Diamine oxidase

Diamine Oxidase (DAO): A Comprehensive Reference

1. Identity and Nomenclature

Diamine oxidase (DAO) is an enzyme formally classified under Enzyme Commission number EC 1.4.3.22. Also known as amine oxidase, copper-containing, 1 (AOC1), and formerly called histaminase, DAO is an enzyme involved in the metabolism, oxidation, and inactivation of histamine and other polyamines such as putrescine or spermidine. DAO (EC 1.4.3.22), also called histaminase or amiloride-binding protein, is a copper-dependent amino oxidase encoded by the AOC1 gene located on chromosome 7 (7q34–36).

Humans have three functioning genes that encode copper-containing amine oxidases; the product of the AOC1 gene is the so-called diamine oxidase (hDAO), named for its substrate preference for diamines, particularly histamine. The enzyme is expressed in bilateria, a biological group of animals, and is encoded by the AOC1 gene — a gene highly conserved across the bilateria group, which includes mammals, birds, reptiles, fish, and insects.

Amine oxidase (copper-containing) (AOC) is a family of amine oxidase enzymes that includes both primary-amine oxidase and diamine oxidase; these enzymes catalyze the oxidation of a wide range of biogenic amines including many neurotransmitters, histamine, and xenobiotic amines.

Common Names and Synonyms

• Diamine oxidase (DAO) — the predominant scientific designation
• Histaminase — historical name reflecting its primary substrate
• AOC1 (Amine Oxidase Copper-Containing 1) — gene product name
• Amiloride-binding protein — an alternative designation used in older literature

2. Natural Sources and Forms

Endogenous (Human) Sources

DAO is predominantly synthesized in the villous enterocytes of the small intestine, and to a lesser extent in the kidneys and placenta. It is secreted into the extracellular space within the intestinal lumen, enabling the inactivation of dietary histamine prior to systemic absorption. This contrasts with histamine-N-methyltransferase (HNMT), which operates intracellularly in tissues such as the liver and brain, illustrating a parallel yet distinct pathway for histamine degradation.

Plant Sources

Copper-containing amine oxidases occur in microorganisms, plants (in particular in the family Fabaceae), and animals, and use putrescine, cadaverine, and histamine as substrates. Edible legume sprouts have been proposed as a promising plant-based source of the enzyme diamine oxidase (DAO), which plays a key role in degrading histamine at an intestinal level and preventing the development of histamine intolerance symptoms.

Vegetal diamine oxidase (vDAO), an enzyme proposed to relieve symptoms of histaminosis, shows better reactivity with histamine and aliphatic diamines, as well as higher enzymatic activity than DAO of animal origin. Scientific research has evaluated enzyme activity of vDAO from germinating grains of Lathyrus sativus (grass pea) and Pisum sativum (pea).

In an evaluation of vDAO enzymatic activity from seedling extracts, it was revealed that vDAO is most active in L. sativus, followed by the pea cultivar CDC Amarillo. During the first 10 days of germination of CDC Amarillo seedlings, the vDAO enzyme activity and its specific activity were higher on the 8th day (1.08 U/mL and 0.81 U/mg protein) of germination, after which they decreased.

Vegetal DAO was specifically characterized for its much higher specific activity compared to that of animal DAO (i.e., pig kidney), and because enzymes from vegetal sources are more accepted by regulatory authorities.

Animal (Porcine) Sources

Different sources of DAO enzyme have been characterized in the literature — animal, vegetal, and microbial — although only the porcine kidney protein extract has been approved as a food supplement. Despite optimistic clinical results of DAO supplementation, variability in enzymatic activity of this type of active ingredient is reported, potentially linked to manufacturing processes.

Higher temperatures (70°C, 80°C, and 90°C) during the extract drying process cause a significant decrease in enzymatic activity, with losses of 10–20% for every 10°C increase in temperature; refrigeration is the only storage method capable of preserving enzymatic activity for at least 24 months. Manufacturing and storage of porcine kidney extract are therefore crucial steps in the formulation of DAO supplements suitable for the treatment of histamine intolerance.

Common Supplement Forms

• Porcine kidney extract: Commercially available DAO food supplements are often prepared from pig kidney.
• Plant-based (vegan) extract: The DAO extract used in recent Phase I clinical trials is a plant-based ingredient obtained from pea sprout dehydrated powder, covered with a gastro-resistant coating that protects the active ingredient from gastric acid.
• Dosage unit: The dosage of supplemental DAO is defined in "histamine degrading units" or HDU.

