Low DAO Levels: What Does a Low DAO Blood Test Mean?

By Sarah Mitchell, NTC · Last updated April 2026 · 8 min read

You’ve received the results of a DAO blood test and the report shows low DAO levels. This result can feel alarming – or, if you’ve been struggling with unexplained symptoms for years, it can feel like finally finding a key piece of the puzzle. Either way, understanding what low DAO levels actually mean, what causes them, and what to do next is essential for moving forward effectively.

What Low DAO Levels Mean

Low DAO levels indicate reduced activity of diamine oxidase – the primary enzyme responsible for breaking down histamine in the gut lumen and intestinal epithelium. With low DAO levels, dietary histamine from high-histamine foods passes through the gut wall into the bloodstream in greater quantities than normal, accumulating to levels that produce histamine intolerance symptoms.

The standard reference threshold for low DAO levels is below 10 HDU/mL (histamine degrading units per millilitre), though reference ranges vary between laboratories. Some labs use different units – always check with the specific lab what their reference range represents.

What Causes Low DAO Levels?

Low DAO levels can have several distinct underlying causes, and identifying which applies to you influences the treatment approach:

Genetic low DAO levels

Variants in the AOC1 gene (which encodes DAO) can result in inherently reduced DAO expression and activity. Genetic low DAO levels are present from birth and represent a constitutional predisposition to histamine intolerance. These individuals may tolerate a normal diet for years until additional gut damage pushes them below their functional threshold.

Acquired low DAO levels from gut damage

This is the most common cause of low DAO levels in clinical practice. DAO is produced primarily by the enterocytes (epithelial cells) lining the small intestine. Any condition that damages these cells reduces DAO production:

• IBS and IBD (Crohn’s disease, ulcerative colitis)
• Coeliac disease – gluten-induced villous atrophy dramatically reduces DAO-producing epithelium
• SIBO (small intestinal bacterial overgrowth) – disrupts the mucosal environment
• Post-infectious gut damage (gastroenteritis, food poisoning)
• Chronic NSAID use – directly damages intestinal mucosa

Nutritional causes of low DAO levels

DAO requires cofactors to function – deficiency in any of these produces functionally low DAO levels even when DAO enzyme is structurally present:

• Vitamin B6 deficiency – P5P is an essential DAO cofactor. See our guide on vitamin B6 for histamine intolerance.
• Copper deficiency – DAO is a copper-containing enzyme
• Vitamin C deficiency – supports DAO activity independently

Medication-induced low DAO levels

Several commonly prescribed medications inhibit DAO activity and can produce low DAO levels in people who would otherwise test normal:

• Some antidepressants (particularly MAOIs and certain SSRIs)
• NSAIDs (ibuprofen, naproxen, aspirin)
• Some antihistamines (paradoxically – at high doses)
• Proton pump inhibitors
• Metoclopramide

Important Nuance: Low DAO Levels Don’t Always Equal Histamine Intolerance

Low DAO levels are not automatically diagnostic of histamine intolerance – and normal DAO levels don’t rule it out. Some people with confirmed low DAO levels eat a normal diet without symptoms (their histamine intake is simply low enough to stay under their threshold). Conversely, some people with clear histamine intolerance symptoms have normal DAO levels – because their problem is excess histamine production from mast cell overactivation (MCAS) rather than impaired breakdown. The DAO test provides one piece of evidence – it should be interpreted alongside symptoms and dietary response. See our complete guide to histamine intolerance testing.

What to Do If Your DAO Levels Are Low

1. Start a low-histamine diet immediately – see the complete guide. Reducing dietary histamine load is the most direct way to manage the consequences of low DAO levels.
2. Address nutritional cofactors – supplement vitamin B6 (P5P), consider copper levels, add vitamin C. These support whatever DAO you do produce to work at full capacity.
3. Take DAO enzyme supplements before meals – exogenous DAO directly compensates for low endogenous DAO levels at the most critical point (during digestion). See our DAO supplement reviews.
4. Investigate and treat underlying gut conditions – if low DAO levels are driven by coeliac disease, IBD, or SIBO, treating the underlying condition can restore DAO production over time. See our guide on gut health and histamine intolerance.
5. Review medications – discuss with your doctor whether any current prescriptions may be contributing to low DAO levels.

Medical disclaimer: This content is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider before starting any supplement regimen.