may help a person feel full longer. Insoluble fiber may also help reduce the absorption of dietary fat, which is dense with calories. 

Fiber also contributes toward good digestive health. Insoluble fiber helps move waste through the intestinal tract, which helps prevent harmful substances from lingering in the intestines and coming into contact with the intestinal lining. Recent research has found that fiber causes a release of mucus from the intestinal lining that assists food in passing through the digestive system. In addition, insoluble fiber adds bulk and softness to stool, which promotes regularity and prevents both constipation and hemorrhoids (dilated veins in swollen anal tissue). Fiber can also reduce the risk of irritable bowel syndrome (disorder of the lower intestinal tract) and diverticulosis (condition in which small pockets develop in the colon).

In addition, foods rich in fiber are full of vitamins, minerals, phytochemicals (plant chemicals), carbohydrates and protein, all of which contribute to their significant impact on prevention and treatment of chronic diseases.

Fiber appears to be involved in the prevention or management of chronic diseases. Research conducted by the U.S. Department of Agriculture has found that people who eat three servings or more of whole-grain foods each day have a lower risk of metabolic syndrome, a condition marked by abdominal obesity and poor triglyceride, cholesterol, blood pressure and blood sugar readings. Metabolic syndrome is a risk factor associated with the development of both diabetes and heart disease.

Other illnesses that may be influenced by a high-fiber diet include:

• Cancer. There is some controversy about the potential role of fiber in preventing cancer. Numerous studies appear to show a strong link between a diet rich in fiber and a reduced risk of cancer. Experts have speculated that fiber’s role in speeding waste through the digestive tract may prevent harmful substances from lingering in the intestinal tract and contacting the intestinal walls. In addition, fiber’s role in bulking up stools may dilute the concentration of potential carcinogens in food. Finally, fiber may keep the body’s pH levels in a range that reduces the ability of intestinal microbes to produce carcinogens.
  
  However, some recent studies have found less certainty in the previously reported links between a high-fiber diet and colorectal cancer prevention. For now, the role of fiber in cancer prevention remains an open question.
  
  
• Heart disease. The health benefits of dietary fiber can be partially explained by its ability to dissolve in liquids. For example, soluble fiber forms a gel when mixed in a liquid. The viscosity (a measure of a fluid’s resistance to flow) of soluble fibers in foods has been credited with its cholesterol-lowering effect. When people eat, bile acids rich in cholesterol are secreted into the intestine and mix with the food to aid in digestion and absorption. As food is absorbed, the remaining viscous soluble fiber interferes with the re-absorption of these cholesterol-rich bile acids from the intestine. The end result is that the cholesterol is removed from the body, reducing blood cholesterol levels. High levels of low density lipoprotein (LDL) cholesterol can forum into plaques that cause the coronary arteries to harden and narrow (atherosclerosis). Eventually, this process can trigger a heart attack.
  
  There is also some evidence that the change in bile acid content in the body due to dietary intake of fiber may slow the liver’s production of cholesterol, as well as change the LDL cholesterol particles to make them less damaging. Scientific studies, clinical trials and population-based studies have documented the total cholesterol and LDL cholesterol-reducing effects of whole grains, as well as the reduction in heart disease risk due to consumption of a high-fiber diet rich in whole grains, fruits and vegetables.
  
  
• Diabetes. Diabetes is a condition that develops when the body can no longer produce enough insulin or access the insulin that is produced. Insulin is a hormone necessary to control blood sugar levels. Dietary fiber may slow the movement and absorption of foods from the intestine, resulting in favorable effects on blood sugar levels after eating. Studies have also demonstrated that a high-fiber diet may reduce insulin requirements and increase insulin resistance, improving blood sugar levels overall. The secondary benefits of a high-fiber diet for diabetes include its lower calorie (energy), fat and simple carbohydrate content, which may explain its beneficial effects in weight management.