Unit 4 – Carbohydrates

4.5 Fiber: Types, Sources, Whole Grains, and Health

Dietary fiber  defined as nondigestible carbohydrates and lignin that are intrinsic and intact in plants.” Fiber plays an important role in giving plants structure and protection, and it also plays an important role in the human diet.

   Figure 4.22. Cellulose structure.

image is a schematic showing multiple green hexagons, each representing a glucose molecule, arranged in rows with lines linking them both vertically and horizontally, as in aCellulose is one type of fiber and is composed of long chains of glucose, similar to starch.  However,  humans cannot digest the special bonds between these glucose units in the digestive tract, and therefore, fiber passes undigested to the colon or large intestine.

You might be wondering how fiber has any benefit to us if we can’t digest it. However, it doesn’t just pass through the digestive tract as a waste product. Instead, it serves many functions on its journey, and these contribute to our health. Let’s explore the different types of fiber, where we find them in foods, and what benefits they provide!

Types of Fiber

Whole plant foods contain many different types of molecules that fit within the definition of fiber. One of the ways that types of fiber are classified is by their solubility in water. Whole plant foods contain a mix of both soluble and insoluble fiber, but some are better sources of one than the other.

Insoluble Fiber – These fibers typically do not dissolve in water and are nonviscous. Some are fermentable by bacteria in the large intestine but to a lesser degree than soluble fibers. Insoluble fibers help prevent constipation, as they create a softer, bulkier stool that is easier to eliminate. Lignin, cellulose, and hemicellulose are common types of insoluble fibers, and food sources include wheat bran, vegetables, fruits, and whole grains.

Soluble Fiber – These fibers dissolve in water, forming a viscous gel in the GI tract, which helps to slow digestion and the absorption of glucose and dietary cholesterol. This means that including soluble fiber in a meal helps to prevent sharp blood sugar spikes, instead making for a more gradual rise in blood glucose, while  lowering blood cholesterol levels. Pectins and gums are common types of soluble fibers, and good food sources include oat bran, barley, nuts, seeds, beans, lentils, peas, and some fruits and vegetables.

NOTE:  Psyllium fiber supplements like Metamucil are composed mainly of soluble fiber, so if you’ve ever stirred a spoonful of this into a glass of water, you’ve seen the viscous consistency characteristic of soluble fiber.

Food Sources of Dietary Fiber

Since fiber provides structure to plants, fiber can be found in all whole plant foods, including whole grains (like oatmeal, barley, rice and wheat), beans, nuts, seeds, and whole fruits and vegetables.

Figure 4.23. Oatmeal topped with blueberries and sunflower seeds.

An example of a fiber packed meal. A bowl of oatmeal topped with blueberries and sunflower seeds.


This meal is packed with fiber from the oatmeal, blueberries, and sunflower seeds.

When foods are refined, parts of the plant are removed, and during this process, fiber and other nutrients are lost. For example, fiber is lost when going from a whole fresh orange to orange juice. A whole orange contains about 3 grams of fiber, whereas a glass of orange juice has little to no fiber. Fiber is also lost when grains are refined. We will discuss this more a little later.

Take a look at the list of foods below to see the variety of foods which provide dietary fiber.

Table 4.4. Common foods listed with standard portion size, calories and fiber 




Grams Fiber

Shredded wheat ready-to-eat cereal (various)

1-1 ¼cup



Wheat bran flakes ready-to-eat cereal (various)

¾ cup



Lentils, cooked

½ cup



Black beans, cooked

½ cup



Refried beans, canned

½ cup




½ cup



Pear, raw

1 medium



Pear, dried

¼ cup



Apple, with skin

1 medium




½ cup



Mixed vegetables, cooked from frozen

½ cup



Potato, baked, with skin

1 medium



Pumpkin seeds, whole, roasted

1 ounce



Chia seeds, dried

1 Tbsp



Sunflower seed kernels, dry roasted

1 ounce




1 ounce



Plain rye wafer crackers

2 wafers



Bulgur, cooked

½ cup



Popcorn, air-popped

3 cups



Whole wheat spaghetti, cooked

½ cup



Quinoa, cooked

½ cup



Although you can get fiber from supplements, whole foods are are a better source, because the fiber comes packaged with other essential nutrients and phytonutrients.

