What Is Dietary Fiber?

Short answer

Dietary fiber is a type of carbohydrate the body can't fully digest. Under FDA's regulatory definition, it includes both fiber naturally present in plants and certain isolated or synthetic non-digestible carbohydrates that FDA has determined have beneficial physiological effects.

Why it matters

"Fiber" gets used loosely in everyday conversation, but it has a specific legal definition that determines what can actually be labeled as dietary fiber on a US product.

What the evidence says

Per eCFR Title 21, Part 101.9, dietary fiber is defined as non-digestible soluble and insoluble carbohydrates that are intrinsic and intact in plants, along with isolated or synthetic non-digestible carbohydrates FDA has determined have beneficial physiological effects on human health. The FDA's Daily Value for fiber is 28 grams, based on a 2,000-calorie diet.

The golp take

The technical definition matters less day to day than a simpler idea: fiber is the part of food your body doesn't fully break down, and that's exactly why it does what it does.

What to do

No specific action needed, this is a reference definition
See "Soluble vs Insoluble Fiber" for how fiber's two main types function differently
See "How Much Fiber Do You Need Each Day?" for your personal target

Sources

eCFR, Title 21, Part 101.9, Nutrition labeling of food, ecfr.gov
U.S. Food and Drug Administration, How to Understand and Use the Nutrition Facts Label, fda.gov

FAQs

Is all fiber the same?

No, it includes both naturally occurring and certain added forms, and functions differently depending on type (soluble vs insoluble).

Does fiber have calories?

Fiber is generally not fully digested or absorbed, so it contributes fewer calories than most other carbohydrates.

Where is dietary fiber defined legally?

In the U.S., under eCFR Title 21, Part 101.9, which FDA enforces for food labeling.