3. Historical and Traditional Use

Diamine oxidase does not have a history of traditional ethnobotanical or folk medicine use in the manner of herbal remedies, because it is an enzyme rather than a plant constituent used medicinally in classical traditions. Its history is instead rooted in the trajectory of early biochemistry and physiology research.

The enzyme was identified in the early 20th century as a product of the study of amine catabolism. Copper-containing amine oxidase enzymes, as a family, catalyze the oxidation of a wide range of biogenic amines including many neurotransmitters, histamine, and xenobiotic amines. Its former name, "histaminase," reflects the longstanding recognition of its role as the principal enzyme responsible for inactivating dietary and circulating histamine — a function recognized by mid-20th-century pharmacology. A 1963 study in Biochimica et Biophysica Acta already applied rigorous kinetic analysis to the enzyme, studying the effect of variations in concentrations of both substrates (amine and oxygen) and all products (aminoaldehyde, hydrogen peroxide, and ammonia) on the reaction to develop a formal mechanism.

The dietary supplementation application of DAO emerged only in the late 20th and early 21st centuries, growing from an understanding of histamine intolerance as a clinical entity. The first commercial DAO supplements were derived from porcine kidney and used primarily in Europe, particularly in Germany, Austria, and Spain, where research on histamine intolerance has been most active. The concept of supplementing exogenous DAO to compensate for endogenous deficiency in histamine-intolerant patients began appearing in the clinical literature in the early 2000s. Treatment with diamine oxidase (DAO) for histamine intolerance has become an emerging tool for addressing both gastrointestinal and systemic symptoms derived from poor histamine metabolism. DAO is the main enzyme responsible for the degradation of dietary histamine in the digestive tract, and its oral supplementation seeks to compensate for the endogenous deficit characteristic of patients with histaminosis.

In plants, the functional role of DAO is physiological rather than medicinal: copper-containing amine oxidases in plants mainly catalyze the oxidation of putrescine to produce ammonium, 4-aminobutyrate, and hydrogen peroxide, with 4-aminobutyrate further converting to γ-aminobutyric acid (GABA). This function in plant stress physiology was described independently of any traditional medical use.

4. Biochemistry: Structure, Cofactors, and Mechanism of Action

Molecular Structure

Human DAO (hDAO) has been cloned and expressed in insect cells and its structure determined by X-ray crystallography to a resolution of 1.8 Å. The homodimeric structure has the archetypal amine oxidase fold. Two active sites, one in each subunit, are characterized by the presence of a copper ion and a topaquinone residue formed by the post-translational modification of a tyrosine.

The copper amine oxidases occur as mushroom-shaped homodimers of 70–95 kDa, each monomer containing a copper ion and a covalently bound redox cofactor, topaquinone (TPQ). Topaquinone is formed by post-translational modification of a conserved tyrosine residue. The copper ion is coordinated with three histidine residues and two water molecules in a distorted square pyramidal geometry, and has a dual function in catalysis and topaquinone biogenesis.

Copper-containing amine oxidases (CAOs) require a protein-derived topaquinone cofactor (TPQ) for activity. TPQ biogenesis is a self-processing reaction requiring the presence of copper and molecular oxygen.

Substrate Specificity

DAO, the enzyme that is responsible for amine biodegradation in animals, plants, and humans, catalyses the biotransformation of amines such as histamine (HA), putrescine, 1-phenylethylamine, tyrosine, tryptamine, serotonine, and spermine.

Although hDAO shares 37.9% sequence identity with another human copper amine oxidase, semicarbazide-sensitive amine oxidase or vascular adhesion protein-1, its substrate binding pocket and entry channel are distinctly different, in accord with different substrate specificities. An aspartic acid residue, conserved in all diamine oxidases but absent from other amine oxidases, is responsible for the diamine specificity by interacting with the second amino group of preferred diamine substrates.

Catalytic Reaction

The enzyme catalyzes the oxidation of primary amines to aldehydes, with the subsequent release of ammonia and hydrogen peroxide, requiring one copper ion per subunit and topaquinone as a cofactor. The three substrates are primary amines (RCH₂NH₂), H₂O, and O₂, whereas the three products are RCHO, NH₃, and H₂O₂.

