Fiber is what type of nutrient?

Fiber is classified as a "non-essential substance" thus far, as most dietary fiber is not absorbed by the digestive tract. It is not classifed under any nutrient. It does however, aid in absorbing and digesting other substances.

Debates are ongoing as to whether or not fiber should be classified as a nutrient in and of itself. Perhaps one day, it will be, but it is unlikely.