Eat more fiber. You've probably heard it before. But do you know why fiber is so good for your health?

Dietary fiber -- found mainly in fruits, vegetables, whole grains and legumes -- is probably best known for its ability to prevent or relieve constipation. But foods containing fiber can provide other health benefits as well, such as helping to maintain a healthy weight and lowering your risk of diabetes, heart disease and some types of cancer.

Selecting tasty foods that provide fiber isn't difficult. Find out how much dietary fiber you need, the foods that contain it, and how to add them to meals and snacks.

What is dietary fiber?

Dietary fiber, also known as roughage or bulk, includes the parts of plant foods your body can't digest or absorb. Unlike other food components, such as fats, proteins or carbohydrates -- which your body breaks down and absorbs -- fiber isn't digested by your body. Instead, it passes relatively intact through your stomach, small intestine and colon and out of your body.

Fiber is commonly classified as soluble, which dissolves in water, or insoluble, which doesn't dissolve.

• Soluble fiber. This type of fiber dissolves in water to form a gel-like material. It can help lower blood cholesterol and glucose levels. Soluble fiber is found in oats, peas, beans, apples, citrus fruits, carrots, barley and psyllium.
• Insoluble fiber. This type of fiber promotes the movement of material through your digestive system and increases stool bulk, so it can be of benefit to those who struggle with constipation or irregular stools. Whole-wheat flour, wheat bran, nuts, beans and vegetables, such as cauliflower, green beans and potatoes, are good sources of insoluble fiber.

The amount of soluble and insoluble fiber varies in different plant foods. To receive the greatest health benefit, eat a wide variety of high-fiber foods.

Benefits of a high-fiber diet

A high-fiber diet:

• Normalizes bowel movements. Dietary fiber increases the weight and size of your stool and softens it. A bulky stool is easier to pass, decreasing your chance of constipation. If you have loose, watery stools, fiber may help to solidify the stool because it absorbs water and adds bulk to stool.
• Helps maintain bowel health. A high-fiber diet may lower your risk of developing hemorrhoids and small pouches in your colon (diverticular disease). Studies have also found that a high-fiber diet likely lowers the risk of colorectal cancer. Some fiber is fermented in the colon. Researchers are looking at how this may play a role in preventing diseases of the colon.
• Lowers cholesterol levels. Soluble fiber found in beans, oats, flaxseed and oat bran may help lower total blood cholesterol levels by lowering low-density lipoprotein, or "bad," cholesterol levels. Studies also have shown that high-fiber foods may have other heart-health benefits, such as reducing blood pressure and inflammation.
• Helps control blood sugar levels. In people with diabetes, fiber -- particularly soluble fiber -- can slow the absorption of sugar and help improve blood sugar levels. A healthy diet that includes insoluble fiber may also reduce the risk of developing type 2 diabetes.
• Aids in achieving healthy weight. High-fiber foods tend to be more filling than low-fiber foods, so you're likely to eat less and stay satisfied longer. And high-fiber foods tend to take longer to eat and to be less "energy dense," which means they have fewer calories for the same volume of food.
• Helps you live longer. Studies suggest that increasing your dietary fiber intake -- especially cereal fiber -- is associated with a reduced risk of dying from cardiovascular disease and all cancers.

How much fiber do you need?

The Institute of Medicine, which provides science-based advice on matters of medicine and health, gives the following daily fiber recommendations for adults:

Fiber: Daily recommendations for adults

Your best fiber choices

If you aren't getting enough fiber each day, you may need to boost your intake. Good choices include:

• Whole-grain products
• Fruits
• Vegetables
• Beans, peas and other legumes
• Nuts and seeds

Refined or processed foods -- such as canned fruits and vegetables, pulp-free juices, white breads and pastas, and non-whole-grain cereals -- are lower in fiber. The grain-refining process removes the outer coat (bran) from the grain, which lowers its fiber content. Enriched foods have some of the B vitamins and iron added back after processing, but not the fiber.

Fiber supplements and fortified foods

Whole foods rather than fiber supplements are generally better. Fiber supplements -- such as Metamucil, Citrucel and FiberCon -- don't provide the variety of fibers, vitamins, minerals and other beneficial nutrients that foods do.

Another way to get more fiber is to eat foods, such as cereal, granola bars, yogurt and ice cream, with fiber added. The added fiber usually is labeled as "inulin" or "chicory root." Some people complain of gassiness after eating foods with added fiber.

However, some people may still need a fiber supplement if dietary changes aren't sufficient or if they have certain medical conditions, such as constipation, diarrhea or irritable bowel syndrome. Check with your doctor before taking fiber supplements.

Tips for fitting in more fiber

Need ideas for adding more fiber to your meals and snacks? Try these suggestions:

• Jump-start your day. For breakfast choose a high-fiber breakfast cereal -- 5 or more grams of fiber a serving. Opt for cereals with "whole grain," "bran" or "fiber" in the name. Or add a few tablespoons of unprocessed wheat bran to your favorite cereal.
• Switch to whole grains. Consume at least half of all grains as whole grains. Look for breads that list whole wheat, whole-wheat flour or another whole grain as the first ingredient on the label and have at least 2 grams of dietary fiber a serving. Experiment with brown rice, wild rice, barley, whole-wheat pasta and bulgur wheat.
• Bulk up baked goods. Substitute whole-grain flour for half or all of the white flour when baking. Try adding crushed bran cereal, unprocessed wheat bran or uncooked oatmeal to muffins, cakes and cookies.
• Lean on legumes. Beans, peas and lentils are excellent sources of fiber. Add kidney beans to canned soup or a green salad. Or make nachos with refried black beans, lots of fresh veggies, whole-wheat tortilla chips and salsa.
• Eat more fruit and vegetables. Fruits and vegetables are rich in fiber, as well as vitamins and minerals. Try to eat five or more servings daily.
• Make snacks count. Fresh fruits, raw vegetables, low-fat popcorn and whole-grain crackers are all good choices. A handful of nuts or dried fruits also is a healthy, high-fiber snack -- although be aware that nuts and dried fruits are high in calories.

High-fiber foods are good for your health. But adding too much fiber too quickly can promote intestinal gas, abdominal bloating and cramping. Increase fiber in your diet gradually over a few weeks. This allows the natural bacteria in your digestive system to adjust to the change.

Also, drink plenty of water. Fiber works best when it absorbs water, making your stool soft and bulky.

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