Histamine, a biogenic amine, undergoes metabolism through three distinct enzymatic pathways. The first involves the deamination of histamine by the enzyme diamine oxidase to form imidazole acetaldehyde. In the second pathway, histamine is metabolized into Nτ-methylhistamine by histamine N-methyltransferase (HNMT). The Nτ-methylhistamine, unless excreted by the kidney, is subsequently oxidized into Nτ-methylimidazoleacetic acid (Nτ-MIAA) by monoamine oxidase (MAO). This two-step process effectively reduces the activity of histamine in the body and is particularly important for quick deactivation of histamine in the brain.

Hydrogen Peroxide as a Byproduct

A toxicologically important feature of the DAO reaction is the generation of hydrogen peroxide (H₂O₂) as a byproduct. The rationale for using DAO in combination with catalase relies on the ability of catalase to decompose H₂O₂ into O₂ and H₂O — a consideration relevant when DAO is applied therapeutically, as accumulation of H₂O₂ in the intestinal environment could cause oxidative stress to enterocytes.

Diamine Specificity and the AOC1 Gene

DAO is a copper-dependent amino oxidase encoded by the AOC1 gene located on chromosome 7 (7q34–36). This functional enzyme, a homodimer with two isoforms, catalyzes the oxidative deamination of the primary amine group of histamine. Mammalian copper-containing amine oxidases (CAOs), encoded by four genes (AOC1–4), catalyze the oxidation of primary amines to aldehydes, regulate various biological processes, and are linked to diseases like inflammatory conditions and histamine intolerance.

Copper Dependency

The amount of DAO in young leaves of clover grown with a range of copper treatments was highly correlated with the copper concentration of the leaf. Anti-DAO antibodies reacted against apo-DAO prepared in vitro, indicating that apo-DAO was absent from copper-deficient leaves. These results suggest that the biosynthesis of DAO in young clover leaves is controlled by the copper concentrations of the leaves. This tight coupling of DAO activity to copper availability is conserved across species, consistent with the enzyme's copper-containing active site.

5. Physiology: DAO as the Intestinal Histamine Barrier

The principal physiological function of DAO is the oxidative deamination of biogenic amines — most notably histamine — into aldehydes, hydrogen peroxide, and ammonia. In the gut, DAO acts as a protective barrier against histamine overload from exogenous sources, such as fermented foods, aged cheeses, cured meats, and alcoholic beverages.

Plasma DAO levels can be used to monitor the function of the small intestinal mucosal barrier under noninvasive conditions. This measure has gained increasing clinical attention in recent years in the diagnosis of gastrointestinal diseases, histamine intolerance (HIT), migraine, abnormal pregnancy, and the prognosis of tumors.

The serum level of diamine oxidase (DAO) reflects the integrity and maturation of the small intestinal mucosa. This measure is important in diagnosing various diseases, including chronic urticaria, multiple organ dysfunction syndrome, preterm abortion, and migraine.

Histamine is mainly produced by mast cells, platelets, basophils, histaminergic neurons, and enterochromaffin cells, where it is stored in vesicles and released in response to stimulation. It is synthesized from the amino acid histidine by pyridoxal phosphate-containing L-histidine decarboxylase (HDC), and functions by binding to four receptors on target cells in various tissues. It causes smooth muscle cell contraction, vasodilation, increased vascular permeability and mucus secretion, tachycardia, arrhythmias, altered blood pressure, stimulated gastric acid secretion, and nociceptive nerve fiber activation.

6. Deficiency: Causes and Consequences

Definition and Prevalence

Diamine oxidase (DAO) is a key enzyme responsible for the metabolism of histamine, preventing its excessive accumulation and thereby maintaining physiological homeostasis. When DAO activity is insufficient, histamine intolerance (HIT) arises, manifesting as migraines, gastrointestinal disturbances, and allergy-like reactions, among other disorders.

The clinical manifestations of the disorder are diverse and include a plethora of unspecific digestive, neurological, dermatological, respiratory, and cardiovascular complaints. Recent studies have shown that patients with histamine intolerance often suffer a complex combination of symptoms, the most frequently reported being abdominal distention, diarrhea, constipation, headache, migraine, flushing, pruritus, low blood pressure, and rhinorrhea.

Genetic Causes

Certain genetic variants in the AOC1 gene, such as rs1049793 (Ser332Phe), rs10156191, and rs2052129, can reduce DAO enzyme activity by affecting its efficiency or gene expression.

