Fiber is actually calorie-free because it is composed of undigestable carbohydrates. Simply put, fiber is not absorbed or digested (hence, it is not considered a true nutrient).
If you see a food label showing 15 grams of carbohydrates and five grams of fiber in one serving of a given food, those five grams of fiber are not contributing calories and considered separate from the 15 grams.
Although calorie-free, fiber is plenty useful. Once it hits our large intestine, the bacteria present there ferment it to produce short-chain fatty acids, which help lower cholesterol, reduce inflammation of the colon, stabilize blood sugar, and stimulate the production of antibodies and other disease-fighting cells.
This is precisely why fiber is so crucial in helping lower the risk of developing certain diseases. Without it, our bacteria are unable to support our immune systems to their full extent.