Do the new synthetic non-digestible fibers count as dietary fiber? (August 2022)
Carrie Dennett, MPH, RDN
You probably know that dietary fiber is “good for you,” but what is it, exactly? Dietary fiber refers to non-digestible types of carbohydrates found in plant foods like fruits, vegetables, beans and whole grains. Because fiber can’t be broken down and absorbed by the body, it passes through the digestive tract undigested and is mostly intact when reaches the intestines. Most of fiber’s beneficial effects happen in the large intestine (also called the colon).
There are two main types of dietary fiber, insoluble and soluble. Soluble fiber—found in oatmeal, beans and fruits like apples, berries and oranges—is able to absorb water, forming a gel-like substance in your digestive tract that can help lower cholesterol and blood sugar. Insoluble fiber—found in whole grains, nuts, seeds, beans and some vegetables—doesn’t dissolve in water but helps keep things moving through the digestive tract. When can be fermented by the bacteria and other microorganisms in your colon (your gut microbiota), it’s also considered fermentable fiber. Fermentable fiber has benefits for gut health and your immune system, since many of the cells that make up the immune system are in your gut.
Since they’re found naturally in foods, both soluble and insoluble fibers are “intact” fibers. And both are important for health. That’s why it’s recommended that adult females eat 25 grams of fiber per day, and adult males eat 38 grams per day. Because the health benefits of dietary fiber are well established, the Food and Drug Administration (FDA) has approved many fiber-focused health claims. However, the FDA didn’t release an official definition of dietary fiber until 2016. This definition identified two general classes of dietary fiber:
- Naturally occurring fibers that are “intrinsic and intact” in plants
- “Isolated or synthetic non-digestible carbohydrates” that the FDA has determined have “physiological effects that are beneficial to human health”
Note that “intrinsic and intact” fiber comes from foods like those mentioned above, and because the fiber hasn’t been removed from the food, there’s no need to demonstrate that it benefits human health. We know it does. But isolated or synthetic fibers, to be classified as dietary fiber, would need to have at least one demonstrated health benefit, such as lowering blood sugar, cholesterol or blood pressure, relieving constipation, or increasing absorption of minerals in the intestines. That’s why isolated or synthetic fiber that meets these criteria is called functional fiber. In recent years, more and more food manufacturers have been adding isolated functional fiber—either extracted from plants or made in a lab—to foods such as snack bars, yogurt, pasta and cereal. You can also find it in fiber supplements.
Isolated or synthetic non-digestible carbohydrates currently identified by the FDA as meeting the definition of dietary fiber are beta-glucan soluble fiber, psyllium husk, cellulose, guar gum, pectin, locust bean gum and hydroxypropyl methylcellulose. Based on its review of the science, the FDA stated at the end of 2021 that it intends to include mixed plant cell wall fibers (a broad category that includes fibers like sugar cane fiber and apple fiber, among many others), arabinoxylan, alginate, inulin and inulin-type fructans, high amylose starch (resistant starch 2), galactooligosaccharide, polydextrose, resistant maltodextrin/dextrin, cross linked phosphorylated RS4, glucomannan, and acacia (gum arabic).
It’s important to note that not all synthetic fibers are truly synthetic. While some don’t have any plant materials as their starting point, others do—they just undergo a significant amount of processing or modification from their original form. For example, polydextrose is synthesized from glucose, and Hydroxypropyl methylcellulose—used widely as a vegan alternative to gelatin in medicines and supplements, and as a replacement for gluten in gluten-free bread —is synthesized from wood cellulose.
Though the definition of what “counts” as dietary fiber may be evolving, the bottom line is that it is important for health. While isolated and synthetic fibers can play a role, we know for sure that not only does the fiber found in fruits, vegetables, legumes, nuts and seeds, offers health benefits, but that these foods also offer you vitamins, minerals, and other nutrients. It’s a complete package. The bottom line is that a nutritious diet is composed of many different types of foods, which will provide you with different types of fiber. Variety is one element of a “healthy diet,” and a healthy relationship with food.