A retrospective pilot study investigated the prevalence of four variants of the AOC1 gene in Caucasian adults with symptoms of histamine intolerance. In a cohort of 100 patients and 100 healthy individuals, DAO-encoding gene non-synonymous single nucleotide variations (SNVs) were genotyped; the study found that 79% of individuals with symptoms of histamine intolerance harbored one or more of the four SNVs associated with reduced DAO activity.

Variants of AOC1 are associated with reduced DAO activity, resulting in accumulation of high levels of histamine and causing a wide range of neurological, gastrointestinal, and epidermal disorders.

Acquired Causes

Deficiency in DAO — whether due to genetic variation, acquired disease, pharmacologic inhibition, or nutritional insufficiency — can result in histamine intolerance, a condition characterized by symptoms that resemble allergic reactions but are independent of immunoglobulin E (IgE).

In addition to genetic factors, DAO deficiency can result from inflammatory conditions or impaired intestinal function, which may restrict DAO secretion. It can also be induced by certain drugs known to inhibit DAO activity, including commonly used medications such as acetylcysteine, clavulanic acid, metoclopramide, and verapamil.

In addition to these etiological factors, intestinal dysbiosis has been proposed as a potential contributor to DAO deficiency. Although research in this area remains limited, studies suggest that alterations in gut microbiota composition may reduce DAO activity or that an overabundance of histamine-secreting bacteria could lead to histamine accumulation in the gut.

Diagnosis

Diagnosis of histamine intolerance is currently based on the appearance of symptoms affecting two or more organs or body systems and their improvement or remission after the dietary exclusion of histamine-containing foods. Additionally, food allergies or underlying systemic mastocytosis must be ruled out. A range of complementary tests have been proposed to obtain a marker to confirm the diagnosis.

In a retrospective study, 13 out of 14 patients with histamine intolerance subjectively reported a benefit in at least one disturbance following diamine oxidase supplementation. The mean value of diamine oxidase activity in the cohort of patients with histamine intolerance symptoms was 7.04 ± 6.90 U/mL compared to 39.50 ± 18.16 U/mL in 34 healthy controls (P = 0.0031).

7. Scientific Evidence by Area of Application

7.1 Histamine Intolerance (HIT) — General Symptom Relief

Evidence strength: Preliminary to moderate; limited by small sample sizes and lack of placebo controls in most studies.

To date, around twenty studies have investigated the effectiveness of a low-histamine diet or DAO supplementation in reducing the frequency and/or intensity of histamine intolerance symptoms, with promising results.

One pivotal study evaluated DAO supplementation in a clinical setting: in an open-label interventional pilot study, 28 patients with HIT were identified. For 4 weeks, they were instructed to take DAO capsules before meals; then, throughout a follow-up period, they were instructed not to take the DAO. A questionnaire that included 22 symptoms, divided into 4 categories, plus a symptom severity score, was used. All symptoms improved significantly during the oral supplementation of DAO. During the follow-up period, without DAO supplementation, the symptoms sum scores increased again, and the symptom intensity score was reduced for all symptoms. The authors demonstrated a significant reduction of every HIT-related symptom and its intensity due to DAO oral supplements.

However, a placebo effect influencing the improvement of symptoms in this study cannot be ruled out. Additional studies, including placebo-controlled randomized trials with high numbers of patients and long treatment periods, are needed to fully evaluate oral DAO supplementation.

A subsequent observational study over two months reported: notable reductions in the prevalence of migraines (−30.4%), gastrointestinal symptoms (−47.1%), fatigue (−72.2%), and cutaneous manifestations (−64.4%). Furthermore, 55.8% of participants reported a decrease in their medication needs, while 53.3% experienced improved tolerance to histamine-rich foods. The results underscore the need for further studies involving larger populations to confirm these findings and explore the long-term effects of DAO supplementation on symptom management in HIT patients.

7.2 Migraine and Headache

Evidence strength: Moderate; one randomized double-blind trial with clinically significant, though modest, results.

One hundred patients with confirmed episodic migraine according to International Headache Society (IHS) criteria and DAO deficiency (levels below 80 HDU/ml) were randomized into two groups. One group received DAO enzyme supplementation and the other received placebo for one month. Clinical outcomes assessed were duration and number of attacks, perception of pain intensity, and adverse effects during treatment. The use of triptans was also recorded.

Great variability was found in the duration of migraine attacks reported by placebo and DAO groups. A significant reduction (p = 0.0217) in hours of pain was achieved in patients treated with DAO supplement, with mean durations of 6.14 (±3.06) and 4.76 (±2.68) hours before and after treatment, respectively. A smaller reduction without statistical significance was also observed for this outcome in the placebo group, from 7.53 (±4.24) to 6.68 (±4.42) hours.

