DAO (diamine oxidase)
A gut enzyme that breaks down dietary histamine, and whose deficiency is put forward to explain the symptoms of histamine intolerance.
DAO (diamine oxidase) is an enzyme produced mainly by the cells of the small intestine. Its role is to break down histamine from food before it enters the bloodstream.
When its activity is insufficient for the amount ingested, histamine builds up and can cause symptoms resembling an allergy without being one: headaches, flushing, nasal congestion, digestive upset, hives. This is what is called histamine intolerance.
It is not an allergy, and the distinction has practical consequences: no IgE antibodies are involved, allergy tests are negative, and the reaction depends on the cumulative dose rather than on sensitisation. A food tolerated in small amounts can trigger symptoms in larger amounts, or when combined with other sources.
Several factors can reduce DAO activity: certain medicines, alcohol, and intestinal inflammation. Diagnosis remains tricky, as blood DAO levels correlate poorly with symptoms, and relies mainly on supervised elimination followed by reintroduction. The foods richest in histamine are fermented, matured or not-so-fresh products: mature cheeses, cured meats, wine and poorly stored fish.