What is Fiber?
Fiber is a plant substance that is indigestible and passes through the digestive tract without being broken down or absorbed. Fiber is able to tolerate the high acidity of the stomach. Fiber resists digestion by intestinal and pancreatic enzymes. Fiber passes through the small intestines unabsorbed and reaches the large intestines mostly intact. Fiber is fermented by the bacteria that colonize the large intestines. Fiber is fermented by bacteria to short-chain fatty acids which are mainly used as fuel by the bacteria and for the cells that line the large intestines.
Recommended Intake of Fiber
The recommendation is:
- 30-38g/day for men
- 21-25g/day for women
Insoluble vs Soluble Fiber
Most foods have both soluble and insoluble fiber in them. Even oatmeal and oat bran are about half soluble and half insoluble fiber. The types of fibers will create different effects in foods and in addition will have different physiology effects. For examples, let’s looks at soluble fiber in oatmeal versus insoluble fiber in brown rice
Oatmeal will turn into a gel and the individual grains bind together when water and head are added. This is characteristic of a fiber that is soluble. It is soluble in water, and water binds the grain together. Oatmeal markets it’s product based on studies concluding that soluble fiber helps reduce the risk of heart disease. The theory is that fiber binds to bile (made from cholesterol) and traps it in the intestines. It is then excreted and not reabsorbed, reducing total blood cholesterol. If cholesterol is reduced, the risk of heart disease is reduced. This is the theory for why soluble fiber decreases heart disease risk.
Brown rice is a good example of an insoluble fiber is brown rice. You can add water, cook it for a few hours and it will not form a gel, the grains stay separate. It is not soluble in water. But, notice the grains become larger as they absorb water. This is characteristic of insoluble fiber, you can think of it as trapping and holding water. The main effect in the body is to add weight to the stool and speed the passage of stool.
Brown rice is a good example of an insoluble fiber is brown rice. You can add water, cook it for a few hours and it will not form a gel, the grains stay separate. It is not soluble in water. But, notice the grains become larger as they absorb water. This is characteristic of insoluble fiber, you can think of it as trapping and holding water. The main effect in the body is to add weight to the stool and speed the passage of stool.
Soluble Fiber
Function |
Health Benefit |
slows absorption of glucose |
decreases the risk of diabetes |
lowers blood cholesterol |
decrease risk of heart disease |
slows transit of food in small intestines |
assists with weight management |
Insoluble Fiber
Function |
Health Benefit |
increases fecal weight |
alleviate constipation, lower risk of diverticulosis |
speed passage in colon |
decreases risk of hemorrhoids and appendicitis |
Provides bulk and a feeling fullness (satiety) |
assists with weight management |
Good Source and High in Fiber
A good source is 10-19% and high in is 20% of more of a nutrient per serving. Fiber has specific definitions that are easier to remember.
- Good source of fiber is 2.5-4.9 grams per serving
- High in fiber is 5 grams of fiber or more per serving
Whole Grain Versus Refined Grain
Whole grains appear similar to how they occurred in nature. Refined grains have gone through steps of processing. For example, brown rice is a whole grain and wheat flour is a refined grain. During the refining process fiber is lost and nutrients are lost.
In the image to the right, a wheat kernel is illustrated with the endosperm, bran and germ labeled. The endosperm is mostly starch, although it contains some fiber protein and iron. The bran contains most of the fiber, in addition it contains protein and fat and iron. The germ has most of the protein and micronutrients, although it also contains some fat and fiber.
During the refining of wheat, the husk which is the toxic outer shell, the bran and the germ are removed. This leaves the endosperm, which is mostly starch. All wheat products, except wheat berries which are a whole grain, are refined. All of them.
Whole grains will have the husk removed, but otherwise the kernel remains intact, which retains the fiber in the bran and the micronutrients and protein in the germ.
“Wheat bread” is unbleached, refined wheat flour (no bran or germ). Most flour comes from the wheat plant.
It is common to add molasses to bread to darken it and make it appear healthier. Multi-grain describes products that include more than one grain, this does not mean it is a whole grain. It's common for a manufacturer to sprinkle a few types of seeds on top of a loaf of bread and voila, it’s now Multi-grain bread! Fiber additives, such as cellulose, added to food do not function the same way naturally occurring fiber does in a food.
In the image to the right, a wheat kernel is illustrated with the endosperm, bran and germ labeled. The endosperm is mostly starch, although it contains some fiber protein and iron. The bran contains most of the fiber, in addition it contains protein and fat and iron. The germ has most of the protein and micronutrients, although it also contains some fat and fiber.
During the refining of wheat, the husk which is the toxic outer shell, the bran and the germ are removed. This leaves the endosperm, which is mostly starch. All wheat products, except wheat berries which are a whole grain, are refined. All of them.
Whole grains will have the husk removed, but otherwise the kernel remains intact, which retains the fiber in the bran and the micronutrients and protein in the germ.
