Like a sponge, fiber works by soaking up water, he explains. Fiber also adds bulk to the stool.
"If you have hard constipation stools, fiber pulls water out of the colon to make stools more normal, softer, give them more bulk," says McNeely. "When stools are bulkier, the colon wall can more efficiently push against them when it contracts -- so elimination is smoother."
When there's too much water in the stool, you've got diarrhea. Fiber can help loose stool. "Fiber can't work miracles," McNeely says, "but if you have a loose stool, a lot of excess liquid in the stool, the fiber in your colon will absorb and firm up the stool -- which helps diarrhea."
The thing is, most people don't get nearly enough fiber-rich foods in their diet. In fact, most people are hard-pressed to name a handful of foods with fiber -- except for bran cereal.
Yet we need at least 20 to 30 grams of dietary fiber every day to keep things rolling. If you've got a sensitive stomach, it's especially important that you get enough fiber, he advises. Don't worry about getting too much fiber -- any extra will simply be eliminated.
When you start eating more foods with fiber, start slowly -- so your system can get used to it. Some people get gas and bloating with a sudden boost in dietary fiber.
High-Fiber Food Basics
How to get enough fiber-rich foods? Eat an apple, a side of baked beans, and steamy stir-fried zucchini over brown rice and you've treated your intestines to a nice batch of dietary fiber.
All plant foods -- fruits, vegetables, grains, nuts, seeds, beans -- contain fiber. But some have more of the roughage type of fiber (called insoluble fiber) that aids digestion.
The basics about fiber:
Soluble fiber is soft stuff -- the insides of beans, peas, lentils, mangoes, apples, peaches, figs, pineapple, pears, etc. Oatmeal and oat bran are also high in soluble fiber. Soluble fiber helps reduce cholesterol and control blood sugar levels. It also acts to slow down food as it passes through your intestines, which can help prevent diarrhea.
Insoluble fiber is the roughage type of fiber your digestive system needs. It's the indigestible part of the plant -- like strawberry seeds, apple and bean skins, the bran shell on brown rice and wheat grains. Insoluble fiber helps prevent constipation. People who have irritable bowel syndrome (IBS) may find that too much insoluble fiber worsens diarrhea. If you have IBS, you may want to opt for more soluble fiber.
One note: If a fruit or vegetable has a peel, don't remove it -- or you'll lose important fiber. Important nutrients and insoluble fiber are found in potato and fruit peels.