The reduction of pain hours observed in the placebo group (0.9 hours) could explain the lack of significant differences between both study groups. One month of DAO supplementation demonstrated a positive trend in the improvement of migraine, but more studies with a longer treatment period are needed to better assess the efficacy of DAO supplementation.

Genetic associations with migraine have also been investigated. Investigators evaluated the association between four SNPs in the AOC1 gene linked to decreased DAO activity and susceptibility to developing migraine in a Caucasian Spanish cohort. The study identified the rs10156191 SNP and gender as significant factors in migraine risk. Subgroup analysis revealed that male homozygotes with the rs2052129G allele and female carriers of the rs10156191T allele had a higher risk of developing migraine. The rs10156191T allele, which is associated with reduced DAO enzyme activity, was more prevalent among individuals with migraine, particularly in women.

DAO deficiency has also been evaluated in migraine sufferers in a separate study, where a high prevalence of DAO deficiency was observed in the migraine group, with 87% of participants displaying this deficiency, compared to 44% in the control group.

With respect to drug interactions relevant to migraine care: findings from a preclinical and human study assessing several anti-inflammatory and anti-migraine drugs — including acetylsalicylic acid, zolmitriptan, paracetamol, a combination of acetylsalicylic acid with paracetamol and caffeine, ibuprofen, and sumatriptan — did not demonstrate any inhibitory effects on the activity or levels of DAO, in either in vitro experiments or in human subjects.

7.3 Chronic Spontaneous Urticaria (CSU)

Evidence strength: Preliminary; one small randomized controlled crossover trial showing effect only in the DAO-deficient subgroup.

Diamine oxidase (DAO) catabolizes and inactivates histamine, a key player in chronic spontaneous urticaria (CSU). The highest expression of DAO occurs in the gastrointestinal tract, possibly to control the burden of histamine intake from food. A hypothesis was tested that a 30-day oral supplementation with DAO (1 capsule b.i.d., 15 minutes before a meal) could reduce the severity of CSU as estimated by the 7-Day Urticaria Activity Score (UAS-7). The study was designed as a double-blind, placebo-controlled, crossover investigation of 22 patients with CSU incompletely controlled by first-line antihistamine therapy.

Twenty patients completed the study. Supplemental therapy with DAO caused a 3.8 ± 1.2 point mean ± SEM UAS-7 score reduction in patients with low serum DAO levels at baseline (p = 0.041 compared to placebo). These data suggest that DAO may be involved in the pathogenic cascade of CSU and that DAO supplementation could be effective for symptom relief in patients with low DAO levels in serum.

7.4 Fibromyalgia

Evidence strength: Preliminary; one randomized double-blind placebo-controlled trial, results pending full analysis.

Fibromyalgia (FM) is characterized by chronic musculoskeletal pain, muscle tension, joint mobility loss, and several psychological symptoms severely affecting patient well-being. Histamine is naturally degraded in the small intestine by diamine oxidase (DAO). Hereditary or acquired DAO deficiency causes extracellular histamine accumulation, leading to symptoms similar to those of individuals diagnosed with FM.

One study aimed to assess the efficacy of adding DAO supplementation for 8 weeks to standard fibromyalgia therapy. One hundred women with FM (age: 33–61 years) were randomly assigned to supplementation and control groups. The Fibromyalgia Impact Questionnaire (FIQ), the Pain Catastrophizing Scale (PCS), and intensity scales were applied for a series of clinical symptoms together with the Bristol scale to assess the added value of DAO supplementation.

Variants of AOC1 are associated with reduced DAO activity, resulting in accumulation of high levels of histamine and causing a wide range of neurological, gastrointestinal, and epidermal disorders, which are present in people with fibromyalgia.

7.5 Inflammatory Bowel Disease (IBD) and Intestinal Barrier Integrity

Evidence strength: Observational; serum DAO used as a diagnostic biomarker of mucosal integrity in IBD.

Crohn's disease (CD) is a chronic intestinal inflammatory disease. In a study of 59 CD patients enrolled along with 28 healthy controls, ELISA was used to quantitatively detect D-lactate and diamine oxidase (DAO) levels in sera. The correlation between these two biomarkers and disease activity scores was assessed.

