Scientists may have figured out how high-fiber diets help digestion.

Basically, fiber (sometimes called "roughage") roughs up the outer membrane of gut cells a bit. As those cells heal, they release mucus, which helps move food along the digestive tract.

The process is detailed by Paul McNeil, PhD, and Katsuya Miyake, PhD, in the early online issue of Public Library of Science Biology.

McNeil and Miyake work at the Medical College of Georgia, located in Augusta, Ga.

"When you eat high-fiber foods, they bang up against the cells lining the gastrointestinal tract, rupturing their outer covering," McNeil explains, in a Medical College of Georgia news release.

"What we are saying is this banging and tearing increases the level of lubricating mucus. It's a good thing," he adds.

How Much Fiber?

McNeil and Miyake studied roughage in a lab. But what about the real world, where fiber goes on your plate, not into test tubes?

Fiber is a cornerstone of the latest dietary guidelines issued by the U.S. Department of Agriculture (USDA).

In those guidelines, issued in 2005, the USDA recommends getting 14 grams of dietary fiber for every 1,000 calories you consume.

So if you consume 2,000 calories per day, you should get 28 grams of dietary fiber that day.

Fiber is found naturally in fruits, vegetables, whole grains, and legumes. How much fiber will you get from different foods? Here's a list from the USDA:

Navy beans, 1/2 cup, cooked: 9.5 grams
100% bran cereal, 1/2 cup, ready to eat: 8.8 grams
Sweet potato, medium-sized and baked, including peel: 4.8 grams
Whole-wheat English muffin: 4.4 grams
Mixed vegetables, 1/2 cup: 4.0 grams
Raspberries, 1/2 cup, raw: 4.0 grams
Apple, medium-sized, including skin: 3.3 grams
Whole-wheat spaghetti, 1/2 cup, cooked: 3.1 grams
Banana, medium-sized: 3.1 grams
Broccoli, 1/2 cup, cooked: 2.8 grams
Collard greens, 1/2 cup, cooked: 2.7 grams
For instance, a breakfast of a medium-sized banana and half a cup of 100% bran cereal yield 11.9 grams of dietary fiber.

Add an apple as a morning snack, a half cup of Navy beans with an equal amount of mixed vegetables at lunch, and whole-wheat spaghetti at dinner, and you've reached 31 grams of dietary fiber -- more than the 28 grams recommended for a 2,000-calorie diet.

Of course, that's just an example, not a menu recommendation. Each person is different, and not every food that you eat has to be loaded with fiber.

The take-home message: Mix and match fruits, vegetables, whole grains, and legumes to help you get recommended amounts of dietary fiber as part of a healthy diet.