Back to Basics: Dietary Fiber

This post is part of a blog series called “Back to Basics.” This series aims to combat the confusion exacerbated by flashy headlines and social media over what constitutes a nutritious diet by providing simple, sound nutritional science. It emphasizes that the concepts of a nutritious diet haven’t changed dramatically over the years but have simply been fine-tuned.

back to basics fiber

“Make at least half of your grains whole.”

This phrase is one of the basic components of the Dietary Guidelines for Americans, which are scientifically-based dietary recommendations released every five years as a partnership between the USDA and HHS. If you’re anything like me, you’ve been familiar with this phrase from learning about the food pyramid in elementary school, reading it on posters in school cafeterias, and seeing it promoted on food packages. But what exactly do we mean when we advise people to make half of their grains whole? And why do we make this recommendation? Read on to find out!

The Anatomy of Grains

Grains consist of an outer husk (the bran) and an inner endosperm, which contains the germ. The germ is the embryo, and the endosperm is the germ’s food supply. These components contain different nutritional qualities.

  • Bran: fiber, iron, magnesium, copper, phosphorus, potassium, selenium, zinc, B vitamins
  • Endosperm: starch, proteins, small amount of vitamins and minerals
  • Germ: unsaturated fat, B vitamins (especially thiamin!), vitamin E, phosphorus, magnesium, zinc

Whole grains contain the entire grain — bran, endosperm, and germ. Refined grains are processed to remove the bran and germ, leaving only the endosperm. Thus, refining grains greatly decreases their vitamin, mineral, and fiber content and increases their caloric density. Refined flours are usually enriched with riboflavin, niacin, thiamin, iron, and folate; however, all other nutrients remain at lower levels. The Whole Grains Council created two great graphics that show how much key nutrients are reduced when whole wheat and brown rice are refined.

A Focus on Fiber

As you can gather from the previous paragraph, whole grain consumption is encouraged because it provides more vitamins, minerals, and fiber. While the loss of vitamins and minerals is an important consequence to consider in making the decision to consume whole grains over refined, the fiber is an especially important component. The vitamins and minerals in whole grains are found in a variety of other foods, but the fiber is specific to just carbohydrate foods. Fiber plays an important role in carbohydrate digestion and absorption and is associated with a variety of positive health outcomes such as preventing or reducing the severity of conditions including diabetes, heart disease, cancer, obesity, and diverticulosis. 

What is fiber?

According to the FDA, fiber is “non-digestible soluble and insoluble carbohydrates (with three or more monomeric units) and lignin that are intrinsic and intact in plants.” In a nutshell — fiber is a non-digestible component of plant foods such as grains, nuts, seeds, fruits, and vegetables.

In addition to the naturally occurring fiber in plant foods, there are also functional fibers. These are defined by the FDA as “isolated or synthetic non-digestible carbohydrates (with three or more monomeric units) determined by the FDA to have physiological effects that are beneficial to human health.” These are the fibers that are extracted from plant foods and added to other foods such as granola bars, cereals, baked goods, jams, jellies, sauces, and frozen foods. The FDA recently changed its definition of functional fiber, thus excluding various sources that food manufacturers currently include as fiber on the food label (read more about this here). The functional fibers that still meet the new FDA definition are:

  • Beta-glucan soluble fiber
  • Psyllium husk
  • Cellulose
  • Guar gum
  • Pectin
  • Locust bean gum
  • Hydroxypropylmethylcellulose

Whatever the source, fiber consumption is encouraged due to its many  beneficial physiological effects. Before diving its qualities and benefits, first is a brief review of carbohydrate digestion and metabolism.

Carbohydrate Digestion & Metabolism

After eating carbohydrates, enzymes produced by the mouth and pancreas break the bonds holding large carbohydrate molecules together. The end result is the three simple sugars: glucose, fructose, and galactose. Since these three molecules have different structures, they are metabolized slightly differently. However, the end result of carbohydrate digestion and absorption is a rise in blood glucose, which stimulates the pancreas to release insulin.

Insulin travels through the blood and aids in the transfer of blood glucose into body cells. If blood glucose rises rapidly, insulin levels will also rise rapidly. One consequence of this is that the glucose will enter and be metabolized by the body cells quickly, leading to a rapid decrease in blood glucose. This leads to an energy crash, which many people experience after eating a sugary treat or a meal high in refined carbohydrates. Since the body cells (especially the brain cells) prefer glucose as an energy source, the quick drop in blood glucose will trigger a cascade of events to increase blood glucose again. Often, this manifests as hunger and a craving for carbohydrate-rich foods. Thus, the goal is to keep glucose and insulin levels steady.

