HIT and MCAS: similar symptoms, different mechanisms
Flushing, itching, diarrhoea or a racing heart can raise a question: is this histamine intolerance (HIT), or mast cell activation syndrome (MCAS)? Symptoms can overlap, but a checklist cannot tell the conditions apart. The circumstances, timing and pattern of reactions matter, together with a focused medical assessment. [1] [3]
HIT: when your body struggles with histamine from food
Histamine is a substance that naturally occurs in our bodies and in some foods. Problems may occur when we take in more histamine than the body can handle. This is commonly associated with histamine intolerance (HIT). [1]
An enzyme called DAO, found mainly in the gut, helps break down histamine from food. In people with HIT, DAO activity is often thought to be too low. However, the situation is more complicated, and this mechanism cannot be confirmed in everyone with similar symptoms. HIT is also not the same as a food allergy. [1]
Higher levels of histamine can be found in aged and fermented foods, as well as certain fish. The amount can also vary significantly depending on freshness and storage. This means that two servings of the same food may contain different amounts of histamine. That is one reason why there is no single universal list of foods that everyone with HIT needs to avoid. [2]
MCAS: when mast cells become too easily activated
With MCAS (mast cell activation syndrome), the problem is different. Mast cells are part of our immune system. When activated, they release histamine and other substances.
In MCAS, repeated episodes occur in which too many of these substances are released. Symptoms typically affect several parts of the body at the same time – for example, flushing or hives together with diarrhea, abdominal cramps, shortness of breath, or a drop in blood pressure. A severe reaction can resemble anaphylaxis. [3]
HIT and MCAS are therefore not the same condition. HIT is mainly associated with reactions to histamine consumed in food. In MCAS, histamine and other substances are released by the body's own mast cells. Feeling unwell after eating is therefore not enough on its own to tell which condition may be responsible.
Diagnosis is not straightforward
Unfortunately, there is currently no single simple and reliable test for HIT. DAO levels in the blood are sometimes measured, but a low DAO level alone does not confirm HIT. The result needs to be considered together with symptoms and other medical findings. [1] [6]
Similarly, feeling better on a low-histamine diet does not automatically confirm HIT. Symptoms can have many causes, and restricting your diet may change several things at once. This is why it is useful to monitor your individual reactions and approach food elimination systematically. [5]
MCAS also has specific diagnostic criteria. Doctors consider the pattern of repeated reactions, laboratory evidence of mast cell activation, and the response to treatment. One commonly used test measures tryptase, ideally during or shortly after a significant reaction, and compares it with the person's usual baseline level. [7] [8]
What should you do if you suspect HIT?
A good place to start is to keep track of what you eat and how your body reacts. A simple diary can help you record foods, portion sizes, meal times, and any symptoms that follow.
If histamine seems to be a problem, you may try a low-histamine diet for a limited period and then gradually reintroduce foods. The goal is not to live permanently on just a handful of “safe” foods. Instead, the aim is to understand your individual tolerance while keeping your diet as varied as possible. [9]
The same caution applies to DAO supplements. They may help some people, but current research is not strong enough to say that they work for everyone. Feeling better after taking DAO also does not confirm a diagnosis of HIT. [10]
If your reactions are severe, keep returning, or affect several parts of your body at once, it is a good idea to discuss them with an allergist. Similar symptoms can also be caused by food allergies, celiac disease, lactose intolerance, chronic hives, or other conditions. [1] [11]
If you suddenly develop difficulty breathing, swelling of the tongue or throat, or collapse, seek emergency medical help immediately. If you have been prescribed an epinephrine auto-injector, use it according to your doctor's instructions. [12]
This article is for educational purposes and does not replace individual medical advice or examination.
Scientific sources
- Reese et al. (2021). Guideline on management of suspected adverse reactions to ingested histamine.
- Sánchez-Pérez et al. (2021). Low-Histamine Diets.
- Castells et al. (2024). Mast cell activation syndrome: Current understanding and research needs.
- ASCIA (2025). Diagnosis and Investigation of Mast Cell Activation Disorders and Syndrome.
- Bent et al. (2023). Placebo-Controlled Histamine Challenge Disproves Suspicion of Histamine Intolerance.
- Evaluation of Serum Diamine Oxidase as a Diagnostic Test for Histamine Intolerance (2023).
- Weiler et al. (2019). MCAS diagnosis and management.
- Gülen (2024). Using the Right Criteria for MCAS.
- Jackson et al. (2025). Evidence for Dietary Management of Histamine Intolerance.
- Schnedl et al. (2019). Diamine oxidase supplementation improves symptoms in patients with histamine intolerance.
- Lee & Picard (2025). Diagnosis and management of MCAS in Canada.
- World Allergy Organization (2020). Anaphylaxis Guidance 2020.