“Part of the answer is that fiber helps normalize transit time, or the rate at which food moves through the digestive tract,” says Joanne L. Slavin, PhD, RD, professor of food science and nutrition at the University of Minnesota.
Fiber Regulates Digestion
Constipation happens when food moves too slowly through the large intestines, often resulting in hard stool that is difficult to pass. Eating fiber-rich foods helps move the contents of the large intestine along more quickly. Fiber also absorbs water, softening stools so that they pass more easily.
Diarrhea occurs when undigested food moves too fast, before the intestines can absorb water, resulting in loose stools. Fiber’s ability to absorb water helps make stools more solid. And by slowing transit time, fiber gives the large intestines a chance to absorb additional water. Fiber also helps bulk up the contents of the large intestines, binding indigestible food together.
“Having something left at the end of digestion and absorption turns out to be necessary to form a normal stool,” says Slavin, a leading expert on fiber and digestion.
Types of Fiber: Insoluble and Soluble Fiber
Fiber is the indigestible part of carbohydrates found mostly in plants. Recent research reveals that there are many forms of fiber, each with a unique effect on nutrition and health. Two important categories are soluble and insoluble fiber.
Soluble fiber partly dissolves in water, creating a gel-like substance that helps lower cholesterol. Sources of soluble fiber include oats, barley, rye, dried beans, oranges, and apples.
Insoluble fiber remains more intact as it passes through the digestive system. That makes insoluble fiber especially helpful in preventing or easing constipation. Insoluble fiber may also help with weight loss, by making meals seem more filling without adding calories. Sources of insoluble fiber include wheat, brown rice, celery, carrots, nuts, and seeds.
Foods can contain both soluble and insoluble fiber.
“Cereal grains are typically higher in total fiber than fruits and vegetables, so that may explain why they stand out in research studies,” Slavin says. Cereal fiber comes from oats, wheat, barley, and other grains.