Mast Cell Activation Syndrome (MCAS): Symptoms, Testing & Naturopathic Support

What Is Mast Cell Activation Syndrome (MCAS)?

The reason MCAS can produce such a confusing collection of symptoms becomes clearer when you understand what mast cells actually do.

Mast cells are immune cells found throughout the body, particularly in places that regularly come into contact with the outside world, such as your skin, airways and digestive tract. They’re part of your normal defence system, designed to recognise potential threats and release chemical messengers that help coordinate an immune response.

Histamine is one of those messengers, but it isn’t the only one. Mast cells can release many different mediators, which helps explain why their activation can potentially produce symptoms that seem completely unrelated.

And mast-cell activation itself isn’t abnormal. Your mast cells are supposed to react when there’s a good reason for them to.

The problem is when that response becomes excessive, inappropriate or repeatedly occurs when the apparent trigger doesn’t seem to justify the size of the reaction.

MCAS describes a specific pattern of recurrent symptoms affecting multiple systems of the body, together with evidence that mast-cell mediators are involved.

That means having reactions that look like mast-cell activation isn’t automatically the same thing as having Mast Cell Activation Syndrome.

What Are the Symptoms of MCAS?

One of the things that can make mast-cell symptoms so confusing is how quickly a reaction can seem to gather momentum.

You might start feeling hot and flushed, then notice your heart racing. Your stomach begins cramping, your nose blocks and suddenly you feel light-headed. What looked like one small reaction has become something affecting several parts of your body at once.

Mast-cell activation can cause symptoms such as:

• • flushing, itching, hives or swelling
  • nasal congestion, sneezing or watery eyes
  • wheezing, coughing or throat tightness
  • abdominal cramping, nausea or diarrhoea
  • headaches or migraines
  • palpitations or a racing heart
  • dizziness, light-headedness or fainting
  • changes in blood pressure
  • fatigue or feeling unusually depleted
  • brain fog or difficulty concentrating

Some reactions are relatively mild. Others can be much more significant, and severe mast-cell activation can cause anaphylaxis, including difficulty breathing, significant swelling or a dangerous fall in blood pressure that requires urgent medical treatment.

But here’s where the symptom list becomes less useful than it first appears.

You can have flushing without MCAS. You can have migraines, diarrhoea, palpitations or dizziness without MCAS. Even experiencing several of these symptoms doesn’t automatically establish the diagnosis.

What becomes more interesting is when several of them repeatedly appear together during the same episodes.

Why Can MCAS Reactions Feel So Unpredictable?

One of the most frustrating things about mast-cell reactions is that the rules seem to keep changing.

You eat tomatoes one evening and feel completely fine. A few days later, the same meal leaves you flushed, congested and with your heart racing. You tolerate a supplement for weeks, then suddenly react to it. Or you start avoiding the foods you’ve blamed for your symptoms, only to find yourself reacting to something completely different.

It’s easy to conclude that your list of triggers is simply getting longer.

But sometimes the individual trigger isn’t the whole story.

Think of mast cells as having a threshold for activation. On a day when your system is relatively settled, one potential trigger may not be enough to produce noticeable symptoms. But add poor sleep, an infection, emotional stress, heat, exercise, alcohol, allergens, hormonal changes or several triggers close together, and that same thing may now be enough to tip you over your threshold.

This doesn’t mean those triggers aren’t real. It means your tolerance for them may not be fixed.

That distinction can explain why endlessly searching for the one food, supplement or environmental exposure responsible for every reaction can become so frustrating. The question isn’t always “What was the last thing I ate or took?” Sometimes it’s “What else was happening in my body when I reacted?”

Perhaps your reactions reliably intensify before your period. Maybe they’re worse after several nights of poor sleep, during a stressful period, in hot weather or after an illness. You may even notice that several things you normally tolerate suddenly become problematic at the same time.

Those patterns can tell you something a list of individual triggers can’t.

The thing that tipped you over the threshold isn’t necessarily the thing that created the reactivity in the first place.

