Not All Fiber is Created Equal

By Hayley Philip January 5, 2023 | 6 MIN READ

The Dirt

The typical American, like you or me, is fiber deficient. Fiber is necessary to maintain digestive health, lower cholesterol, and control blood sugar levels. To add more fiber to our diets, many of us seek foods with added fiber but fail to consider its solubility, viscosity and fermentability - the key factors directly impacting our health.


Not All Fiber is Created Equal


Health and Nutrition

By Hayley Philip January 5, 2023 | 6 MIN READ

The Dirt

The typical American, like you or me, is fiber deficient. Fiber is necessary to maintain digestive health, lower cholesterol, and control blood sugar levels. To add more fiber to our diets, many of us seek foods with added fiber but fail to consider its solubility, viscosity and fermentability - the key factors directly impacting our health.

Nana’s homemade sourdough bread, my mid-day protein bar, a whole wheat turkey wrap—all fiber-packed options that are good for me, right?

Until recently, I assumed all fiber was good fiber and that I will take it any way I can get it. But we must consider where we are getting our nutrients from.

Fiber comes in two primary buckets: natural fibers and functional fibers.

Natural fibers naturally occur in foods, like pears and raspberries, versus functional fibers which are derived from a variety of whole and processed foods. 

Though this classification seems to provide a clear-cut differentiation between fiber types, some academics believe we must also focus on three things when considering fiber quality in our diet: solubility, viscosity, and fermentability.

Understanding Solubility, Viscosity, and Fermentability

Solubility is the first important consideration. The term refers to the bioavailability of nutrients in the body. Foods like oats, peas, beans, barley, apples, citrus, and carrots are all great sources of soluble fiber. These fiber nutrients are able to be mixed with water and absorbed by the body. Soluble fibers can aid in digestion and lower blood sugar.

Alternatively, insoluble fibers cannot become available for the body to absorb. Instead, they are considered non-digestible fibers, or more of a bulking agent that simply passes directly through the digestive system. While the FDA has used this as a threshold for including it on the nutritional label (as little to no nutrients can be derived), non-soluble fibers also serve a purpose: they pull water to the colon to help soften stool, thus making it easier to pass.

, Not All Fiber is Created Equal

Viscosity refers to fibers’ ability to thicken when mixed with fluids. This includes polysaccharides like gums, pectins, psylliums, and beta-glucans. Viscous fibers have been credited with many physiological responses such as enhanced feelings of fullness which can aid in weight loss due to appetite control.

Good sources include asparagus, Brussels sprouts, sweet potatoes, barley, oats, oranges, legumes, and mangoes.

Fermentability is precisely as it sounds – the ability to ferment. Fibers that are able to ferment can stimulate the growth and activity of beneficial gut bacteria, called prebiotic fibers. This can improve glycemic control and digestion efficiency, and lower blood cholesterol concentration. Good sources of fermentable fibers are oats, barley, chicory root, leeks, onions, and bananas.

FDA’s updated fiber classification

Fiber has been loosely defined as a group of carbs that humans cannot digest, but does not identify the health effects of the type of fiber. Recently, the FDA changed its framework around what can and cannot be included as fiber on a nutritional label.

Specifically, all naturally-occurring fibers are allowed to be listed; however, only seven out of 27 functional fibers made the cut. This is due to either their solubility, viscosity, and/or fermentability:

, Not All Fiber is Created Equal

The FDA recommends that our total dietary fiber be about 25 grams per day, of which about 6 grams or 25% of our DV should be comprised of soluble fiber. Our nutritional label placed fiber beneath carbohydrates and is noted as Dietary Fiber.

, Not All Fiber is Created EqualRemember that the grams listed next to the nutritional fact are per serving size. Furthermore, the percentage to the right details what percentage of the recommended daily value (DV) each serving size contains.

In the example to the right, there are 6 grams of dietary fiber per ½ cup, which accounts for 24% of your recommended daily value.

In the U.S. products that contain at least 10% DV or over 2.5g of fiber per serving can claim on their packaging that they are a good source of fiber. Foods containing 20% DV or 5g or more can label a product high in fiber.

Fiber and gut-health science

Here is a little bit of science for you to bring the whole picture to life.  Many things impact an individual’s composition of gut microbiota, of which fibers play an important role.

The fiber in the gut provides energy for the microbes to create metabolites, like short-chain fatty acids. These fatty acids are then absorbed into circulation or be utilized as an agent for other microbes to use for digestion.

High dietary fiber consumption is associated with a diverse gut microbiota, which increases microbial activity. Increase microbial activity is directly correlated with a decrease in the prevalence of obesity and other inflammatory diseases. A reduction in inflammatory markers leads to a strengthen and fortified immune system.

Gut-derived short-chain fatty acids have been shown to improve metabolic regulation and insulin sensitivity, regulate weight and reduce inflammation. That said, recent studies support the importance of soluble, fermentable fibers as a priority.

Fermentable fiber specifically can serve as food, that some of your good gut bacteria can feed on, in a process called fermentation, which releases gases like carbon dioxide and hydrogen, which are either released or absorbed into the body. Most especially, the bacteria found in your colon need fiber to function.

, Not All Fiber is Created Equal

Tips to Take Away

While this may seem like a lot to digest, pun intended, here are some easy takeaways that you can start doing today!

  1. Seek 10% whole grains when you can. As you likely discovered when reading over the sources of soluble, fermentable, and viscous fibers is that most whole-grain fibers fall into all three categories. Look for foods like whole barley, oats, brown rice, and whole-wheat products.
  2. Eat fruits and veggies at every meal and as a snack. This is probably the easiest to remember—because fruits and veggies are both high in fiber AND they are packed with other beneficial nutrients. Eating fresh produce every meal will ensure that you are getting in natural fibers.
  3. Gradually increase your fiber, but not all at once. You may be tempted after reading this to go out and pack your diet full of fiber, but be forewarned. Drastic or sudden increases in fiber can cause gastrointestinal issues. Be sure to gradually increase your fiber intake so as not to disrupt your gut microbiome and cause discomfort.
  4. Drink plenty of fluids with your fiber. Fiber draws in water, so without drinking water to accompany our fiber, you risk becoming dehydrated. They have a harmonious relationship, so be sure to include plenty of fluids with your fiber intake.

Whether you are substituting your white bread for a whole wheat wrap, just remember that while not all fibers are created equal, they all serve a purpose. Natural fibers are nutrient dense and are bioavailable for the body to use, while functional fibers can help with digestion.

For your daily recommended value, be sure to consume at least 25g of natural fibers per day—these will be listed on your nutrition labels!

The Bottom Line

In addition to gut health, fiber also improves sleep, skin, heart health, diabetes risk, weight management—the list goes on. Just as you prioritize nutrients like vitamins and minerals, be sure to get your recommended daily fiber intake.