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  • Writer's pictureGeorgina Hardy

What is Mast Cell Activation Syndrome (MCAS)?

Updated: Jun 15, 2022

Firstly, a mast cell is a type of blood cell that contributes to the functioning of the immune system (1). Mast cells contain certain chemicals, such as histamine, that are released during allergic reactions and other immune responses (i.e threats to the body such as infections, toxins, etc.) (2). The activation of mast cells, and the subsequent release of these chemicals, produces symptoms that are typically associated with allergies (3).

In Mast Cell Activation Syndrome (MCAS), it’s thought that mast cells can become activated inappropriately (i.e the body thinks there is a threat when there actually isn’t) (4). This causes the release of the chemicals within the mast cells, resulting in symptoms (4). Symptoms of MCAS There are various symptoms that someone with MCAS can present with. Symptoms can be acute and/or chronic and can vary in intensity (1). Common symptoms include (6):

  • Fatigue

  • Lightheadedness / Fainting

  • Headache

  • Flushing

  • Itching / hives

  • Burning / prickling of the skin

  • Nausea / vomiting

  • Pain all over the body (similar to that in fibromyalgia)

  • Chills

  • Skin swelling prone to moving around different parts of the body

  • Eye irritation

  • Shortness of breath

  • Flushing

  • Bloating, abdominal pain, diarrhoea

  • High heart rate

  • Variable blood pressure

In more serious cases, MCAS can lead to anaphylaxis - a life-threatening allergic reaction, which requires immediate medical treatment (3,5). Triggers For people living with MCAS, there are a variety of potential triggers. These triggers are variable and can be different from one day to the next. Because of this, it can be difficult to identify triggers, but where possible, any known triggers may need to be avoided (3,6). Every MCAS patient will have different ones, however, common triggers include (3, 6):

  • Heat

  • Stress / emotions

  • Exercise

  • Alcohol

  • Medications

  • Odors

  • Insect stings

  • Certain foods


MCAS is difficult to diagnose and is often only diagnosed once other similar conditions (e.g. Allergies, Mastocytosis) have been discounted (7).

Diagnostic criteria for MCAS are (7):

  1. Presentation of typical clinical symptoms (as mentioned above)

  2. During or shortly after (4hrs) a symptomatic episode, there should be an increase in the levels of chemicals that are released by mast cells, found in the blood or urine through testing.

  3. Response to MCAS treatments (antihistamines and/or mast cell stabilisers)

As it can be difficult to access testing facilities during a patient’s symptomatic episode, some doctors diagnose based on symptom history and response to a trial of MCAS medication (3).

How to Treat Mast Cell Activation Sydrome

Although there is no cure for MCAS, there are ways to manage it through the use of medications (either antihistamines or mast cell stabilisers) and by avoiding known triggers (1,8).

Some foods can be triggers for MCAS patients. In this instance, dietary intervention may be appropriate. Although there are no evidence-based recommended diets for MCAS, with the help of a registered dietitian, an elimination diet may be trialed to help determine food triggers.

There are a couple of dietary approaches that may be useful:

Low-Histamine Diet If a person with MCAS is thought to be reacting to foods that are high in histamines, a low-histamine diet may be beneficial if first-line dietary advice (e.g. reducing consumption of ultra-processed foods, cooking with fresh produce) does not improve symptoms (9).

Histamine is one of the chemicals that can be released from the mast cells, causing MCAS patients to become symptomatic (10). It’s thought that by decreasing the amount of histamine in the body, the low-histamine diet could potentially improve histamine-related symptoms (11).

The low-histamine diet involves restricting foods that are high in histamines for two to four weeks. Then, importantly, reintroducing to identify any triggers and assess personal tolerance levels (9).

It’s important to note that there is a lack of consensus on which foods are actually high in histamine, however, histamine is thought to be higher in foods such as tomato, aubergine, avocado, alcohol, cheese, chocolate and fermented foods (12, 13).


If a person with MCAS is presenting with IBS-type symptoms, the low-FODMAP diet may be considered if first-line dietary advice has not improved symptoms (9). FODMAP stands for Fermentable, Oligo-, Di-, Mono-saccharides and Polyols. They are carbohydrates that are not fully or well absorbed in the intestines (14). They can cause gastrointestinal symptoms in some people (15).

The low-FODMAP diet is a 3-phase diet that involves restricting foods that are high in FODMAPs for 4-8 weeks, re-introducing them to assess tolerance, then moving onto the maintenance phase and only avoiding foods that were identified in phase 2 to cause symptoms (15).

This dietary approach may help those with symptoms of bloating, wind, altered bowel habits and pain. One study has also found that histamine levels in the urine reduced eight-fold after patients with IBS symptoms followed a low-FODMAP diet (16), which may have implications for those with MCAS.

Important! Both of these dietary approaches should be undertaken with the support of a registered dietitian, to minimise the risk of any issues occurring (e.g. nutritional deficiencies). Additionally, due to lack of research and the fact that the low-histamine diet only targets one of the chemicals released by mast cells (histamine), neither diet is officially recommended in the treatment of MCAS.

Helpful patient resource (UK):


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