Fiber - Types, Food Sources, Health Benefits, and Whole vs. Refined Grains


Dietary fiber is defined by the Institute of Medicine's Food and Nutrition Board 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.

Cellulose is one type of fiber. The chemical structure of cellulose is shown in the figure below, with our simplified depiction next to it. You can see that cellulose has long chains of glucose, similar to starch, but they're stacked up, and there are hydrogen bonds linking the stacks. The special bonds between these glucose units in fiber are not enzymatically digested in the digestive tract, and therefore, fiber passes undigested to the colon or large intestine.


image shows chemical structure of cellulose, with a total of 16 glucose units arranged in rows of 4 each, with hydrogen bonds linking them vertically, as in a grid.              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 a grid

Figure 6.1 - The chemical structure of cellulose, and a simplified illustration of cellulose.

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.


  1. 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. 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. Consuming a diet high in soluble fiber can also help to lower blood cholesterol levels, because soluble fiber binds cholesterol and bile acids (which contain cholesterol) in the GI tract.  Soluble fiber is also highly fermentable, so it is easily digested by bacteria in the large intestine. 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. (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.)


  1. 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.


Food Sources

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.


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

Figure 6.2 - 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 6.1 Common foods listed with standard portion size, and calories and fiber in a standard portion.

Food

Standard Portion Size

Calories in Standard Portion

Dietary Fiber in Standard Portion (g)

Shredded wheat ready-to-eat cereal (various)

1-1 ¼cup

155-220

5.0-9.0

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

¾ cup

90-98

4.9-5.5

Lentils, cooked

½ cup

115

7.8

Black beans, cooked

½ cup

114

7.5

Refried beans, canned

½ cup

107

4.4

Avocado

½ cup

120

5.0

Pear, raw

1 medium

101

5.5

Pear, dried

¼ cup

118

3.4

Apple, with skin

1 medium

95

4.4

Raspberries

½ cup

32

4.0

Mixed vegetables, cooked from frozen

½ cup

59

4.0

Potato, baked, with skin

1 medium

163

3.6

Pumpkin seeds, whole, roasted

1 ounce

126

5.2

Chia seeds, dried

1 Tbsp

58

4.1

Sunflower seed kernels, dry roasted

1 ounce

165

3.1

Almonds

1 ounce

164

3.5

Plain rye wafer crackers

2 wafers

73

5.0

Bulgur, cooked

½ cup

76

4.1

Popcorn, air-popped

3 cups

93

3.5

Whole wheat spaghetti, cooked

½ cup

87

3.2

Quinoa, cooked

½ cup

111

2.6

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:


Whole vs. Refined Grains

Figure 6.3 - Wheat growing in a field.

 

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.


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.

Figure 6.4 - The anatomy of a wheat kernel which includes the bran, endosperm, and germ.

  1. The bran is the outer skin of the seed. It contains antioxidants, B vitamins and fiber.

  2. 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.

  3. 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.

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


"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.


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.

Figure 6.5 - The nutrient content of refined wheat and enriched wheat as compared to whole wheat flour.


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

 

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 (ex.100%  whole wheat), and it should not be followed by a bunch of refined grains (like enriched wheat flour).

Getting familiar with the name 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 are: 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.


This is an example of a product that is 100% whole wheat, but does not appear to be a good source of fiber, because the serving size is small. The nutrition facts lists 2g of fiber and 9%DV for fiber (for 1 slice or 26g), therefore this whole wheat bread is not a good source of fiber.