DAO levels were significantly higher in patients with active CD compared to controls, and DAO levels in CD patients were positively correlated with disease activity (r = 0.53, P < 0.05). The area under the ROC curve (AUC) when CD activity was diagnosed with DAO alone was 0.748. The diagnostic efficacy was not significantly different from that of erythrocyte sedimentation rate (ESR) and hypersensitive C-reactive protein (CRP).

A study indicated that the levels of serum DAO and D-lactate are higher in IBD patients compared with those in healthy groups, and serum DAO and D-lactate could be decreased after treatment, suggesting that IBD patients suffer intestinal barrier dysfunction.

Diamine oxidase (DAO) is a cytoplasmic enzyme found primarily in the villus epithelial cells of the small intestine. Serum DAO levels have been evaluated as a potential marker of intestinal disease in a variety of disorders, including gut atrophy, ischemia, and inflammation.

7.6 Pregnancy and DAO Activity

Evidence strength: Observational/biological; DAO as a biomarker of placental function. Supplementation in pregnancy explored but not yet well established.

Some studies have reported an increase in DAO activity during pregnancy. It has been suggested that the placenta enhances DAO production, acting as a metabolic barrier to prevent excessive histamine from entering the fetal-maternal circulation. High levels of histamine in pregnant women have been linked to adverse outcomes, likely due to its vasoactive properties. Thus, maintaining a balance between histamine and DAO is crucial for a healthy pregnancy.

DAO activity is transiently elevated during pregnancy; placental DAO production rises dramatically in the second trimester, which is why many women report histamine intolerance improving during pregnancy.

7.7 Attention Deficit Hyperactivity Disorder (ADHD) — Exploratory

Evidence strength: Very preliminary; single pilot study, no interventional data available.

A DAO deficiency may influence some processes in the brain, such as learning and memory, and the development of some disorders such as ADHD by allowing for the permeability of histamine into the central nervous system during critical developmental periods. One pilot study reported compelling evidence suggesting that children and adolescents with ADHD have a very elevated prevalence (78.8%, nearly four out of five) of having at least one AOC1 gene allele associated with DAO deficiency.

However, if most of the population has at least one AOC1 gene variant associated with DAO deficiency, these findings may not be as critical as expected. If most of the population does not have AOC1 gene variants related to DAO deficiency, results may suggest that the DAO enzyme could play a critical role in the pathophysiology of ADHD. No interventional supplementation trials in ADHD populations have been published as of the time of this article.

8. Dosage Forms and Dosages Reported in Studies

The dosage of supplemental DAO is defined in "histamine degrading units" or HDU.

• Pilot HIT study (Schnedl et al., 2019): 28 patients with HIT were instructed to take DAO capsules before meals for 4 weeks. Specific mg or HDU dosage was not detailed in the publicly available abstract.
• Migraine RCT (Izquierdo-Casas et al., 2018–2019): Participants were treated with DAO 20 minutes before breakfast, lunch, and dinner for a total of one month. The trial registration specified 14 mg DAO per dose.
• Chronic spontaneous urticaria crossover RCT: A 30-day oral supplementation with DAO (1 capsule b.i.d., 15 minutes before a meal) was administered.
• Fibromyalgia RCT (Duelo et al., 2023): DAO supplementation was added for 8 weeks to standard therapy in 100 women with FM.
• Phase I Safety Trial (adiDAO® Veg, 2025): A randomized, double-blind, single ascending dose (SAD) Phase I clinical trial evaluated the safety of escalating doses of DAO supplementation in healthy volunteers. Thirty participants were randomly assigned to receive single doses of 42 mg, 84 mg, or 210 mg of DAO extract (adiDAO® Veg) or placebo under fasting conditions.

Across studies, DAO is consistently administered before meals (typically 15–20 minutes prior), to ensure the enzyme is active in the intestinal lumen during food-derived histamine exposure. There are no established regulatory limits for plant-sourced DAO supplementation.

9. Safety Considerations

Clinical Safety Evidence

A randomized, double-blind, single ascending dose (SAD) Phase I clinical trial evaluated the safety and tolerability of escalating doses of DAO supplementation in healthy volunteers. Vital signs, laboratory parameters, and adverse events (AEs) were monitored. No serious adverse events or clinically significant changes in vital signs, ECGs, or laboratory parameters were observed.

Reported animal studies — including an acute oral study and a 90-day sub-chronic toxicity study in rats — corroborate the safety of orally administered diamine oxidase extract.