Health Benefits of Dietary Fiber

A high-fiber diet has many benefits, which include:

  • Helps prevent constipation. Many fibers (but mostly insoluble fibers) help provide a softer, bulkier stool which is then easier to eliminate.
  • Helps maintain digestive and bowel health. Dietary fiber promotes digestive health through its role in supporting elimination and fermentation, and it’s positive impact on gut microbiota. Since fiber provides a bulkier stool, this helps keeps the digestive tract muscles toned and strong which can help prevent hemorrhoids and hemorrhoids and diverticulosis.
  • Lowers risk of cardiovascular disease. Higher fiber intake has been shown to improve blood lipids by reducing total cholesterol, triglycerides, and low density cholesterol (“bad cholesterol,” associated with a higher risk of cardiovascular disease), and increasing high density cholesterol (“good cholesterol,” associated with lower risk of cardiovascular disease). Higher fiber intake has also been associated with lower blood pressure and reduced inflammation.
  • Lowers risk of type 2 Diabetes. Higher fiber intake (especially viscous, or soluble fibers) has been shown to slow down glucose digestion and absorption, benefiting glucose metabolism. A higher fiber diet may also decrease diabetes risk by reducing inflammation.
  • Lowers risk of colorectal cancer. More evidence is supporting the idea that higher fiber intake lowers the risk of colorectal cancer, although researchers aren’t sure why. One hypothesis is that dietary fiber decreases transit time (the time it takes for food to move through the digestive tract), thereby exposing the cells of the gastrointestinal tract to carcinogens from food for a shorter time.
  • Helps maintain a healthy body weight. Research has shown a relationship between higher dietary fiber intake and lower body weight. The mechanisms for this are unclear, but perhaps high-fiber foods are more filling and therefore keep people satisfied longer with fewer calories. High-fiber foods also tend to be more nutrient-dense compared to many processed foods, which are more energy-dense.

What Are Whole Grains?

Before they are harvested, all grains are whole grains. They contain the entire seed (or kernel) of the plant. This seed is made up of three edible parts: the bran, the germ, and the endosperm. The seed is also covered by an inedible husk that protects the seed.

Figure 4.24. Wheat growing in a field.

Wheat growing in a field.

Figure 4.25. The anatomy of a wheat kernel which  includes the bran, endosperm, and germ.

The anatomy of a wheat kernel is illustrated showing that the largest part of the wheat kernel is the endosperm located in the middle of the seed. The germ is a smaller part at the bottom of the seed, and the bran is the outer covering.


  • The bran is the outer skin of the seed. It contains antioxidants, B vitamins and fiber.
  • The endosperm is by far the largest part of the seed and provides energy in the form of starch to support reproduction. It also contains protein and small amounts of vitamins and minerals.
  • The germ is the embryo of the seed—the part that can sprout into a new plant. It contains B vitamins, protein, minerals like zinc and magnesium, and healthy fats.




Differences Between Whole and Refined Grains

The Dietary Guidelines for Americans define whole grains and refined grains as the following:

Whole Grains—Grains and grain products made from the entire grain seed, usually called the kernel, which consists of the bran, germ, and endosperm. If the kernel has been cracked, crushed, or flaked, it must retain the same relative proportions of bran, germ, and endosperm as the original grain in order to be called whole grain. Many, but not all, whole grains are also sources of dietary fiber.”

Whole grains include foods like barley, corn (whole cornmeal and popcorn), oats (including oatmeal), rye, and wheat. (For a more complete list of whole grains, check out the Whole Grain Council.)

Refined Grains—Grains and grain products with the bran and germ removed; any grain product that is not a whole-grain product. Many refined grains are low in fiber but enriched with thiamin, riboflavin, niacin, and iron, and fortified with folic acid.”

