Diamine Oxidase Enzyme (DAO)

Low DAO may lead to increased blood histamine levels.

DAO is an enzyme found in the blood and the digestive system.

LocationWhat does it do?Consequence of reduced activity
Digestive SystemBreaks down histamine in the digestive system (from food or that is released).The excess digestive histamine will enter the blood stream.
BloodBreaks down histamine circulating in the blood.Histamine is not broken down, so it continues to build-up (see below).

Normal Diamine Oxidase Enzyme maintains blood histamine levels


Reduced Diamine Oxidase Enzyme may allow histamine to increase in the blood


Many factors have been suggested to reduce DAO activity. However, there is no research to support these claims.

The potential factors to reduce DAO activity include:

  • Genetics: some people may be genetically programmed to produce less DAO enzyme.
  • Intestinal damage: Damage or inflammation to the inner lining of the digestive system may decrease DAO (digestive DAO is in the lining).  However, if this were true, people with undiagnosed celiac disease (which causes significant inflammation in the digestive system) would experience histamine related symptoms. However, these symptoms are no more common in undiagnosed celiac disease than the general population. This theory still makes sense and may occasionally account for low DAO, but is not a given fact.
  • Potential DAO blocking factors, including:
    • Certain medication: There are long lists of “DAO blocking” medications on the internet. Similar to problematic dietary lists, the medication lists are primarily based on anecdotal reports (i.e., people assume that problematic medications are DAO blockers). However, there has only been one research study that has investigated this topic. The study demonstrated that some medications inhibit DAO in a test tube.  Further studies are needed to see how this applies to real life situations. Also, only a handful of medications were studied. The effect of other medications is not known. The right medication can be more important than potential DAO inhibition. For example, one client attributed his symptoms to an antibiotic that he received in the hospital. However, that was the best antibiotic to cure his infection. Without it, he may have died.
    • Alcohol: Most clients report significantly worse symptoms with alcohol intake. There is no direct evidence, but alcohol is thought to inhibit diamine oxidase.
    • Foods with elevated diamine content:  Histamine is the focus, but many other diamines form in food alongside histamine (see Related Diamine Compounds in Histamine in Food). The other diamines are also broken down by diamine oxidase enzyme (DAO) – reducing histamine breakdown.
    • High body histamine level: Chronic histamine excess in the body (e.g., allergies, stress, mast cell disease, etc.) may reduce DAO production. Lowering body histamine may increase DAO production, which possibly explains why some clients find benefit from a low histamine diet, but can gradually return to their usual diet after about four weeks.

There are no validated tests for diamine oxidase status.

Tests that measure the activity of DAO in the blood are available in Europe (to my knowledge there are no labs outside of Europe that offer this test). The results from the few research studies are interesting but are not enough to establish it as a validated test. Additionally, it is unknown if blood DAO activity correlates with digestive levels (and digestive levels is the foundation of the histamine intolerance theory). In the United States, some labs offer DAO testing, but this is a different technique than the European tests, and there is no research to support them.

Diamine oxidase enzyme supplements are available in tablet form.

This supplement helps break down histamine in the digestive system, reducing absorption into the blood stream.


DAO supplementation has been shown to improve histamine associated symptoms in a few clinical trials, but more research is needed. It is difficult to predict which clients will benefit, so the best thing is to try it (without making any other treatment changes).


Take the DAO supplement about 15 minutes before a meal. The capsule needs time to breakdown in the stomach so the enzyme can mix with the food. Finish your meal within 15 minutes (and don’t snack afterwards!) The number of tablets depends on several factors and everyone is different. It takes some experimentation.


Sciotec (an Austrian company) produces the diamine oxidase supplement and sells it under different brand names throughout the world.

Europe: The brand name is DAOsin.

Canada/United States: Several brands that were previously available in the United States have been discontinued. Diamine oxidase will be distributed directly by a Sciotec JV partner under the brand name Umbrellux DAO  with the typical 10,000 HDU per capsule. Other supplements that either feature DAO in the product name or claim to contain DAO but don’t show any HDU are misleading consumers.


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