In the last twenty years or so, a significant number of studies have been done on the value of dietary fiber; exactly what it is and does for the human body. Fiber, which used to be simply dismissed as that part of the diet the body did not digest (and therefore was only useful for dealing with such problems as constipation and diarrhea), has taken on whole new aspects. Instead, we now know that several types of dietary fiber exist that all provide a variety of vital functions for healthy living, including appearing to regulate cholesterol and blood glucose levels, help with weight control, and reduce cancer risks.
Roughly speaking—and we do mean “roughly”— dietary fiber is what your grandmother used to refer to as “roughage,” that part of all plant foods that the human body does not fully digest and that provides the bulk eliminated from the body in the stool. But recent studies have distinguished various kinds of dietary fibers, leaving the single designation “dietary fiber” in a state of flux.
Currently, for the purpose of labeling food products in the United States, dietary fiber is defined as “the material isolated by analytical methods approved by the Association of Official Analytical Chemists (AOAC). However, food scientists have developed a variety of low molecular weight carbohydrates (sugar alcohols such as sorbitol and mannitol, polydextrose, and various fructo- and galactooligosaccharides) that are not digested by human digestive enzymes, but are digested by friendly bacteria in the human digestive tract, and are not measured by the AOAC’s methods. Processed foods increasingly use these carbohydrates because people demand satisfaction for their sweet tooth without adding calories to their diet.
The Institute of Medicine at the National Academy of Sciences has proposed a new definition that differentiates between dietary fiber and added fiber. This definition describes dietary fiber as nondigestible carbohydrates and lignin that are intrinsic and intact in plants and added fiber as isolated nondigestible carbohydrates added to foods during food processing that have proven beneficial physiological effects in humans. The Institute of Medicine would define total fiber as the sum of dietary fiber and added fiber.
By changing the definition of dietary fiber, these groups want to recognize the physiological actions of fiber and its effects on health and reduce the emphasis on simply quantifying total dietary fiber. They hope to reinforce the idea that more than one kind of dietary fiber exists, and that adequate human nutrition requires fiber from a variety of plant sources each day, not just a single fiber supplement that matches the required number of fiber grams recommended.
FIBER RECOMMENDATIONS
Today, the Food and Nutrition Board of the National Academy of Sciences recommends that adults consume from 20 to 35 grams of fiber per day. Some health professionals recommend that dietary fiber intake should be based on energy intake—10 to 13 grams of dietary fiber per 1,000 calories consumed. American nutrition facts on food labels use 25 grams of dietary fiber per day for a 2,000 calorie-per-day diet or 30 grams per day for a 2,500 calorie-per-day diet.
In much of the industrialized world, however, people do not get as much fiber as recommended. In the United States, for instance, the average adult gets only 14 to 15 grams of fiber per day, even though 73% of these people say their fiber intake is “about right.”
This could be due to consumption of too many processed foods. While many foods contain more than five grams of fiber in their whole, unprocessed form, most or all of this fiber is often lost when processed. For example, the process used to produce most bread flours in the United States retains only 60% of the original wheat grain in the flour. The remaining 40%, which includes the bran and the germ, is discarded. The bran and the germ, however, contain virtually all of wheat’s fiber. As a result, 60% extraction wheat flour (white flour) contains almost no fiber, while whole wheat flours contain an ample amount. Other examples can include fruit juices and vegetable juices which may start out high in fiber in their whole, unprocessed state but end up with virtually no fiber after processing.
TOO MUCH OF A GOOD THING
While an adequate amount of fiber is a dietary necessity, excessive amounts of fiber—generally more than 50 grams daily—can promote constipation, diarrhea, or spastic bowel disorder, especially if you don’t drink enough water. Providing your body with enough water is essential because fiber attracts and absorbs water as it progresses through the digestive tract.
Although fiber can bind minerals and prevent their absorption by the body, this property does not appear to interfere with mineral balances because most sources of fiber provide enough minerals to compensate for any binding that occurs. In addition, beneficial bacteria in the digestive tract feed on some types of fiber, aiding the release of necessary minerals, such as calcium, which can be absorbed in the colon.
Increasing your fiber intake too quickly may cause gastrointestinal problems, such as cramps, bloating, and flatulence, because your body is not used to handling that much fiber. To reduce the amount of GI distress, increase fiber intake slowly, and be sure to increase your fluids as well, since fiber absorbs large amounts of water. Your body will adjust to the increase in fiber, and the GI problems will subside as you continue to eat the fiber you need.
