Hair is mostly keratin which is actually a helical protein.

Is interesting that while we don’t directly digest and breakdown fiber … our gut microbes do and in ferment them into short chain fatty acids which are also important for gut health. Having a masters degree in nutritional sciences I approach the question more in regards from the biochemistry of the molecule. If you revisit your nutrition facts label you will indeed find fiber as a sub component of carbohydrate. Under carbohydrate it lists dietary fiber, total sugars, and added sugars. Some products like cheerios list soluble fiber as well.