Drug Interactions: Inhibitors of Endogenous DAO Activity

In a study quantifying the influence of active drug ingredients on the activity of human diamine oxidase (DAO), DAO was identified as the main enzyme in catabolism of biogenic amines in the intestine. Ingestion of food containing high amounts of biogenic amines in the case of reduced DAO activity leads to accumulation of histamine causing symptoms of histamine intolerance. Many drugs are suspected to inhibit DAO activity; however, only few scientific data were available to support this before these measurements.

Specific drug inhibition findings from in vitro study:

• Chloroquine and clavulanic acid showed the greatest inhibition potential on diamine oxidase (>90%).
• Cimetidine and verapamil showed inhibition of about 50%.
• Moderate influence on DAO was caused by isoniazid, metamizole, acetylcysteine, and amitriptyline (>20%).
• Diclofenac, metoclopramide, suxamethonium, and thiamine have very low inhibition potential (<20%), while cyclophosphamide and ibuprofen displayed no effect on DAO.
• Since even levels of about 30% inhibition may be critical, most of the observed substances can be designated as DAO inhibitors.

Porcine Allergenicity

The commercial sources of DAO are derived from pig kidney, and those allergic to pork tend to develop quite severe allergic reactions to the products. Plant-based (pea sprout-derived) DAO preparations have been developed in part to address this limitation for individuals with pork allergy.

Supplement Quality Variability

Variability in supplement quality, dosing standardization, and labeling accuracy remains an issue in the nutraceutical market. Additionally, there is a need for broader awareness and diagnostic criteria for histamine intolerance.

Limitations of Supplementation

DAO supplements are not a panacea. They do not address histamine produced internally due to mast cell activation or allergic reactions.

Pregnancy

While DAO levels naturally increase during pregnancy — a phenomenon believed to protect the fetus from histamine — the safety of exogenous DAO supplementation during pregnancy or breastfeeding has not been established.

10. Body Systems and Health Areas

Based on the peer-reviewed literature, DAO is associated with the following body systems and health areas:

• Gastrointestinal system: Primary site of DAO synthesis and action; reduced DAO correlates with diarrhea, bloating, constipation, and other GI complaints in HIT; serum DAO is an established biomarker of small intestinal mucosal integrity.
• Nervous system / Headache: When DAO activity is insufficient, histamine intolerance (HIT) arises, manifesting in part as migraines; genetic variants in AOC1 have been associated with migraine risk; one RCT demonstrated reduced migraine duration with DAO supplementation.
• Dermatology: The reduced histamine degradation in the gut leads to its accumulation in plasma, causing clinical manifestations including urticaria.
• Cardiovascular system: Clinical manifestations of HIT include tachycardia.
• Respiratory system: Manifestations of HIT include dyspnea.
• Reproductive system: Placental DAO production rises substantially in the second trimester; serum DAO levels have been investigated as a marker of pregnancy complications.
• Musculoskeletal system: Exploratory links between AOC1 variants and fibromyalgia symptoms have been investigated.
• Neurodevelopmental: Pilot data link AOC1 variants and DAO deficiency to a higher prevalence in ADHD, though this remains highly preliminary.

11. Evidence Gaps and Research Needs

Around twenty studies have investigated the effectiveness of dietary strategies — including DAO supplementation — in reducing the frequency and/or intensity of symptoms, with promising results. However, the limitations of these studies (small patient cohort, lack of control group, and short dietary intervention periods) highlight the need for more ambitiously designed research.

In patients with symptoms triggered by histamine-rich food, measuring the serum diamine oxidase activity can help identify subjects who can benefit from a histamine limitation diet and/or diamine oxidase supplementation. Properly designed, controlled studies investigating histamine intolerance that include histamine provocation are indispensable for providing insight.

Currently, the only evidence-based strategy for alleviating or preventing symptoms is adherence to a low-histamine diet, often supplemented with exogenous DAO to enhance histamine breakdown at the intestinal level. However, while this dietary approach can improve symptoms, it does not appear to influence serum DAO activity.