TYPES OF FIBER
Dietary fiber can be separated into two basic types: soluble and insoluble. Soluble fibers, which include gums, pectins, and mucilages, dissolve and thicken in water to form a gel. Because it slows the passage of food through the digestive system making you feel full longer, this type of fiber may help with weight loss. Research also indicates that this type of fiber may help lower blood cholesterol and regulate blood glucose levels by affecting the rate at which nutrients are absorbed. Good sources of soluble fiber include dried beans and legumes, psyllium, oats, barley, citrus fruits, apples, and bananas.
Insoluble fibers include cellulose, hemicellulose, and lignin, which are found in the woody or structural parts of plants, such as fruit and vegetable skins, wheat bran, and whole grains. This type of fiber tends to speed up the passage of material through the digestive tract and may reduce the risk of colon cancer, as well as diverticular disease. Good sources of insoluble fiber include apples, pears, bananas, and whole grains.
Polysaccharides are also fibers. They contain many glucose units combined into a long complex of several hundred to several thousand sugar (glucose) molecules. Most natural carbohydrates occur as polysaccharides, such as starches, glycogen, and cellulose—the complex carbohydrates. The polysaccharides found in mushrooms have been reported by scientists to lower blood pressure, cholesterol, and blood sugar concentrations, and have shown anti-tumor activity.
Both types of fiber—soluble and insoluble—are beneficial to the body. Nature usually provides whole plant foods complete with both types of fiber, which is why you will see apples, bananas, several of the whole grains and other foods listed as good sources of both soluble and insoluble fibers. The body needs a healthy balance of both. Each provides a different set of benefits for continued health.
HOW FIBER WORKS
Scientists want to know exactly how dietary fiber is used by the body to maintain health. Their research so far has indicated two aspects of fiber that appear to have significant impact on health: viscosity4 and fermentation.
Viscosity
Viscosity refers to soluble fiber and the gel it forms when dissolved in water. Different types of soluble fibers have different degrees of viscosity, and there appears to be some correlation between the degree of viscosity and the benefit derived from the fiber—the more viscous the fiber, the more benefit to the body
A daily intake of more than three grams of soluble fiber, along with a total fiber intake of more than 25 grams, appears to result in
- Reduction in blood cholesterol levels;
- Stimulation of appetite regulating hormones for an increased feeling of fullness;
- Stabilization of blood sugar and insulin levels; and
- Protection against gastrointestinal diseases such as diverticulitis.
Fermentation
Insoluble fibers, like cellulose, hemicellulose, and lignins, combine with soluble fibers like pectin to make up plant cell walls. These compounds do not
break down easily; otherwise, the plants would not be able to survive.
Animals need simple sugars for energy, but they cannot break the complex plant fiber compounds into simple sugars. However, microbes living
in their digestive tracts can. The digestive systems of ruminants, like cattle, sheep and goats, act like fermentation vats where microbes break down complex plant fibers into simple sugars and fatty acids, which the animal absorbs and uses for energy. Human digestive systems aren’t that good; however, with our own healthy intestinal microbes, we can use the fiber from whole grains, fruits, and vegetables to provide important health
benefits.
While intestinal microbes may ferment both soluble and insoluble fiber, soluble fiber usually has a higher rate of fermentation. These
microorganisms also metabolize potentially carcinogenic nitrogenous wastes in the fermentation process that otherwise would accumulate in the colon. Fermentation produces propionate, butyrate, and acetate, which are
absorbed by cells in the colon. The body uses butyrate as a fuel source for colon cells, while propionate is transported to skeletal muscles, and
acetate is transported to the liver where it helps inhibit the synthesis of cholesterol.
Dietary fibers also bind bile acids, which ferment in the colon to short-chain fatty acids. Cells in the colon absorb these fatty acids, which help in the
absorption of sodium and water and improve the body’s fluid balance. These fatty acids also lower colonic pH and inhibit the conversion of primary bile acids to secondary bile acids, which are thought to be carcinogenic.
BENEFITS OF FIBER
The United States Food and Drug Administration believes that a diet low in saturated fat and cholesterol and high in fruits, vegetables, and grain products containing fiber, particularly soluble fiber, reduces the risk of coronary heart disease. In addition, recent research shows a possible correlation between dietary fiber intake and colorectal cancers and diabetes.
