oat bran

Bran, also known as miller's bran, is the component of a cereal grain consisting of the hard layers – the combined aleurone and pericarp – surrounding the endosperm. Corn (maize) bran also includes the pedicel. Along with the germ, it is an integral part of whole grains, and is often produced as a byproduct of milling in the production of refined grains. Bran is highly nutritious, but is difficult to digest due to its high fiber content; its high fat content also reduces its shelf life as the oils/fats are prone to becoming rancid. As such, it is typically removed from whole grain during the refining process – e.g. in processing wheat grain into white flour, or refining brown rice into white rice. — Wikipedia