The health benefits of dietary fiber have been so highly touted that few people know where the scientific evidence ends and the hype begins.
What you may not know: Scientists who have studied the link between fiber (in both foods and supplements and health have sometimes found only modest benefits-or no benefits at all.
Reason: To get the optimal benefits of fiber, it must be combined with other healthful substances, such as the antioxidants, vitamins and minerals found in fiber-rich plant foods, including fruits, vegetables and whole grains.
That's why fiber supplements, in the form of powders, tablets and capsules, provide little overall health benefits, other than promoting bowel regularity.
Other important facts…
Types Of Fiber
Fiber has long been divided into two main categories…
- Soluble fiber, which dissolves in liquid, is found in such foods as oats, beans, psyllium (a plant-derived product) and citrus fruits.
- Insoluble fiber, which does not dissolve in liquid, is mainly found in grain products, such as whole wheat...most nuts and vegetables, such as beets, brussels sprouts and cauliflower.
Now: The National Academy of Sciences discourages the use of the terms "soluble" and "insoluble"-partly because distinctions between the two types of fiber and their physiological effects aren't as clear-cut as scientists once thought. What's more, researchers have discovered subtypes of fiber, including beta glucan and arabinoxylan, both of which show promise for lowering both total and LDL "bad" cholesterol and reducing the risk for coronary artery disease.
Even though soluble fiber and insoluble fiber have different chemical properties and effects in the body, they both provide similar health benefits, such as reducing the risk for heart disease.
The latest research shows that everyone should get a mix of different fibers daily. Unfortunately, the average American's total daily fiber consumption is only about 15 g (the equivalent of one medium apple and one cup each of raspberries and cooked oatmeal). The recommended daily amount is 38 g for men up to age 50...and 30 g at age 51 or older. For women, it's 25 g up to age 50...21 g at age 51 and older.
Because the optimal mix of the different fibers has not been determined, all adults should eat a wide variety of foods that contain both soluble and insoluble fibers, such as grains, fruits and vegetables, including beans. Latest research on fiber's health effects…
- Colon cancer. In theory, fiber should prevent colon cancer by speeding the elimination of stools. The faster that stools—and the bile acids and other potential carcinogens that are present in stools-exit the intestines, the less risk of cell damage that can lead to cancer.
Scientific evidence: The research on the effects of dietary fiber on colon cancer and polyps
growths that often turn into cancer) is mixed. Some studies have shown no benefit-although it's possible that the levels of fiber in these studies, and the length of time the studies were conducted, may have been insufficient.
New theory: As fiber is broken down by bacteria in the colon, it creates substances called butyrates-short-chain fatty acids that "bathe" cells in the colon and are thought to reduce cancer risk.
Helpful: Eat at least five servings daily of fruits and vegetables, including several servings of legumes, such as pinto beans.
- Constipation and other digestive problems. Fiber not only decreases the time it takes for stools to exit the body (transit time), it also absorbs water in the colon, which increases the weight and size of stools, making them softer and easier to eliminate.
Scientific evidence: Researchers agree that people who consume the recommended daily amount of fiber are far less likely to suffer from constipation than those who eat less. Fiber also reduces the constipation and diarrhea that can accompany irritable bowel syndrome.
Helpful: Eat three to six servings daily of high-fiber foods, such as bran cereals, wholegrain breads, brown rice, barley and wholewheat pasta.
- Diabetes. There's very strong evidence that a high-fiber diet-particularly the fiber in whole grains—both reduces the risk of developing diabetes and improves insulin control in those who already have diabetes.
Scientific evidence: Research shows that people who eat the most whole grains (three servings daily) can reduce their risk of developing diabetes by 26%, compared with those who eat the least (less than one serving daily).
Helpful: Eat three to four servings daily of whole grains, such as brown rice, whole-grain crackers or bread. Grains that have been processed, such as those used in white rice and white bread, don't have the same benefits.
- Heart disease. Whole grains may promote blood vessel health and are rich in antioxidants, which may inhibit cellular damage. Whole oats and barley are rich in soluble fiber, including beta-glucans that help control levels of LDL cholesterol. The fiber binds to bile acids and pulls cholesterol out of the intestines and bloodstream. It's also thought to lower blood sugar and improve insulin sensitivity (the body's response to insulin)—both of which reduce the risk for heart disease.
Scientific evidence: People who eat whole grains and whole-grain breakfast cereals can reduce their risk for heart disease by 35% or more. Other studies have shown that the fiber in oat cereals can lower blood cholesterol by 5%, which results in about a 10% reduction in heart disease risk.
Helpful: Eat three servings daily of whole grains, such as oats and/or barley.
- Obesity. Fiber not only absorbs water in the stomach but also promotes a feeling of fullness (satiety)-causing people to consume fewer calories overall. High-fiber foods also appear to increase the body's production of cholecystokinin (CCK), a hormone involved in appetite control.
Scientific evidence: Studies at Harvard have shown that people who consume the most fiber are less likely to gain weight than those who get lower amounts. In one study, women with the greatest increase in fiber intake over 12 years had a 49% lower risk for major weight gain than did women with the lowest increase. An eightyear study of men found that for every 40 g per day of whole-grain and cereal fiber consumed, weight gain dropped by about one pound.
Helpful: Eat at least five servings daily of vegetables including beans)...three servings of whole grains (including cereals, bread and brown rice)...and about four servings of fruits.
Can Flaxseeds Prevent Heart Disease?
Claims that flaxseeds can prevent heart disease and cancer are not yet proven. But like other seeds and nuts, flaxseeds contain many healthful components, including lignan, which may have antioxidant properties, as well as fiber. Flaxseed oil contains omega-3 fatty acids, which may lower total cholesterol levels and LDL ("bad") cholesterol.
How to use flaxseeds: Grind flaxseeds in a coffee grinder. If you eat them whole, flaxseeds may pass through your body undigested and you won't benefit as much. Sprinkle ground flaxseeds on hot or cold breakfast cereals...stir a few tablespoons into muffin, cookie, waffle or pancake batter. Or use them to replace eggs in recipes-whip one tablespoon of finely ground flaxseeds with one-quarter cup of water to replace one whole egg.
Caution: Flaxseeds are high in fiber, so if you add flaxseeds to your diet, be sure to drink plenty of fluids to maintain normal bowel function. High doses of flaxseed oil from supplements are not recommended-they have no proven benefit and could interfere with some medications.