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[NUTRITION ADVISER]

Fiber Fundamentals: Up-to-Date Answers to Common Questions

Susan M. Kleiner, PhD, RD

THE PHYSICIAN AND SPORTSMEDICINE - VOL 26 - NO. 3 - MARCH 2021


Some of the most common nutrition questions are about fiber: "Am I eating enough?" "How do I get more?" "Don't only older people have to worry about fiber?" "I don't have bowel problems—why do I need more fiber?" "Won't a high-fiber diet make me too full to exercise?" Because research continues to confirm fiber's benefits, here are up-to-date answers and practical ways you can increase the fiber in your diet.

What Is Fiber?

Fiber, found only in plant foods, is an indigestible form of carbohydrate that provides plants with structural rigidity. Fiber is classified by its ability or inability to dissolve in water. Most plant foods contain both types. (See "Soluble and Insoluble Fibers," below.)

What Are the Benefits?

Both soluble and insoluble fibers enhance the work of the intestines, but in different ways. Following are some of the health benefits of these types of fiber.

Blood sugar regulation. Soluble fiber slows the absorption of sugars and starches from the small intestine into the bloodstream. This action helps smooth out the peaks and valleys in blood sugar levels, possibly helping to ward off type 2 ("adult onset") diabetes. For someone who already has diabetes, soluble fiber helps control blood sugar levels.

Cholesterol reduction. Cholesterol made by the body is an ingredient in bile, a substance that is used in digestion and is recycled. Soluble fiber binds to bile acids in the intestines, thereby lowering the body's cholesterol pool. Soluble fiber can lower blood cholesterol levels by at least 5% in people with healthy cholesterol levels, and even more in those who have elevated cholesterol.

Regularity and cancer prevention. Insoluble fiber provides bulk that helps move food residues through the intestine, which helps prevent constipation and diverticular disease. Insoluble fiber also flushes carcinogens, bile acids, and cholesterol out of the system. Studies of total fiber intake (soluble and insoluble) show a decreased risk of colon, rectal, breast, prostate, and other cancers with consumption of a high-fiber diet.

Weight control. Dietary fiber plays an important role in weight management. Because fiber helps you feel full and slows the emptying of your stomach, you eat less. Also, high-fiber diets tend to be low in calories and less likely to contribute to obesity. By avoiding obesity, you lower your risks for the development and progression of heart disease, cancer, high blood pressure, and diabetes.

Who Needs Fiber?

Everyone needs a high-fiber diet to stay healthy, but many of us are not eating enough fiber. The American Dietetic Association (ADA) recommends that adults eat between 20 and 35 grams of fiber each day (10 to 13 grams per 1,000 calories). Unfortunately, most Americans eat only 14 to 15 grams a day.

Kids need fiber, too. New ADA recommendations for children over 2 years old suggest a daily intake equal to or greater than their age, plus 5 grams. By age 20 they should be consuming 20 to 35 grams a day. There are no recommendations for children younger than 2 years, but a variety of fruits, vegetables, and easily digested cereals should be introduced as solids are begun in the diet.

Fiber recommendations for the elderly are based on calories consumed, since many have a reduced calorie intake. It is safe to encourage 10 to 13 grams per 1,000 calories (the same as for all adults), along with adequate fluids.

How Do I Get More Fiber?

To increase your fiber intake, make plant foods the foundation of your diet. For packaged foods, read nutrition labels for the amount of fiber per serving—a good source of fiber contains more than 1 gram per serving. Refined bread and cereals usually contain less than that, and beans, whole grains, and fiber-fortified bread and cereals usually have more (table below). Be sure to get plenty of fluid with a high-fiber diet.


Common Fiber-Containing Foods

Food Dietary Fiber
Content (grams)

Kidney beans, cooked (3/4 c)9.3
Cereal, All Bran (1/3 c)8.5
Prunes, dried (3 medium)4.7
Popcorn, air popped (3 1/2 c)4.5
Pear (1 medium)4.1
Apple (1 large)4.0
Orange (1 large)4.0
Potato, baked, with skin (1 medium)4.0
Spinach, cooked (1 c)4.0
Sunflower seeds (1 oz)4.0
Banana (1 medium)3.8
Rice, brown, long-grain, cooked (1 c)3.3
Carrots, cooked (1/2 c)3.2
Barley, cooked (1/2 c)3.0
Strawberries (1 c)2.8
Bread, whole wheat (1 slice)2.4
Cranberries (1/2 cup)2.0
Cereal, wheat flakes (3/4 c)1.8
Oatmeal, cooked (3/4 c)1.6
Seaweed, nori or kombu (1 c)1.0
Bread, white (1 slice)0.6

Increase your fiber slowly to prevent cramping, bloating, and other unpleasant symptoms. Be aware, too, that you can get too much fiber. Excess fiber decreases the absorption of minerals, and large amounts over a short time—as in supplements—can lead to a serious intestinal obstruction. More than 50 grams per day is probably too much.

Does Fiber Affect Sports Performance?

The timing of high-fiber meals is important. To avoid intestinal discomfort during exercise, don't eat a high-fiber meal or snack just before a workout. If you exercise early in the day, eat most of your fiber after you exercise. If you exercise at midday or later, allow at least 3 hours after a high-fiber meal.

Don't increase your fiber just before a race or other competition. Your body needs time—perhaps weeks—to adjust to increases.

What About Supplements?

It's best to get your fiber from food. You can't easily match the fiber in a healthy diet by using supplements, and supplements don't supply the many other nutrients in food. But for those who don't eat enough food to meet fiber requirements, fiber-fortified foods and fiber supplements in moderation can help.

Can I Still Eat French Bread?

You don't have to swear off refined foods, but you might swap some for higher-fiber fare. Include at least 2 to 3 servings of whole grains among the 6 to 11 grain servings recommended in the US Department of Agriculture's food pyramid. Eat at least 5 servings of fruits and vegetables daily, and eat beans, peas, or lentils at least 2 to 4 times a week.


Soluble and Insoluble Fibers

There are two basic types of edible fiber: water insoluble and water soluble. Food sources for the two types differ, as do their actions in the body.

Water-insoluble fiber absorbs lots of water and acts like a stool softener and bulk former, keeping things moving through the gut. Good sources are root and leafy vegetables, whole grains (such as wheat, barley, rice, corn, and oats), beans, unpeeled apples and pears, and strawberries.

Water-soluble fiber is generally sticky and viscous, slowing the movement of food through the digestive tract, but also inhibiting the body's absorption of some undesirable substances. Good sources are barley, rice, corn, oats, beans, apples, pears, citrus fruit, bananas, carrots, prunes, cranberries, seeds, and seaweed.


Remember: You, your physician, and your nutritionist need to work together to discuss nutrition concerns. The above information is not intended as a substitute for appropriate medical treatment.

Dr Kleiner is a private nutrition consultant to athletes in the Seattle area. She is a member of the American College of Sports Medicine; a member of the American Dietetic Association and its practice group, Sports and Cardiovascular Nutritionists (SCAN); and a fellow of the American College of Nutrition.


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