With the help of new research, our understanding of all nutrients increases over time. However, it would be hard to find a nutrient that we have learned more about in the past ten years as fiber. Everywhere you look, you will find debates about how the term “fiber” should best be defined, and about which substances should and shouldn’t be classified as types of fiber.

Recommendations For Fiber

Food groups richest in fiber include Beans & Legumes, Vegetables, Fruits, Grains, and Nut & Seeds. Because different proportions of insoluble/soluble fiber, viscous/non-viscous fiber, and fermentable fiber can be found within each of these different food groups.

Public Health Recommendations

In 2005, the National Academy of Sciences (NAS) updated its recommendations for fiber intake. These recommendations appear below.

  • 0—1 year: Not determined
  • 1—3 years: 19 grams
  • 4—8 years: 25 grams
  • 9—13 years, female: 26 grams
  • 9—13 years, male: 31 grams
  • 14—18 years, female: 26 grams
  • 14—18 years, male: 38 grams
  • 19—50 years, female: 25 grams
  • 19—50 years, male: 38 grams
  • 51+ years, female: 21 grams
  • 51+ years, male: 30 years
  • Pregnant women: 28 grams
  • Lactating women: 29 grams

The Daily Value (DV) for fiber is 25 grams per day. The DV is the standard that you will see on food labels.

Key Features of Fiber

First, fiber has always been recognized as a plant-based nutrient. To make a simple comparison, plants have fiber in the same way that animals have muscles and bones. Fiber allows plants to maintain their shape and structure. And even though fungi (including mushrooms) are classified by scientists as belonging to their own separate category of living things, we would want to include them alongside of plants in this fiber discussion since they often contain chitooligosaccharides (CHOS) which most researchers consider a type of fiber.

Second, fiber does not digest in the same way that most other nutrients digest. Most nutrients undergo full digestion as they get chewed, chemically transformed in our stomachs, and then combined with enzymes and digestive fluids in our small intestine. This combination of chewing plus digestive fluids plus digestive enzymes is usually sufficient to allow our bodies to digest and absorb nutrients from food. In the case of fiber, however, the above processes above do not substantially alter the fiber’s basic nature. Fiber passes all the way through our stomach and small intestine and then proceeds on to our large intestine, still largely recognizable as dietary fiber. Our chewing and digestive fluids and enzymes are enough to dramatically change it. It is only in the last portion of our digestive tract—our large intestine—that fiber can undergo a major transformation; if this transformation occurs, it is not brought about by human enzymes or human digestive fluids but rather by trillions of bacteria living in our large intestine.

Most Common Classification Systems for Fiber

Soluble versus Insoluble Fiber. This distinction between soluble and insoluble fiber is probably the most familiar fiber classification system, and it also has the advantage of being the simplest. Fiber can be analyzed to determine how easily it dissolves in water, and fibers with a strong tendency to dissolved get classified as “soluble” while ones that don’t get classified as “insoluble.” In addition, as a very general rule, insoluble fibers tend to provide more benefits in the area of preventing constipation due to increased stool bulk and speeding up the rate of food passage through our digestive tract, while soluble fibers tend to provide better support for blood sugar balance, cardiovascular health, and satisfaction of appetite. However, there are also some important disadvantages to this simple classification system.

  • First, it is possible for fibers to have soluble and insoluble components. In this case, the distinction tends to be less helpful.
  • Second, the vast majority of foods providing fiber not only contain both soluble and insoluble types, but they often contain both in nutritionally significant amounts. So this distinction isn’t always helpful in making food choices.
  • Third, these two categories are often insufficient to account for important health benefits. For example, within the soluble fiber group can be found both viscous and non-viscous soluble fiber. Viscous soluble fibers are gel-forming and much more closely associated with cardiovascular and blood-sugar regulating benefits than non-viscous soluble fibers.

Types of Fiber

  • Non-Starch Polysaccharides (NSP) Resistant Oligosaccharides Resistant Starches Lignin*
  • Celluloses fructo-oligosaccharides (FOS) numerous dextrins
  • Hemicelluloses galacto-oligosaccharides numerous maltodextrins
  • Pectins chito-oligosaccharides (CHOS) some high-amylose starches
  • Hydrocolloids (gums, mucilages, beta-glucans) There is only one basic molecule with a single uniform structure called “lignin,” even though its three basic components (three different cinnamyl alcohols) vary in their proportion.) Our recommendation for obtaining fiber from all five major fiber-containing food groups (Beans & Legumes, Vegetables, Fruits, Grains, and Nut & Seeds) is largely based on distribution of the ESFA fiber types throughout these five groups.

Role in Health Support

Optimal Food Passage Through the Digestive Tract

Between the small and large intestine, food and other waste products need to travel more than 25 feet to get to the end of the journey. This process is controlled by very carefully choreographed muscle movement, which in turn is partly triggered by the presence of food inside of our digestive tract. So, in a real sense, the amount of food inside our intestine partly controls the rate at which our muscles move.

