Ah, Roughage

You really can't say enough great things about fiber. What can fiber do for you? Lower cholesterol; control body weight; improve blood sugar; prevent constipation, diarrhea, hemorrhoids, diverticulosis, gallstones, irritable bowel syndrome, syndrome X, and colon and rectal cancer; and improve immunity and liver detoxification.

What we call fiber, plants call structure. Fiber is comprised of carbohydrate and lignin that are resistant to human digestion. The recommended daily intake of fiber is 25 grams for women and 38 grams for men. The average American eats a meager 10-15 grams of fiber a day. In contrast, hunter-gatherer populations elsewhere in the world who eat and live now as they did thousands of years ago, eat as much as 100 grams of fiber a day. There are estimates that if Americans ate 13 grams extra of fiber per day (about 25 grams total), it would decrease their risk of colorectal cancer by about one third!

Location, Location, Location

It comes as no great surprise that fiber naturally occurs in large amounts in healthy foods. Animals do not produce fiber, so we can only get it by eating plant foods. Fiber is abundant in fruits, veggies, whole grains and beans. So when you choose high fiber foods, you are also choosing nutrient dense, low-calorie foods that improve your health beyond just the effects of the fiber.

Mind the Gap
If you are going to consciously increase your fiber intake, do it slowly and drink lots of water! Your intestines need some time to adapt to the changes in stool bulk and speed so you don't get constipated or diarrhea. Soluble fiber will bind up a lot of water, therefore you need to drink a lot of water to prevent becoming constipated dehydrated.

The Details
Although the definition of fiber is not entirely agreed upon, it is often defined as: a combination of cellulose, hemicelluloses, and lignins (not technically carbohydrates) that are resistant to digestion. Cellulose and hemicellulose are plant polysaccharides (complex carbohydrates) that make up the plant cell wall. This is where the vast majority of fiber is found.

The major types of fiber are found in the following foods:

• Cellulose - bran, beans, peas, root vegetables, cabbage family of vegetables, outer layer of seeds, and apples
• Hemicellulose - bran and whole grains
• Lignin - root vegetables, whole wheat, fruits with edible seeds (such as strawberries)
• Gums - oatmeal, barley, and beans.
• Pectins - apples, strawberries, and citrus fruits

You'll notice in the list above, which is not exhaustive, that multiple types of fiber are found in the same foods, e.g. apples are good sources of both pectin and cellulose. This is because plants are mixtures of different types of fiber, and the amounts of each type can even vary based on the part of the plant and its age.

Different types of fibers are further categorized by their physical qualities to differentiate between their effects on the body: soluble, insoluble, fermentable or viscous. Again, the major types of fiber have various combinations of these physical qualities, sometimes contradictory. However, sometimes one type is dominant and the food is known for that property, such as prunes and stool softening.

The major types of fiber primarily have the following qualities:

• Cellulose - insoluble and non-fermentable
• Hemicellulose - some are soluble and viscous, some are insoluble
• Lignin - insoluble and non-fermentable
• Gum - soluble, fermentable and viscous
• Pectin - soluble, fermentable and viscous

Generally, soluble and viscous fibers slow down the foods transit through the digestive system, while at the same time physically blocking the absorption of some nutrients, including glucose and fats. They also increase the sensation of fullness. They also physically trap enzymes responsible for the digestion. Insoluble fibers bulk the stool and speed up the foods transit through the digestive system. Sped up transit also decreases absorption of some nutrients simply because they lack the time to come into contact with the appropriate part of the intestines.

Below, the types of fiber are organized by their health-promoting properties:

1. Lower Cholesterol: Soluble and Viscous Fiber
   
   
   • Soluble and viscous fiber bind to the cholesterol you have eaten, therefore reducing its absorption in the intestines.
   • Viscous fibers bind to bile acids, which are made by the liver from cholesterol and are necessary for fat digestion. The bound cholesterol is excreted in the feces. As a result, the liver must use additional body stores of cholesterol to manufacture new bile acids.


2. Stable Blood Sugar Levels: Viscous Fiber
   
   • Viscous fiber slows the rate at which food leaves the stomach, which delays the absorption of sugars after a meal. This effect also creates a feeling of fullness, which helps to prevent overeating and weight gain.
   
   
   • The improved insulin sensitivity caused by viscous fiber is useful in the prevention and treatment of type 2 diabetes.