However, studies also suggest that whole foods offer more protection against chronic diseases than dietary fiber, antioxidants, or other biologically active components alone. Associations between dietary fiber and disease may reflect a synergy between dietary fiber and other substances in the whole foods, which suggests that adding purified dietary fiber may be less beneficial than changing your diet to include whole foods. A varied diet of whole foods takes advantage of this possible synergy to improve overall health.
The Cardiovascular System
Studies seem to indicate that dietary fiber’s primary benefit to the cardiovascular system is its ability to decrease serum cholesterol and LDL cholesterol concentrations. Fibers with the greatest effects on blood cholesterol are highly viscous, soluble fibers, such as oats and buckwheat. Oats have been associated with a lower body mass index (BMI), lower blood pressure, and lower serum cholesterol. Lower serum cholesterol, LDL cholesterol, and a higher ratio of HDL to total cholesterol have been associated with buckwheat. Psyllium has shown the largest cholesterol lowering effect, decreasing serum cholesterol by 32% and liver cholesterol by 52% in one study.
Secondary benefits may come from the lower fat and simple sugar contents of fiber-rich foods, which help alleviate obesity and high triglyceride levels in the blood. In addition, antioxidants found in the bran and germ of some cereal grains may also have health benefits.
Cancers
The current belief is that dietary fiber and dietary fat are the most significantly associated dietary factors in colorectal cancer. However, since the initial studies in the 1990s that seemed to show this correlation, further research has led many to question the protective effect of dietary fiber on colorectal cancer. The protective effects may not be based solely on the balance of fiber and fat in the overall diet, but also on the interaction of specific dietary components, such as the type of fiber, associated nutrients, and other food interactions.
In 73% of recent studies, as total dietary fiber intake increased, the incidence of colorectal cancers decreased. When the total dietary fiber intake was broken down, fruit fiber was found to be protective in 77% of the studies, vegetable fiber in 66% of the studies, and grain fiber in 36% of the studies. Therefore, these studies seem to indicate that increasing fruit fiber, vegetable fiber, and total dietary fiber appears to protect against the development of colorectal cancers.
Fruit, vegetable, and grain fibers probably work together with other substances in fruits and vegetables to affect such things as bacterial flora, bile acid composition, pH, and fecal bulk to help protect against cancer.
Diabetes Mellitus
The mechanisms by which fiber may affect insulin requirements or insulin sensitivity are not clear, but dietary fiber, particularly grain fiber, appears
to decrease the risk for developing type II diabetes. In particular, eating fiber with a higher degree of fermentability—such as the soluble fiber in oats—may reduce gastric emptying rates, promote glucose uptake, enhance insulin-dependent glucose disposal, inhibit the body’s secretion of glucagons6
, and reduce the liver’s output of glucose in both animals and humans.
Considerable experimental evidence indicates that soluble fibers slow gastric emptying rates, digestion and the absorption of glucose to help immediate
glucose metabolism after eating, and long-term glucose control for those with diabetes. In studies, eating high amounts of dietary fiber per day for
24 weeks significantly improved glycemic control and reduced the number of times people with type I diabetes experienced low blood sugar. Among
pregnant women with type I diabetes, a higher fiber intake lowered daily insulin requirements. Other studies with type II diabetics also suggest
that high fiber intake diminishes the demand for insulin.
Multiple effects of fiber on the digestive tract may reduce the amount of external insulin required by individuals with either type I or type II diabetes
when consuming a high fiber diet.
TAKING FIBER OUT OF FOOD
Two basic problems exist with taking fiber out of food. In the first place, overly refined foods remove most of the fiber before the food is eaten. White flour is a good example. As we have mentioned, bread made from white flour contains significantly less fiber than bread made from whole wheat flour, which includes the germ and bran—the highest concentrations of fiber in grains. So, even though you may be eating grain products, you may not be getting the fiber you think you are.
The second problem with taking fiber out of food comes from isolating fiber to create fiber supplements. Although fiber-rich foods seem to offer significant protection from a variety of diseases, these effects may be minimized when purified fibers are used. For example, studies show that guar gum, a dietary fiber obtained from the Indian cluster bean, has significant cholesterol lowering effects, but that hydrolyzed7 guar gum lacks these effects.8 This seems to indicate that extracting fibers for use as supplements may alter their effectiveness.