Refined grains include foods like white rice and white flour. According to the Whole Grain Council, “Refining a grain removes about a quarter of the protein in a grain, and half to two thirds or more of a score of nutrients, leaving the grain a mere shadow of its original self.”

Refined grains are often enriched with vitamins and minerals, meaning that some of the nutrients lost during the refining process are added back in after processing. However, many vitamins and minerals are not added back, and neither are the fiber, protein, and healthy fats found in whole grains. In the chart below you can see the differences in essential nutrients between whole wheat flour, refined wheat flour, and enriched wheat flour.

Figure 4.26. The nutrient content of refined wheat and enriched wheat as compared to whole wheat flour.

This figure is illustrating the nutrient content of refined wheat and enriched wheat as compared to whole wheat flour. Refined wheat flour has 8% of vitamin E, 11% of vitamin B6, 16% of magnesium, 24% of thiamin, 24% of riboflavin, 25% of niacin, 25% of fiber, 29% of potassium, 33% of iron, 59% of folate, and 78% of protein as compared to whole wheat flour which has 100% of all these nutrients. Enriched wheat flour has 156% thiamin, 299% riboflavin, 119% niacin, 129% iron, and 661% folate compared to whole wheat flour which has 100% of all these nutrients.

Because whole grains offer greater nutrient density, MyPlate and the Dietary Guidelines recommend that at least half of our grains are whole grains. Yet current data show that while most Americans are eating enough grains overall, they’re eating too many refined grains and not enough whole grains, as shown in this graphic from the Dietary Guidelines:

This figure shows that average intakes of whole grains are far below recommended levels across all age-sex groups, and average intakes of refined grains are well above recommended limits for most age-sex groups.

Figure 4.27. Average Whole & Refined GrainIntakes in Ounce-Equivalents per Day by Age-Sex Groups, Compared to Ranges of Recommended Daily Intake for Whole Grains & Limits for Refined Grains.

Shopping for Whole Grain Foods

Looking for whole grain products at the grocery store can be tricky, since the front-of-package labeling is about marketing and selling products. Words like “made with whole grain” and “multigrain” on the front of the package make it appear like a product is whole grain, when in fact there may be very few whole grains present.

The color of a bread can be deceiving too. Refined grain products can have added caramel color to make them appear more like whole grains.

To determine if a product is a “good source” of whole grain, the best place to look is the ingredient list on the Nutrition Facts panel. The ingredients should list a whole grain as the first ingredient (e.g., 100% whole wheat), and it should not be followed by a bunch of refined grains such as enriched wheat flour.

Getting familiar with the names of whole grains will help you identify them. Common varieties include wheat, barley, brown rice, buckwheat, corn, rye, oats, and wild rice. Less known varieties include teff, amaranth, millet, quinoa, black rice, black barley, and spelt.


Most, but not all, whole grains are a good source of fiber, and that is one of the benefits of choosing whole grains. Keep in mind that some products add extra fiber as a separate ingredient, like wheat bran, inulin, or cellulose. These boost the grams of fiber on the Nutrition Facts label and may make the product a good source of fiber, but it doesn’t mean it’s a good source of whole grains. In fact, it may be a product made mostly of refined grains, so it would still be missing the other nutrients that come packaged in whole grains and may not have the same health benefits. Therefore, just looking at fiber on the Nutrition Facts label is not a good indicator of whether or not the product is made with whole grains.

Also, some products that are 100% whole wheat but do not appear to be a good source of fiber, because the serving size is small. The bread label below is an example of this. The first ingredient is “stone ground whole wheat flour” with no refined flours listed, but it still has only 2g of fiber and 9% DV. But of course, that still contributes to your fiber intake for the day, and if you made yourself a sandwich with two slices of bread, that would provide 18% of the DV.

For more information about dietary fiber on food labels, go to  USDA;s  Interactive Nutrition Facts Panel.


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Introduction to Nutrition and Wellness Copyright © 2022 by Janet Colson; Sandra Poirier; and Yvonne Dadson is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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