Histamine Intolerance, MCAS, or an Allergy: What Could Be Making My Child Sick?

Written by student dietitian Sam Coates, checked by registered dietitian Nishti

Imagine this scenario: Your child eats a snack at school, a piece of cheese or an apple. Then within hours they’re complaining of a headache, their cheeks are flushed and they’ve developed an itchy rash. You’re left wondering—was it the food, an environmental trigger, or something else entirely?

One day it’s a rash, then it’s tummy pain plus having mood swings and becoming extremely irritable. You’ve cut out countless foods, swapped detergents, and even changed shampoo’s — and yet the symptoms persist. You take to the internet and come across a vast array of explanations, but what is really going on? Is it histamine intolerance, MCAS, or a tricky allergy? How do you even begin to figure it out?

While each of these conditions are unique, with possibly different causes and different treatments. The symptoms can overlap in confusing ways, meaning for many parents, the search for answers can feel exhausting, overwhelming and feel never-ending. To make it harder, the symptoms of Histamine Intolerance, Mast Cell Activation Syndrome (MCAS) and Allergies all typically begin shortly after exposure to a trigger - making it harder to identify and differentiate between them without professional guidance. Nevertheless, understanding their differences and similarities is a critical first step in helping your child feel better.

Breaking It Down: Each Condition Explained

Histamine Intolerance

Histamine is a natural chemical involved in immune responses, digestion, and more. However, when the body struggles to break it down—usually due to a deficiency or lack of activity of the enzyme Diamine Oxidase (DAO), histamine builds up in our bodies, causing symptoms.

Common Triggers: Aged cheeses, fermented foods (e.g. yogurt or sauerkraut), cured meats, seafood and even some fruits and vegetables like tomatoes, aubergine or spinach.

Typical Symptoms: Symptoms often vary depending on the amount of histamine consumed and the individual’s unique threshold, however they are always attributed to the consumption of high histamine foods.

• Skin: Flushing, hives, or itching.
• Digestive System: bloating, diarrhoea, stomach cramps, and nausea.
• Nervous System: headaches, migraines, dizziness, or feeling fatigued.
• Respiratory System: nasal congestion, sneezing, or difficulty breathing in severe cases.
• Cardiovascular System: rapid heartbeat or low blood pressure in severe cases.
• Behavioural Changes: irritability, mood swings, or unexplained fatigue - particularly after eating high-histamine foods.
• Joint Issues: aching or stiffness in joints, which may flare after consuming high-histamine foods.

Mast Cell Activation Syndrome (MCAS)

Mast cells are part of the immune system and play a key role in releasing histamine and other chemicals during allergic or inflammatory reactions. In MCAS, these cells become overly reactive, releasing too many chemicals in response to many different things, that are not actually a threat to the body.

Common Triggers: heat, stress, infections, exercise, and certain foods (including high-histamine foods).

Symptoms: MCAS symptoms are diverse and impact multiple systems in the body;

• Skin: hives, flushing, or persistent itching.
• Digestive System: abdominal pain, diarrhoea, nausea, and sensitivity to certain foods.
• Cardiovascular System: rapid heartbeat, low blood pressure, fainting, or palpitations.
• Nervous System: brain fog, difficulty concentrating, dizziness, or anxiety.
• Respiratory System: wheezing, nasal congestion, or shortness of breath.
• Behavioural Changes: increased anxiety, irritability, or difficulty focusing, often linked to the unpredictability of symptoms or physical discomfort.
• Joint Issues: pain or swelling in joints, which may be intermittent and triggered by inflammation.
• Headaches and Migraines: persistent or episodic

Because the symptoms are so varied, MCAS is often misdiagnosed, requiring careful evaluation by a specialist to look at the body as a whole.

Allergies

Unlike histamine intolerance or MCAS, allergies are specific immune system responses to certain proteins, such as those in peanuts, shellfish or pollen. When exposed to these allergens, the immune system mistakenly identifies them as threats and releases histamine to combat them.

Common Allergens: foods (nuts, dairy, eggs), environmental triggers (dust, pollen, mould) or insect stings.

Symptoms: allergic reactions can range from mild to severe, depending on the allergen and the individual:

• Skin: hives, swelling, redness, or itching.
• Digestive System: stomach cramps, vomiting, or diarrhoea (especially with food allergies).
• Respiratory System: sneezing, wheezing, nasal congestion, or difficulty breathing.
• Eyes: redness, itching, or watering.
• Severe Cases: anaphylaxis, characterized by swelling of the throat, difficulty breathing, a drop in blood pressure and potential loss of consciousness. This is a medical emergency and requires immediate attention.
• Behavioural Changes: increased clinginess, irritability, or restlessness in children, especially if they are experiencing discomfort or swelling.
• Joint Issues: rarely, allergic reactions may contribute to joint pain or swelling due to inflammatory responses.
• Headaches and Migraines: may occur in response to exposure to allergens, especially environmental ones.

Take home message

Navigating your child’s health when faced with unexplained symptoms can be both frustrating and worrying. While histamine intolerance, MCAS and allergies share similarities, understanding their unique features is the first step toward finding clarity.

Due to the significant overlap in symptoms, distinguishing between these conditions can be incredibly challenging, often requiring professional guidance. However, if we break it down, here are the key take home messages…

• Histamine Intolerance: symptoms such as flushing, nasal congestion, rashes, digestive issues (like nausea or diarrhoea) and headaches occur after consuming high-histamine foods (e.g. aged cheese, fermented products). These symptoms result from the body’s inability to break down histamine efficiently.
• MCAS: symptoms such as skin rashes, abdominal pain and dizziness can occur after eating, However, MCAS reactions effect multiple different body systems and tend to be triggered by various factors beyond food, such as temperature, physical activity or stress. Behavioural changes, headaches, and joint pain are also more common in MCAS.
• Allergies: Allergic reactions are typically more localised. For example (i) eating peanuts might lead to the lips swelling (ii) exposure to pollen might trigger sneezing and itchy eyes. Severe allergies can escalate to anaphylaxis, characterized by difficulty breathing and a life-threatening drop in blood pressure.

Each condition requires different approaches and finding the right support e.g. from a paediatrician, allergist or specialist dietitian.

You are not alone in this journey, with the right support, community and information you can help your child feel better and regain some peace of mind along the way.

Need help? Book a free inquiry call with Nishti here.