DAO enzyme (diamine oxidase)

The main enzyme for breaking down histamine in the gut. When DAO activity is low, histamine from histamine-rich meals is broken down more slowly and can accumulate. The reference range for DAO activity is set out in your findings report.

What diamine oxidase does in the body

Diamine oxidase (DAO) is a copper-containing enzyme produced mainly by the mature cells at the tips of the small intestinal villi; further sites of production are the kidneys and, in pregnancy, the placenta. The gut cells release DAO into the gut contents and into the blood. There it breaks down histamine and related amines such as putrescine and cadaverine before they pass through the gut wall in larger amounts. DAO is therefore the most important barrier against histamine from food and from gut bacteria; histamine inside cells, by contrast, is broken down by another enzyme, histamine N-methyltransferase.

Low DAO – what happens in the body?

When DAO activity is low, histamine from food and from gut bacteria is broken down more slowly. A larger share passes through the gut wall into the blood and acts via the histamine receptors: blood vessels widen (skin flushing, headaches, drop in blood pressure), the nasal lining swells, the skin itches, the heart beats faster, and cramps and diarrhoea increase in the gut. Typically the complaints occur after histamine-rich meals. Because DAO is made at the villus tips, activity falls when the small intestinal lining is damaged – for example with coeliac disease, chronic inflammatory bowel diseases or after gut infections. Alcohol and some medications inhibit the enzyme, other biogenic amines compete with histamine for breakdown, and variants in the DAO gene are associated with lower activity.

High DAO – what does it mean?

High DAO activity shows that the small intestinal lining produces enough enzyme to break down histamine from food and gut flora promptly. In pregnancy the value rises sharply, because the placenta produces large amounts of DAO and so protects the child from histamine in the mother's blood.