DAO Deficiency

Diamine oxidase (DAO) is the main enzyme that breaks down dietary histamine in the small intestine. DAO deficiency means that DAO activity in the gut is too low to clear histamine efficiently, so histamine from food can build up and trigger histamine intolerance (HIT) symptoms.

In most people with DAO deficiency, the problem is reduced enzyme activity or expression, not complete lack of DAO.¹

Low DAO activity is associated with histamine intolerance (HIT), which can cause symptoms in body systems where histamine receptors are located: gastrointestinal, skin, respiratory, cardiovascular, and nervous system.

Symptoms include:

Gastrointestinal 

• Abdominal pain and intestinal colic
• Bloating and distension
• Diarrhea and constipation
• Nausea and belching
• Vomiting
• Post-meal fullness

Neurological/central nervous system

• Headache and migraine
• Vertigo and dizziness

Dermatological 

• Urticaria (hives)
• Pruritus (itching)
• Flushing/redness
• Swollen/reddened eyelids 

Respiratory 

• Rhinorrhea (runny nose)
• Nasal congestion
• Sneezing
• Asthma-like symptoms (bronchoconstriction)

Cardiovascular 

• Palpitations 
• Hypotension (low blood pressure) 
• Arrhythmia/tachycardia

DAO deficiency can be primary (mainly genetic) or secondary (due to gastrointestinal disorders, medications, dysbiosis, or nutrient deficiencies.)

• AOC1 mutation
  Genetic mutations of the AOC1 gene (which encodes the DAO enzyme) are one of the main known causes of low DAO activity. Studies in selected patient groups have found that a high proportion (often above 70%) carry at least one AOC1 variant associated with reduced DAO activity. Several single nucleotide polymorphisms (SNPs) are more common in people with reduced DAO activity and HIT symptoms.²
• Gastrointestinal inflammation
  Basal DAO levels correlate with the integrity of the small‑intestinal mucosa. Many laboratories consider serum DAO concentrations between about 15 and 50 U/mL as a normal range.³
  Inflammatory gut disorders such as celiac disease, inflammatory bowel disease (IBD), and lactose intolerance can damage the intestinal villi, especially the villous tips where DAO‑producing enterocytes are concentrated. This structural damage reduces DAO expression and activity and impairs histamine breakdown in the gut. The degree of epithelial damage broadly correlates with the reduction in DAO activity.⁴
• Dysbiosis
  Gut microbiota imbalances (dysbiosis) have been reported in people with histamine intolerance. Studies show lower levels of beneficial bacteria and higher abundance of histamine‑producing species such as Staphylococcus, Proteus, Enterobacteriaceae, Clostridium perfringens, and Enterococcus faecalis.
  Increased numbers of histamine‑producing bacteria can contribute to histamine accumulation in the gut and may worsen symptoms, even in people without primary DAO deficiency.⁵
• Medications
  Medications that may impair DAO activity include antibiotics, antimalarials, H2 receptor antagonists, antihypertensives, analgesics, mucolytics, antidepressants, antiemetics, and muscle relaxants.⁶ The effect depends on the specific drug and dose, so any concerns should be discussed with a healthcare professional. 
• Nutrient deficiencies
  DAO’s enzymatic activity depends on cofactors including copper ions, calcium ions, and topaquinone (TPQ), which are involved in certain reactions and also support enzyme stability. At present, human evidence directly linking copper deficiency to clinically relevant DAO deficiency is limited.Some research suggests that functional vitamin B6 status may influence DAO efficiency, as vitamin B6 is a cofactor in many amine‑oxidase‑related reactions. However, more studies are needed to confirm whether B6 deficiency significantly impairs DAO function.⁷

DAO blood tests are available, but results can vary over time and between laboratories, so they should not be used as a definitive diagnostic test.

DAO testing is available at some clinics and laboratories, often as part of a histamine intolerance panel. Home test kits can also be purchased online.  

One common method is a quantitative enzyme‑linked immunosorbent assay (ELISA) that reports histamine‑degrading units (HDU) per mL. Reference ranges can vary, but some consider 80 HDU/mLas reduced DAO activity.⁸

A radioimmunoassay uses a different unit (U/mL), with values under about 12.5 U/mL considered reduced DAO.⁹

Some labs also test for genetic variants of the DAO gene using a DNA test from white blood cells.

There is no scientific evidence that DAO deficiency causes weight gain. Clinical studies show that DAO deficiency and histamine intolerance are mostly linked to migraines, gastrointestinal symptoms, skin reactions, fibromyalgia, and sleep disorders, not body weight.

Some online sources speculate that histamine or DAO may be linked to weight gain, but this has not been demonstrated in research.

A low‑histamine diet is considered the first‑line approach for managing histamine intolerance and DAO deficiency. It usually has two phases:

• Elimination phase: Avoid foods that are high in histamine or promote histamine release for a limited period (usually 2-4 weeks)
• Re‑introduction phase: Gradually reintroduce foods one by one while tracking symptoms to identify individual triggers.

This is best done with the help of a healthcare professional to avoid unnecessary long‑term restriction and nutrient deficiencies.

DAO supplements may help support the breakdown of ingested histamine in people with low intestinal DAO activity.

Safety evaluations by the European Food Safety Authority (EFSA) suggest dosages of up to 0.3 mg DAO three times daily (0.9 mg/day total) taken before meals. Supplements should be gastro‑resistant so that the enzyme reaches the small intestine intact. 

Research has shown that DAO supplementation before meals can improve HIT-related symptoms, including bloating, postprandial fullness, and abdominal pain, and their intensity.¹⁰

One study found that taking capsules containing 4.2 mg pig‑kidney protein with 0.3 mg DAO before meals up to three times daily for 4 weeks led to improved symptoms. Symptoms returned after supplementation stopped, but serum DAO activity did not significantly increase, suggesting that the supplementation acted as a replacement enzyme rather than stimulating DAO production.¹¹

Supplementing with DAO enzyme cofactors such as Vitamin C, copper, and/or vitamin B6 may lead to an increase in DAO activity, but evidence is lacking.¹²

Although it has been speculated that MTHFR mutations can worsen histamine intolerance by impairing methylation processes that involve HNMT, there is no evidence that MTHFR is linked to DAO deficiency.