“Wheat bread” is unbleached, refined wheat flour (no bran or germ). Most flour comes from the wheat plant.
It is common to add molasses to bread to darken it and make it appear healthier. Multi-grain describes products that include more than one grain, this does not mean it is a whole grain. It's common for a manufacturer to sprinkle a few types of seeds on top of a loaf of bread and voila, it’s now Multi-grain bread! Fiber additives, such as cellulose, added to food do not function the same way naturally occurring fiber does in a food.
Fiber Additives
Chickory Root Fiber in Double Fiber Bread
Cellulose is one type of insoluble fiber. It is part of the cell walls of all plants and gives plants their structure. When you consume whole fruits or vegetables or grains, you will be consuming naturally occurring cellulose. Cellulose is also a food additive, generally recognized as safe by the FDA. It has many functions.
Cellulose traps moisture, preventing processed food products from tasting dry. Where does cellulose come from? Wood. Yep, you are eating wood pulp.In 1979, an article in the Milwaukee Sentinel revealed the manufacturers secret to giving White Wonder Bread 300% more fiber: enriched white flour, water and added cellulose (aka: wood pulp). Today, as you can see by looking at the ingredient list in the image on the right, bread is much more complex! Although there is no evidence that eating wood is harmful, there is also no evidence that consuming added cellulose (extra fiber as wood) in breads, muffins or other baked goods increases the nutritional value or improves health. It's likely the added cellulose does nothing more than make the product "appear" healthier, and give some individuals gastrointestinal distress and provide a laxative effect.
General Rule: anytime a food manufacturer takes something which occurs in nature out of it's original state, refines it and adds it into packaged food, it's not healthy.
In the image above, you can see that there are two types of "added fiber" in the Double Fiber Bread, in addition to a number of other food additives. Note that sugar is the 3rd ingredient. The reason this food product is "high in" fiber is because of the additive cellulose fiber and inulin, not because of the fiber naturally occurring in the wheat used to make the product.
Too Much Fiber?
More is not always better. Too much fiber can cause dehydration and inhibit mineral absorption. This mainly occurs with fiber additives in processed foods. It's recommended to consume adequate fluids with a high fiber diet.
- The smaller fiber provides a smooth consistency and stickiness to dressings, dips and ice cream.
- The longer fiber provides a firm texture, particularly to breads and other processed carbohydrate baked goods.
Cellulose traps moisture, preventing processed food products from tasting dry. Where does cellulose come from? Wood. Yep, you are eating wood pulp.In 1979, an article in the Milwaukee Sentinel revealed the manufacturers secret to giving White Wonder Bread 300% more fiber: enriched white flour, water and added cellulose (aka: wood pulp). Today, as you can see by looking at the ingredient list in the image on the right, bread is much more complex! Although there is no evidence that eating wood is harmful, there is also no evidence that consuming added cellulose (extra fiber as wood) in breads, muffins or other baked goods increases the nutritional value or improves health. It's likely the added cellulose does nothing more than make the product "appear" healthier, and give some individuals gastrointestinal distress and provide a laxative effect.
General Rule: anytime a food manufacturer takes something which occurs in nature out of it's original state, refines it and adds it into packaged food, it's not healthy.
In the image above, you can see that there are two types of "added fiber" in the Double Fiber Bread, in addition to a number of other food additives. Note that sugar is the 3rd ingredient. The reason this food product is "high in" fiber is because of the additive cellulose fiber and inulin, not because of the fiber naturally occurring in the wheat used to make the product.
Too Much Fiber?
More is not always better. Too much fiber can cause dehydration and inhibit mineral absorption. This mainly occurs with fiber additives in processed foods. It's recommended to consume adequate fluids with a high fiber diet.
References
- Brody, T. Nutrients That Resist or Escape Digestion. In: Nutritional Biochemistry. 2nd ed. San Diego, CA: Academic Press. 1999.
- Savin J. Fiber and Prebiotics: Mechanisms and Health Benefits. Nutrients. 2013 Apr; 5(4): 1417–1435.
- Levine J. High Fiber Bread Boasts Miracle Ingredient: Wood. The Milwaukee Sentinel - Mar 17, 1979.
- Select Committee on GRAS Substances (SCOGS) Opinion: Cellulose, Microcrystalline cellulose. Federal Drug Administration. Page Last Updated October 16, 2015. Last accessed April 1, 2016. http://www.fda.gov/Food/IngredientsPackagingLabeling/GRAS/SCOGS/ucm261287.htm
- Wheat kernel by Jkwchui on Wikipedia, CC BY-SA 3.0.
- Cooked brown rice by jacqueline on FlickrCC, CC BY-NC 2.0.
- Irish Steel Cut Oats by regan76 on FlickrCC, CC BY 2.0.
- Orowheat® bread image is from orowheat.com.
Last updated April 9, 2016.