An Introduction to DAO Deficiency
Author: Histamine Dietitians
Tolerance Residents: January to March 2025
Diamine oxidase deficiency (DAO deficiency) is often one of the main precursors to developing a histamine intolerance.
Some studies estimate that over 10% of the population may have at least a mild DAO deficiency – however many people remain unaware of it.
What does DAO deficiency mean?
Diamine oxidase (DAO) is an enzyme found mostly in the gut, and its job is to break down histamines from the food and drink we consume.
DAO deficiency is where the body lacks sufficient DAO enzyme activity, leading to impaired breakdown of histamine. When this happens, excess histamine can pass through the gut and into our bloodstream.
This can cause a 'histamine overload' throughout our bodies and can trigger certain symptoms like headaches, digestive issues and skin reactions. Diamine oxidase deficiency is therefore a leading cause of histamine intolerance.
Diagnosing DAO Deficiency
Like histamine intolerance, it can also often be difficult to identify DAO deficiency. This is due to a range of factors, including the lack of standardised testing. Some approaches include:
● Blood tests - these tests look at the DAO activity in a blood sample at a given point in time, however most are inaccurate due to DAO levels fluctuating and can be influenced by a host of external factors.
● Genetic testing - for some people, DAO deficiency can be genetic. If this is the case, then a genetic test can be far more accurate than a blood test, however it will not capture a range of potential, non-genetic causes of DAO deficiency such as poor gut health.
An expert dietitian will be able to recommend whether a test might be appropriate, as well as explore other methods like histamine elimination diets, lifestyle changes and DAO supplementation.
Managing DAO Deficiency
There are a range of strategies that nutritional experts can employ to help their patients manage DAO deficiency and any associated reactions.
These include:
● Dietary adjustments - avoiding foods that are high in histamine, in particular those that your body specifically reacts to. Everyone experiences DAO deficiency uniquely, one of the reasons that getting to the ‘root cause’ can be difficult.
● DAO supplementation - taking a DAO supplement prior to consuming high-histamine food or drink can help some people minimise reactions.
● Medication - for some patients, antihistamine medication can block histamine receptors to prevent internally produced, histamine release. This is not the case for all patients however, see a useful article we found on hayfever and antihistamines here.
Drastic dietary changes or long-term removal of certain foods comes with longer-term risks if the body is no longer getting enough essential vitamins and minerals. Low-histamine diets should be explored as a short-term option only during the diagnosis phase, and done so under the supervision of an expert dietitian.
Next Steps
DAO deficiency can for many people be one of the causes of adverse reactions to food and drink, but it is not the only one.
If you think you have a DAO deficiency, you can try the Tolerance Questionnaire, and get in touch with an expert dietitian who can help you better understand this condition and find the right way forward for you.
If you want to know more about how DAO deficiency is linked to histamine intolerance, then you can get instant access to the Histamine Dietitian's' free masterclass here.