Therefore, to get the most nutritional benefit from your food, choose less refined food products that contain more of the nutritional elements the body needs, including fiber. Many nutrients in foods are only available to the body through the actions of fiber in the system. When fiber is processed out of the food, the body often cannot effectively use the nutrients that remain or have been added.
Obviously, science has more to learn about the interactions of dietary fiber, other nutrients, digestive processes, and disease pathology. What is evident, however, is that a diet rich in a variety of whole foods provides the necessary vitamins, minerals, phytonutrients, and fiber the body needs for optimal health and wellness.
An Apple a Day
When it comes to apples, fresh is better, say two Cornell University food researchers who have added scientific weight to that old saying: An apple a day keeps the doctor away. Writing in the journal Nature, Rui Hai Liu and Chang Yong “Cy” Lee report that trace chemicals in apples help protect against cancer. Liu, an assistant professor of food science, used extracts from 25 varieties of apples to suppress liver and colon cancer cells in laboratory tissue samples. An extract from apple skins, the most nutritionally dense part of the fruit, inhibited colon cancer cells by 43 percent and liver cancer cells by 57 percent. Fuji, Spartan and Red Delicious performed better than other varieties in these tests. Lee said Americans should eat about five times as many apples as the 19.7 pounds a year they now consume—which comes out to…. You guessed it! An apple a day! The Cornell researchers found that just one average apple (about 150 grams or 5 ounces) has more antioxidant activity than 1,500 milligrams of Vitamin C. Antioxidants neutralize, the tissue damaging molecules that may contribute to cancer.
FIVE REASONS TO EAT AN APPLE A DAY:
- Diet - Apples taste great, boost energy, and are fat free—and designed for portability.
- Heart - The antioxidants found in apples help fight the damaging effects of LDL cholesterol.
- Digestion - Just one apple provides as much dietary fiber as a serving of cereal— or about one-eighth of the recommended daily intake of fiber!
- Lungs - An apple a day strengthens lung function and may lower the incidence of lung cancer.
- Bones - Apples contain boron, an essential trace element that has been shown to strengthen bones—a good defense against osteoporosis.
Oats—A Great Nutritional Value!
If you are looking for a food with great nutritional impact, you can’t beat oats. A prime source of complex carbohydrates for sustained energy, they also contain about 50% more protein than bulgur and twice as much as brown rice; impressive levels of selenium, thiamin, phosphorus, manganese, magnesium and iron; and respectable quantities of calcium, copper, folic acid, vitamin E, and zinc. Plus, of all the cereal grains and pseudo-grains, only buckwheat, barley and common millet contain more antioxidants than oats.
But it is the high soluble fiber content of oats that has captured the attention of many nutritionists in recent years. A cup of cooked oatmeal provides 4 grams of fiber—16% of the total amount of fiber recommended each day. About half of that is soluble fiber, which has been shown to help lower blood cholesterol levels. Bolstering the claims to lower cholesterol, oats also contain tocotrienols, light yellow, fat-soluble, viscous oils in the vitamin E family. Tocotrienols inhibit cholesterol synthesis and have been found to lower serum cholesterol.
Oats also contain more unsaturated fatty acids than other grains, especially the essential fatty acid linoleic acid. The body uses linoleic acid to make prostaglandins, hormone-like chemicals with a wide range of effects, such as regulating inflammation, stimulating uterine contractions during labor, and protecting the lining of the stomach. To make it even better, these fatty acids are distributed throughout the oat grain, not concentrated in the germ, as in other grains.
Processing removes the hulls from oats, leaving oat groats, which are then milled into steel-cut oats, rolled flakes, quick and instant flakes, oat flour, and oat bran. Only about 10% of the breakfast cereal market is hot cereals, but two thirds of that 10 percent are oatmeal-based. Instant oatmeal accounts for over one-half of the market and continues to increase. Ready-to-eat cereals often contain added oat bran, however; and many popular cold cereals use oat flour. Oats, by far, are the most popular grain for breakfast cereals—hot or cold. Breads, muffins, and the ever-popular oatmeal-raisin cookies also take advantage of nourishing oat flour, bran and flakes. Who knew that cookies could be good for you?
Oats are a wonderful source of the fiber you need daily, and in addition, they supply trace minerals, protein, and antioxidants that the science of Nutritional Immunology has shown are necessary for the health and strength of the immune system. For a mighty nutritional punch in a mighty small package, you can’t beat oats!