Insoluble fiber plays a unique role in this process. Because it cannot be broken down in the small intestine, and because it has the ability to attract water to it, insoluble fiber can help control the consistency of food in our digestive tract as well as the pace at which it passes through. (To put the result in everyday terms, it can help with “regularity.”) However, viscous soluble fiber also has a role to play in this process, alongside of the role played by insoluble fiber. This second type of fiber helps control the “thickness” of your food as it gets digested. It’s especially important when food is in our stomach because it can help pace and regulate the passage of food out of our stomach and along to our small intestine. This moment in the digestive process is called “gastric emptying,” and soluble, viscous fiber is known to help slow gastric emptying, as well as helping keep it on a steady pace. Since rate of gastric emptying is related to our blood sugar and blood insulin balance, you can see how helpful soluble viscous fiber can be in steadying the amount of food that begins its passage through the intestinal tract at any one moment in time.

Cardiovascular Benefits Including Improved Cholesterol Metabolism

Numerous cardiovascular benefits are associated with fiber intake, but we will start with one of the best-documented benefits: improved control of blood cholesterol levels. Viscous soluble fiber is able to bind with cholesterol in the intestine and prevent its uptake into the body by allowing it to be eliminated in the stool. The most viscous fibers—including the beta-glucans found in barley, oats, sea vegetables, shiitake mushrooms, and other foods, as well as the pectins found in the skins of cherries, grapes, berries, citrus fruits, and other foods—have all been shown to have blood cholesterol-lowering effects. Soluble fibers, and especially viscous soluble fibers, appear to be the best type for blood cholesterol benefits. It’s important to note that in addition to the foods rich in pectins and beta-glucans listed above, there are other foods also provide substantial amounts of soluble fiber, including soluble fiber with varying degrees of viscosity. In this extended list would be included numerous other fruits and vegetables. Choosing fiber-rich fruits and vegetables is usually going to helpful in getting the cholesterol-lowering effects of soluble fiber.

Improved cholesterol levels, however, are not the only cardiovascular benefit provided by food fiber. Blood pressure reduction has also been associated with fiber intake, as has reduced overall risk of high blood pressure (hypertension). Overall risk of cardiovascular diseases—and risk of specific diseases including atherosclerosis and coronary heart disease—has been shown to decrease in association with healthy fiber intake.

Stabilization of Blood Sugar

A number of research teams have been able to demonstrate that the addition of high-fiber foods to a regular meal not already rich in fiber can improve blood sugar control if done on a consistent basis over a period of weeks (and in some cases months). In addition, studies on blood sugar problems in whole populations have shown better blood sugar control when healthy levels of fiber are consumed. In other words, fiber-rich meals can help most of us regulate our blood sugars in a healthier way. Interestingly, some studies also show that the blood sugar benefits of a fiber-rich meal can often extend throughout the day, even after a second or third meal is eaten. As an example, a fiber-rich breakfast might be able to help us steady our blood sugar levels even after eating lunch or dinner.

As mentioned earlier, part of the blood sugar-balancing benefits of fiber come from the special ability of soluble, viscous fiber to impart a slow release of food from the stomach (called gastric emptying). But blood sugar benefits from soluble fiber go even further. High-fiber meals significantly increase production of a hormone called glucogon-like peptide 1 (GLP-1), a hormone known to reduce blood sugar levels. Interestingly, one research group has also suggested that this GLP-1 response gets triggered not directly by the presence of soluble fiber, but by the products of fiber fermentation by bacteria in the large intestine. So you can see how the blood sugar benefits of fiber may involve several different factors. It is also important to note that risk of adult onset, type 2 diabetes and insulin resistance have also been found to decrease with healthy fiber intake.

Maintenance of Colon Health

There are two key ways in which fiber helps to maintain colon health. “Colon” in this context simply means all of our large intestine. Both ways involve the bacteria that live in our large intestine.

A first way that we see fiber helping to maintain colon health is by directly acting as fuel for the growth of “friendly” bacteria. Some of the bacterial species in our gut are so specialized that they can digest specific subtypes of fiber. For example, some species of Bacteroides break down hemicellulose as their primary fuel source, while others in the same class (bacteria that share 96% of their DNA) break down pectins as their main fuel. The reason that bacteria can digest nutrients that humans cannot comes down to their production of very specialized enzymes. In other words, there is a very intimate and mutually supportive relationship between fiber in our diet and populations of bacteria in our large intestine. These bacteria do best when our fiber intake is best, and our colon stays healthiest when these bacterial populations are thriving and in balance.

The second way that fiber helps support colon health is also related to bacterial digestion of fiber. As food fibers are digested in the large intestine by bacteria, their metabolism can provide the short-chain fatty acids (SCFAs) that cells along the large intestinal wall use as a fuel source. SCFAs—and in particular, one SCFA called butyrate—are critical for colon health because they provide cells that form the lining of the colon with the fuel they need to carry out their metabolic activities. In addition to this key support of normal intestinal function, healthy intake of fiber has also been associated with reduced risk of colon cancer.