Dietary fiber is largely an indigestible component of fruits, vegetables, grains, nuts, seeds and legumes. Animal products do not contain fiber. Unlike fats, carbohydrates and protein, most fiber is not broken down as it passes through the small intestine; hence, it supplies no nutrients or calories. Even so, health experts continue to extol the virtues of fiber, which your mother or grandmother may have called “roughage”—and which is partly what makes “good” (high-fiber) carbohydrates so good.
Fiber helps reduce certain risk factors for heart disease, including high blood pressure and high cholesterol, and scientists are beginning to study fiber’s potential for reducing inflammation in the body—another way it may reduce the risk of heart disease. Studies have linked low-fiber diets to an increased risk of type 2 diabetes and high-fiber diets to a reduced risk of insulin resistance (sometimes called prediabetes).
There is evidence that both soluble and insoluble fiber are beneficial. Soluble fiber, in particular, improves blood sugar control by slowing the absorption of glucose (sugar) in the intestines. People with diabetes who get enough fiber may even reduce their need for insulin or other medication.
Because fiber increases stool bulk and frequency of bowel movements, it is often recommended as a way to prevent and treat constipation, especially if you get very little fiber to begin with. Fiber, particularly insoluble fiber, also seems to help diverticular disease by preventing constipation and strained bowel movements, thereby reducing pressure in the colon. Whether fiber helps in irritable bowel syndrome or other intestinal conditions is uncertain—some people improve with certain types of fiber, while others feel worse.
Fiber may also protect against colon cancer, but results of studies have been inconsistent—and, more recently, disappointing. Two Harvard studies failed to find a protective benefit of fiber from cereals, vegetables and fruits after controlling for other variables that may increase colon cancer risk, such as smoking and eating a lot of red meat.
Lastly, though not all studies have found this, fiber may play a role in weight control. People with higher intake tend to weigh less. High-fiber foods are often lower in calories than other foods, and they help you feel full so you eat less. Fiber may also reduce insulin secretion and alter hormones that affect weight.