And understanding that difference can shift the focus from continually adding things to your avoidance list towards investigating why your threshold for reacting may have changed.

MCAS vs Histamine Intolerance: What’s the Difference?

If you react to wine, fermented foods or leftovers, become flushed or congested after eating, or notice headaches and digestive symptoms after higher-histamine foods, histamine intolerance can seem like the obvious explanation.

And sometimes it is.

But histamine intolerance and MCAS aren’t simply two names for the same problem.

With histamine intolerance, the issue is thought to involve the amount of histamine your body is dealing with relative to its ability to break it down. This is why factors such as dietary histamine, gut health and the activity of enzymes involved in histamine metabolism, including DAO, can become relevant.

With MCAS, the problem sits further upstream. Your mast cells themselves are being activated and releasing mediators – and histamine is only one of them.

That distinction can help explain why someone with mast-cell activation may react to things that have very little to do with how much histamine was in their last meal. Heat, exercise, hormonal changes, fragrances, alcohol, medications, infections or emotional stress may all be part of the pattern.

It also explains why a low-histamine diet can make someone feel significantly better without necessarily explaining the whole problem. Reducing histamine may lower one part of the overall load, while the question of why mast cells are becoming so readily activated remains.

So feeling better when you reduce histamine doesn’t automatically tell you that histamine intolerance is the entire story.

Sometimes the more useful clue is what happens beyond food – because when reactions are being triggered by multiple, seemingly unrelated things, it may be the behaviour of the mast cells themselves that needs a closer look.

Why Do Some Women Develop Mast Cell Activation Symptoms?

When your body suddenly seems more reactive than it used to be, it’s natural to look back and wonder what changed.

The difficulty is that mast cells don’t operate in isolation. They communicate with the immune and nervous systems, respond to hormonal signals and can be influenced by what’s happening elsewhere in the body.

So identifying mast-cell activation is only part of the investigation.

The next question is what changed around the time your reactions changed.

MCAS and Hormonal Changes

There are weeks when you can eat fairly normally, exercise, have a glass of wine or take your usual supplements without thinking twice.

Then suddenly everything seems to set you off.

Your face flushes after dinner. Your nose blocks for no obvious reason. Your heart starts racing in bed. Foods you ate perfectly happily the week before suddenly make you itchy, bloated or headachy.

And just as you start wondering whether you’ve developed another intolerance, your period arrives – and things begin to settle.

When that pattern repeats month after month, the timing becomes difficult to ignore.

Mast cells can respond to sex hormones, including oestrogen and progesterone. Experimental research suggests oestrogen can influence mast-cell activation and histamine release, while progesterone may have a different, potentially inhibitory influence on mast-cell activity.

This may help explain why the same trigger can affect you differently depending on where you are in your cycle. For some women, that becomes particularly noticeable around ovulation or before a period. During perimenopause, when hormonal fluctuations can become less predictable, the pattern itself may become less predictable too. Changes can also sometimes become apparent after starting or altering hormonal medication.

The important clue isn’t simply that your symptoms are “hormonal”.

It’s that something you tolerate perfectly well at one point in your cycle may become enough to provoke symptoms at another.

So before adding another food, supplement or product to the list of things your body apparently “can’t tolerate”, it can be worth asking a different question:

Where was I in my cycle when I reacted?

That pattern may tell you far more than the trigger alone.

MCAS, Mould and Environmental Exposure

Mould often enters the MCAS conversation as a potential “root cause”. But exposure and causation are not the same thing.

Mast cells are part of the body’s frontline immune defence, particularly at surfaces such as the airways and skin. Mould, fungal components, allergens and other environmental exposures can interact with immune pathways capable of activating them.

So environmental exposure can absolutely be relevant to someone experiencing mast-cell symptoms. What matters is establishing how relevant it is to your presentation.

A history of symptoms beginning or significantly worsening in a particular building, improving when you’re away from that environment, or recurring when you return carries very different clinical weight from simply discovering mould in a home or receiving an abnormal mycotoxin result.