References

• Samonina-Kosicka J, Kańska M. Mechanistic studies of reactions catalysed by diamine oxidase using isotope effects. J Labelled Comp Radiopharm. 2013;49(3):357-64. PubMed
• McGrath AP, et al. Structure and inhibition of human diamine oxidase. Biochemistry. 2009;48(41):9810-22. PubMed
• McGrath AP, et al. The structure and inhibition of human diamine oxidase. PMC Full Text
• Diamine oxidase. Wikipedia (with scientific references)
• Advances in the Clinical Application of Histamine and Diamine Oxidase (DAO) Activity: A Review. Catalysts. 2023;13(1):48. MDPI
• Schnedl WJ, et al. Diamine oxidase supplementation improves symptoms in patients with histamine intolerance. Food Sci Biotechnol. 2019. PMC
• Schnedl WJ, et al. Diamine oxidase supplementation improves symptoms in patients with histamine intolerance. PubMed
• Evaluation of the Safety and Tolerability of Three Single Ascending Doses of Diamine Oxidase (DAO) in Healthy Volunteers: A Randomized Clinical Trial. MDPI. 2025
• Comas-Basté O, et al. Histamine Intolerance: The Current State of the Art. Biomolecules. 2020;10(8):1181. PMC
• Manzotti G, et al. Serum diamine oxidase activity in patients with histamine intolerance. Int J Immunopathol Pharmacol. 2016. PMC
• Study Protocol for a Prospective, Unicentric, Double-Blind, Randomized, and Placebo-Controlled Trial on the Efficacy of a Low-Histamine Diet and DAO Enzyme Supplementation. PMC. 2025
• Izquierdo-Casas J, et al. Diamine oxidase (DAO) supplement reduces headache in episodic migraine patients with DAO deficiency: A randomized double-blind trial. Clin Nutr. 2019;38:152–158. ScienceDirect
• Diamine Oxidase Interactions with Anti-Inflammatory and Anti-Migraine Medicines in the Treatment of Migraine. PMC
• Diamine Oxidase Supplementation in Chronic Spontaneous Urticaria: A Randomized, Double-Blind Placebo-Controlled Study. Int Arch Allergy Immunol. 2018. PubMed
• Exogenous Supplementation with DAO Enzyme in Women with Fibromyalgia: A Double-Blind Placebo-Controlled Clinical Trial. PubMed. 2023
• Cumulative effect of AOC1 gene variants on symptoms and pathological conditions in adult women with fibromyalgia: a pilot study. PMC. 2023
• Pilot Study on the Prevalence of Diamine Oxidase Gene Variants in Patients with Symptoms of Histamine Intolerance. Nutrients. 2024;16(8):1142. PMC
• Prevalence and Clinical Picture of Diamine Oxidase Gene Variants in Children and Adolescents with ADHD: A Pilot Study. PMC. 2024
• Cai J, et al. Diagnostic and Clinical Significance of Serum Levels of D-Lactate and Diamine Oxidase in Patients with Crohn's Disease. Gastroenterol Res Pract. 2019. PMC
• The Correlation between Endotoxin, D-Lactate, and Diamine Oxidase with Endoscopic Activity in Inflammatory Bowel Disease. PMC. 2022
• Evaluation of the inhibitory effect of various drugs/active ingredients on the activity of human diamine oxidase in vitro. PMC
• Improvement of Histamine Intolerance Symptoms in Pregnant Women with Diamine Oxidase Deficiency: An Exploratory Study. J Clin Med. 2025;14(13):4573. PMC
• Diamine Oxidase as a Therapeutic Enzyme: Study of Germination from Vegetal Sources and Investigation of β-ODAP. PMC. 2023
• Diamine Oxidase from White Pea (Lathyrus sativus) Combined with Catalase Protects the Human Intestinal Caco-2 Cell Line from Histamine Damage. PMC
• Lyophilised legume sprouts as a functional ingredient for diamine oxidase enzyme supplementation in histamine intolerance. LWT. 2020;125:109201. ScienceDirect
• Influence of Seed Disinfection Treatments on the Germination Rate and Histamine-Degrading Activity of Legume Sprouts. PMC. 2024
• McGrath AP, et al. Structure and Inhibition of Human Diamine Oxidase. Biochemistry. 2009;48(41):9810-22. ACS Publications
• Observational Study on the Effectiveness of a Diamine Oxidase (DAO) Supplement in a Sample of Consumers with Histaminosis. World J Food Sci Technol. 2026
• Diamine oxidase deficiency implications for health, current management, and future directions in the treatment of histamine intolerance: A review. ScienceDirect. 2025
• Amine oxidase (copper-containing). Wikipedia (with scientific references)
• The tree of life of copper-containing amine oxidases. Front Plant Sci. 2025. PMC
• Correlation of active site metal content in human diamine oxidase with trihydroxyphenylalanine quinone cofactor biogenesis. PubMed. 2010