3. Protect the Intestines: Fermentable and Non-fermentable Fibers
   • Fermentable fiber provides food for the "friendly" bacteria that live in the intestines (see my previous posting on friendly intestinal bacteria). A strong population of friendly bacteria out-competes pathogenic bacteria for residence in the intestines. This also improves detoxification since these good bacteria bind toxins present in the intestines until they are excreted. Fibers also bind to cancer-causing toxins which prevents them from coming into contact with the intestinal cells.
   
   
   • The fermentation of fiber produces a short-chain fatty acid called butyric acid. This is the main fuel for the cells of the large intestine, therefore the presence of butyric acid helps keep the colon healthy and intact.


4. Regularity: Insoluble and Non-fermentable Fibers
   
   • Insoluble and non-fermentable fibers increases the bulk of the feces.
   • They decrease the transit time of feces through the intestines.
   • Having regular bowel movements is associated with a decreased risk for colon cancer and hemorrhoids.


5. Nutrient Absorption
   Unfortunately, most fibers decrease somewhat the absorption of nutrients we want to get from our food, in addition to the nutrients we want to avoid like fat, cholesterol, and sugar.
   
   • There is a watery viscous layer on top of our intestinal cells which nutrients must pass through to be absorbed. Eating lots of viscous fiber is believed to increase the thickness of this layer.
   • Soluble and viscous fibers physically block the absorption of some nutrients by binding to them. They also physically trap enzymes responsible for digestion.
   • Insoluble fibers speed up food's transit through the digestive system, which decreases absorption of some nutrients simply because they lack the time to come into contact with the appropriate part of the intestines.
   • Gums likely decrease the undulating muscle movement of the intestines, which is how nutrients are brought to the intestinal cells in order to be absorbed. The nutrients may miss their optimal absorption locations.
   • Excessive intake of non-fermentable fiber, typically in supplemental form, may lead to mineral deficiencies.


6. Fermentable Fibers
   Some fibers increase absorption of desired nutrients. Fermentable fibers produce short-chain fatty acids which acidify the intestinal environment, thereby improving calcium binding and absorption.

Bottom line, since fiber rich foods are richer in needed nutrients, their decreased absorption is not a problem and more than compensated by fibers inhibition on undesirable substances like excess fat and intestinal toxins.

Conclusion - the Magic Bullet
Rarely in nutrition would we call something a "magic bullet." However, healthy digestion and appropriate passage of food through the intestines is critical to good nutrition. The importance of fiber to the prevention and treatment of the major conditions affecting Americans today (heart disease, diabetes, and cancer), and its integral role in most digestive diseases and symptoms, make it is easy to see why we nutritionists idolize fiber. So do your health a favor and eat a variety of fibers by eating whole plant foods.

~Raven Bonnar-Pizzorno, MS (Nutr), RD candidate

References:

• American Dietetic Association. Health implications of dietary fiber - - Position of the ADA. Journal of the American Dietetic Association 1997; 97:1157-1159.
• Burton-Freeman B. Dietary fiber and energy regulation. J Nutr 2000 Feb;130(2S Suppl):272S-5S. PMID:15360.
• Davy BM and Melby CL. The effect of fiber-rich carbohydrates on features of Syndrome X. J Am Diet Assoc 2003 Jan;103(1):86-96 2003.
• Fernandez ML. Soluble fiber and nondigestible carbohydrate effects on plasma lipids and cardiovascular risk. Curr Opin Lipidol 2001 Feb;12(1):35-40 2001.
• Groff JL, Gropper SS. Advanced Nutrition and Human Metabolism. Wadsworth Thomson Learning, New York, 2000.
• Institute of Medicine. Dietary Reference Intakes: Proposed Definition of Dietary Fiber. National Academy Press, Washington DC, 2001.
• Mahan K, Escott-Stump S. Krause's Food, Nutrition, and Diet Therapy. Saunders; Philadelphia, 2004.
• McIntosh M, Miller C. A diet containing food rich in soluble and insoluble fiber improves glycemic control and reduces hyperlipidemia among patients with type 2 diabetes mellitus. Nutr Rev 2001 Feb;59(2):52-5 2001.
• Pereira MA, Pins JJ. Dietary fiber and cardiovascular disease: experimental and epidemiologic advances. Curr Atheroscler Rep 2000 Nov;2(6):494-502. PMID:15350.
• Swanson KS, Fahey GC. New developments in the area of dietary fiber. Nutrition in Complementary Care Newsletter 2001; 4(1):5,12.
  

Related Topics:

• 4 Warning Signs Your Diet May Lack Fiber
• 6 Foods and Tips for More Fiber

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