There are many factors that contribute to how quickly carbohydrates are absorbed and utilized. When consumed as part of a mixed meal, the carbohydrates will not be absorbed as rapidly as when they are consumed alone; fat is especially effective at preventing rapid rises in blood glucose. Additionally, the cooking method can alter how a carbohydrate is absorbed and utilized. Another great way to prevent the rapid absorption and utilization of carbohydrates is to choose sources that are high in fiber, as explained in the next section.

Soluble Fiber Functions

As the term “soluble” implies, this kind of fiber absorbs liquid in the gastrointestinal tract and swells, forming a gel. This exerts two main effects on food transport and absorption.

Starting in the stomach, carbohydrates containing soluble fiber will swell into a larger bolus than those lacking in this substance. Consequently, the larger bolus of food takes more time to empty from the stomach into the small intestine, which allows for a longer-lasting feeling of satiety. This decreases the probability of overeating or grazing later in the day.

In addition to slowing gastric emptying, the gel formed by the soluble fiber can delay or inhibit the absorption of glucose in the intestines. Since the glucose is bound by the gel, the digestive enzymes have a more difficult time reaching them. Thus, the glucose is released into the blood at a steadier rate. This slower rise in blood glucose isn’t as overwhelming to the pancreas as a spike in blood glucose; the pancreas will release the insulin at a slower rate, thus allowing for a steadier uptake of glucose into the body cells.

Along with promoting steadier blood glucose levels, soluble fiber can also lower cholesterol levels. It exerts this effect by binding to bile acids. Bile acids are synthesized from cholesterol in the liver and released by the gallbladder into the small intestine to aid in the absorption of fats. The soluble fiber binds to the bile acids, which are then excreted in the feces. The liver must then compensate by converting cholesterol to bile acids, consequently lowering cholesterol levels.

Insoluble Fiber Functions

Insoluble fiber differs from soluble fiber in that it does not absorb water in the gastrointestinal tract. Thus, instead of slowing down intestinal transit, insoluble fiber speeds up transit by promoting intestinal motility. This helps prevent constipation by moving things along in the intestines, reducing the risk of developing conditions such as diverticulosis.

Intestinal Degradation/Fermentation

Ever heard of this thing called the gut microbiome? Our intestines, especially our colon, house a large number of bacteria that exert various effects on our body. Research thus far has found that the gut microbiome can play a role in obesity, diabetes, mental health, cancer, and a variety of other conditions; thus, we want to promote having more healthy bacteria in our colon. One nutrient that can help with this is fiber. In addition to promoting the growth of more good bacteria, the fermentation of fiber by colonic bacteria produces short chain fatty acids, which promote the health of the colon by:

  •      Acidifying the lumen (internal space) or the colon, thus preventing the growth of harmful bacteria
  •      Increasing colonic sodium and water absorption
  •      Inhibiting tumor formation & abnormal cell growth
  •      Stimulating mucosal cell proliferation
  •      Improving colonic blood flow
  •      Enhancing immune function (over 50% of the body’s lymphocytes and immune cells are found in the gastrointestinal tract!)

Keep an eye out for more research regarding the many positive benefits of a healthy gut microbiome! Meanwhile…eat some fiber. 🙂

How Much Fiber is Enough?

Currently, Americans ages 2 and older only consume about 16 g of fiber per day, much lower than the current Adequate Intake (AI). The AI is the amount determined to be sufficient for the general population based on observed or experimental intakes. These levels are as follows:

<50 years

  •      Males: 38 g
  •      Females: 25 g

≥50 years

  •      Males: 30 g
  •      Females: 21 g

Sources of Soluble Fiber

Pulses are a prime example of a soluble fiber-rich food. This food group, consisting of beans, legumes, and peas, reduce glucose levels both after the meal in which they are consumed, as well as after the subsequent meal. This phenomenon has become known as the “second meal effect.”

Other great sources of soluble fiber include:

  • Whole grains (especially oats & barley)
  • Fruits & vegetables (especially carrots, apples, bananas, strawberries, & citrus fruits)
  • Nuts & seeds

Sources of Insoluble Fiber

  • Seeds and skins of fruits
  • Whole grains
  • Leafy greens
  • Celery
  • Onions, leeks, scallions, garlic
  • Broccoli & cauliflower

The Takeaway

  •      Make at least half of your grains whole grains.
  •      Eat at least five servings of fruits and vegetables per day.
  •      Try adding some beans & legumes to your diet (perhaps even going meatless for a day). 

Published by Talia Follador

Registered Dietitian Nutritionist (RDN) with a passion for holistic health, intuitive eating, psychology, and cooking. Proud Penn State alumnus, spiritual explorer, nutrition geek, baseball junkie. I like to have my cake and eat it, too . :)

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