By Roberta H. Anding, MS, Baylor College of Medicine and Texas Children’s Hospital

Edited by Kate Findley, The Great Courses Daily

Currently, the health benefits of fiber have been trending in nutrition-related news. While fiber can certainly offer benefits, it can also be problematic for those who do not fully understand what it is. Professor Anding explains.

Fiber Considerations for Diabetics

To understand why fiber can be beneficial, but not in every circumstance, it’s helpful to have a working definition of fiber. First, though, Professor Anding provides an illustration of what can happen when people take nutrition advice out of context.

“I had a patient one time who had pretty advanced diabetes, but he read something in the newspaper that said, ‘You know what, if you have diabetes, a great way to control your blood sugar is additional fiber,’” Professor Anding said. “So in his wisdom, he decided he would start taking fiber supplements.” 

Unfortunately, with longstanding diabetes, your stomach becomes sensitive and doesn’t empty effectively. As this man increased his fiber—through a supplement, not through natural foods—the fiber couldn’t leave the stomach and coalesced in a ball, causing the obstruction of the connection of his stomach to his small intestine. That obstruction is called a bezoar. 

“It’s like a hairball in a cat, but it’s a fiber ball in a human,” Professor Anding said.

This example demonstrates why you should try to get your nutrients through natural food rather than taking a supplement.

Definition of Fiber

What exactly is fiber, though? Many definitions of fiber exist, but the Food Nutrition Board suggests that total fiber is the sum of dietary and functional fiber.

Dietary fiber is the non-digestible carbohydrate and lignins, which are the woody portions of the plant that are intrinsic to plant-based foods. What causes celery to stand erect is oftentimes the non-digestible carbohydrate or the lignin. If you’ve eaten broccoli, you know that the stem of broccoli can be unbelievably woody, as can the stem of asparagus.

Functional fiber is also non-digestible carbohydrate, but it offers some unique health benefits for humans. For example, functional fiber could be the gummy or viscous fiber found in oatmeal, and this specific type of fiber is called “beta-glucan.” 

Beta-glucan can lower your cholesterol and support the immune function. Many studies investigate the role of oatmeal, specifically beta-glucan, in supporting the immune function. 

For example, studies have shown that in athletes or anyone who trains vigorously, intense physical activity actually suppresses the immune system. One solution to that immunosuppression can be found in oatmeal in the form of beta-glucan. 

Physical Properties of Fiber

Another definition of fiber looks at the physical properties alone. These are properties that you as a consumer can detect just by looking at the food.

These categories are based on solubility or whether that fiber actually dissolves in water. Water-insoluble fiber includes celluloses and the lignins—again, that woody portion of food. 

You can pull the strings off of a piece of celery, and you’re pulling off some of that water-insoluble fiber. This doesn’t mean that fibrous foods don’t absorb water—they might become a little firmer—but that’s different than becoming soluble in water.

Water-soluble fibers absorb and hold the water. Oatmeal would be an example, along with red beans, white beans, and garbanzo beans. When you cook them in water, they break apart, absorb and hold the water, and become a little gummy. 

Therefore, when defining fiber, we can look at the technical definition—dietary fiber versus functional fiber—or we can look at the properties of that fiber. Is it soluble in water, or is it insoluble? 

To further that definition, fiber is found in complex carbohydrates. Structurally, complex carbohydrates are composed in long chains. 

Complex carbohydrates are found in beans, whole grains, and vegetables. In contrast with simple carbohydrates, they take longer to digest and do not cause your blood sugar to rise as quickly. 

What can be confusing is that there are other things in our diet that are fibrous. You might have had a piece of meat that’s difficult to chew.

Something might have gristle or connective tissues, but that’s not fiber. There might be fibers in those foods, but that doesn’t fit the above definitions of fiber. 

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.