Fiber

What can high-fiber foods do for you?
  • Support bowel regularity
  • Help maintain normal cholesterol levels
  • Help maintain normal blood sugar levels
  • Help keep unwanted pounds off
What events can indicate a need for more high fiber foods?
  • Constipation
  • Hemorrhoids if related to straining from constipation
  • High blood sugar levels
  • High cholesterol levels
What is dietary fiber?

Dietary fiber is undoubtedly one of the most talked about nutrients for health promotion and disease prevention. In fact, dietary fiber is the focus of two FDA-approved health claims that appear on foods labels touting the benefits of high fiber foods for the prevention of heart disease and certain types of cáncer. Since the early 1950’s, when the term “fiber” first began to be used in scientific journals, there has been considerable controversy among food scientists, nutritionists, and medical experts about the exact definition of dietary fiber. In fact, even the United States Food and Drug Administration, the federal agency responsible for overseeing food labeling, has no formal, written definition of dietary fiber. For food labeling purposes and the determination of health claims, the FDA has adopted the analytical methods that the Association of Official Analytical Chemists uses for defining dietary fiber. Although most experts agree that a key defining characteristic of dietary fiber is that it’s derived from the edible parts of plants that are not broken down by human digestive enzymes, many people believe that this definition is too ambiguous and that a more clear, internationally-accepted definition is needed to ensure that the total fiber counts on food labels are consistent and accurate. In recent years there has been a movement among various organizations to include the physiological benefits of dietary fiber in a new definition. For example, the American Association of Cereal Chemists proposed a new definition of dietary fiber that includes the statement “Dietary fibers promote beneficial physiological effects including laxation and/or blood cholesterol attenuation and/or blood glucose attenuation”. In addition, the Institute of Medicine at the National Academy of Sciences (the organization responsible for issuing Recommended Dietary Allowances) has proposed a new definition that differentiates between dietary fiber and added fiber. According to this definition, dietary fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants. Added fiber, which refers to fiber that is added to foods during food processing, consists of isolated nondigestible carbohydrates that have proven beneficial physiological effects in humans. For food labeling purposes, the Institute of Medicine defines Total Fiber as the sum of Dietary Fiber and Added Fiber. Despite the controversy surrounding the exact definition of dietary fiber, experts agree on one important thing—dietary fiber is an important weapon in the fight against heart disease, colon cancer, diabetes, and obesity.

What is the function of dietary fiber?

Until very recently, the functions of a specific type of fiber were determined by whether or not the fiber was classified as soluble or insoluble. Soluble fibers, such as the type found in oat bran, are known to reduce blood cholesterol levels and normalize blood sugar levels. On the other hand, insoluble fiber, such as the type found in wheat bran, are known to promote bowel regularity. Many commonly used plant sources of fiber contain both soluble and insoluble fibers. Psyllium husks, for example, contain a mixture of 70% soluble and 30% insoluble fibers. Despite the widespread use of the terms “soluble” and “insoluble” to describe the health benefits of dietary fiber, many medical and nutrition experts contend that these terms do not adequately describe the physiological effects of all the different types of fiber. These experts are now proposing the use of the terms “viscous” and “fermentability” in place of soluble and insoluble to describe the functions and health benefits of dietary fiber.

Reducing Cholesterol Levels

Like soluble fibers, viscous fibers lower serum cholesterol by reducing the absorption of dietary cholesterol. In addition, viscous fibers complex with bile acids, which are compounds manufactured by the liver from cholesterol that are necessary for the proper digestion of fat. After complexing with bile acids, the compounds are removed from circulation and do not make it back to the liver. As a result, the liver must use additional cholesterol to manufacture new bile acids. Bile acids are necessary for normal digestion of fat. Soluble fiber may also reduce the amount of cholesterol manufactured by the liver.

Normalizing Blood Sugar Levels

Viscous fibers also help normalize blood glucose levels by slowing the rate at which food leaves the stomach and by delaying the absorption of glucose following a meal. Viscous fibers also increase insulin sensitivity. As a result, high intake of viscous fibers play a role in the prevention and treatment of type 2 diabetes. In addition, by slowing the rate at which food leaves the stomach, viscous fibers promote a sense of satiety, or fullness, after a meal, which helps to prevent overeating and weight gain.

Promoting Bowel Regularity

Certain types of fiber are referred to as fermentable fibers because they are fermented by the “friendly” bacteria that live in the large intestine. The fermentation of dietary fiber in the large intestine produces a short-chain fatty acid called butyric acid, which serves as the primary fuel for the cells of the large intestine and helps maintain the health and integrity of the colon. Two other short-chain fatty acids produced during fermentation, propionic and acetic acid are used as fuel by the cells of the liver and muscles. In addition, propionic acid may be responsible, at least in part, for the cholesterol-lowering properties of fiber. In animal studies, propionic acid has been shown to inhibit HMG-CoA reductase, an enzyme involved in the production of cholesterol by the liver. By lowering the activity of this enzyme, blood cholesterol levels may be lowered. In addition, fermentable fibers help maintain healthy populations of friendly bacteria. In addition to producing necessary short-chain fatty acids, these bacteria play an important role in the immune system by preventing pathogenic (disease-causing) bacteria from surviving in the intestinal tract. As is the case with insoluble fiber, fibers that are not fermentable in the large intestine help maintain bowel regularity by increasing the bulk of the feces and decreasing the transit time of fecal matter through the intestines. Bowel regularity is associated with a decreased risk for colon cancer and hemorrhoids (when the hemorrhoids are related to straining and constipation).

What are deficiency symptoms for dietary fiber?

There is no identifiable, isolated deficiency disease caused by lack of fiber in the diet. However, research clearly indicates that low intake of dietary fiber (less than 20 grams per day) over the course of a lifetime is associated with development of numerous health problems including constipation, hemorrhoids, colon cancer:disease, obesity and elevated cholesterol levels.

What are toxicity symptoms for dietary fiber?

Intake of dietary fiber in excess of 50 grams per day may cause an intestinal obstruction in susceptible individuals. In most individuals, however, this amount of fiber will improve (rather than compromise) bowel health. Excessive intake of fiber can also cause a fluid imbalance, leading to dehydration. Individuals who decide to suddenly double or triple their fiber intake are often advised to double or triple their water intake for this reason. In addition, excessive intake of nonfermentable fiber, typically in supplemental form, may lead to mineral deficiencies by reducing the absorption or increasing the excretion of minerals, especially when mineral intake is too low or when mineral needs are increased such as during pregnancy, lactation, or adolescence.

How do other nutrients interact with dietary fiber?

Foods high in nonfermentable fiber, or the fiber that passes all the way through the intestines unchanged, may reduce the absorption and/or increase the excretion of several minerals, including calcium and iron.

What health conditions require special emphasis on dietary fiber?

A diet high in fiber may play a role in the prevention and/or treatment of the following health conditions:

  • Breast cancer
  • Cardiovascular disease
  • Colon cancer
  • Constipation
  • Diabetes
  • Diverticulitis
  • Gallstones
  • High cholesterol
  • Irritable bowel syndrome
  • Obesity
  • Syndrome X
What are current public health recommendations for dietary fiber?

In its most recent 2005 public health recommendations for fiber (published as the Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients), National Academies Press, 2005), the National Academy of Sciences established an Adequate Intake (AI) level of 38 grams of total daily fiber for males 19-50 years of age and 25 grams for women in this same age range. It also noted that individuals in this age range in the United States only get about half this much fiber each day.

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