Histamine Digest(Formerly Histamine Block) Histamine Digest by Seeking Health contains a patented enzyme formula providing 10,000 HDU of diamine oxidase (DAO) the main enzyme responsible for the degradation of ingested histamine per capsule. Histamine is known as a biogenic amine, an organic nitrogen compound, involved in certain immune responses and found in mast cells. It may be involved in regulating healthy function in the GI tract as well as the nervous system.