And even when mould exposure is relevant, it doesn’t automatically mean it caused MCAS. Mould can affect the body in several different ways, and some of those effects can look very similar to mast-cell symptoms.

So if your symptoms started or worsened in a water-damaged building, that matters – but it doesn’t necessarily tell us that mast cells are the whole explanation.

That distinction prevents two very different questions from becoming confused:

“Has mould exposure affected my health?” and “Is mould the reason my mast cells are behaving this way?”

Sometimes the answer to both may be connected. But they shouldn’t be assumed to be the same.

Can Infections Trigger Mast Cell Symptoms?

Mast cells aren’t just involved in allergies and histamine reactions. They’re immune cells – and responding to infection is part of their job.

They can recognise signals associated with viruses, bacteria and other pathogens, release inflammatory mediators and communicate with other parts of the immune system to help coordinate a defence response.

Usually, that’s exactly what you want them to do.

But some people notice that their pattern of reactivity changes following an infection. The fever and acute illness are long gone, yet months later they’re experiencing flushing, palpitations, digestive reactions, headaches or sensitivities they didn’t have beforehand.

This has become particularly visible following COVID-19, where researchers have been investigating whether persistent immune activation, altered autonomic signalling and mast-cell pathways may contribute to symptoms in a subset of people with Long COVID.

Importantly, post-infectious mast-cell symptoms don’t necessarily mean the original infection is still active.

That’s a meaningful distinction if you’ve been searching for evidence that a virus or infection must still be hiding somewhere because you haven’t felt like yourself since you were sick.

The infection may have changed the immune environment without needing to still be present to influence what happens next.

Why Do MCAS and POTS Sometimes Appear Together?

MCAS and POTS are often spoken about together, particularly online, to the point where having one can start to feel like evidence that you probably have the other.

The relationship is more complicated than that.

POTS is a form of dysautonomia involving abnormal regulation of heart rate and circulation when upright. MCAS involves inappropriate episodes of mast-cell mediator release. They are distinct conditions, although research suggests they may coexist in some people.

Palpitations, dizziness, fatigue, brain fog, digestive symptoms and intolerance to heat or exercise can occur with POTS without mast cells being responsible for them. Equally, mast-cell mediator release can affect blood vessels and contribute to cardiovascular symptoms.

So a racing heart isn’t automatically a mast-cell reaction just because someone also experiences flushing or food reactions.

The circumstances around the symptom become more informative than the symptom itself.

A heart rate that reliably rises when you stand and improves when you lie down points towards an autonomic pattern. An episode involving flushing, itching, swelling, wheezing or gastrointestinal symptoms alongside cardiovascular changes raises a different set of questions.

And some women genuinely appear to have both patterns operating together.

The important distinction is that overlap doesn’t establish causation. Current research has not established that MCAS causes POTS, or that POTS causes MCAS.

Recognising where the two conditions overlap – and where they don’t – can prevent one diagnosis being asked to explain symptoms that may actually be coming from two different systems.

Can Stress, Trauma and the Nervous System Affect Mast Cells?

Being told your symptoms are “just stress” when you’re experiencing flushing, palpitations, digestive reactions or feeling physically unwell is incredibly dismissive.

But there is another problem with that explanation: it treats the nervous system and immune system as though they have nothing to do with each other.

They do.

Mast cells and the nervous system communicate in both directions. Mast-cell mediators can activate nearby nerves, while signals released during a stress response can influence mast-cell activity. This creates the potential for immune activation to affect how the nervous system responds – and for the nervous system to influence how readily the body reacts.

That doesn’t mean stress or trauma “caused your MCAS”. And it certainly doesn’t mean the symptoms are psychological.

It means the state of the nervous system can become one of the variables affecting a very real biological response.

This becomes particularly interesting when reactions have been happening for a long time.

The nervous system is plastic – it learns from repeated experience. If the brain and body repeatedly encounter something alongside a significant physical reaction, they can become increasingly efficient at recognising the circumstances surrounding that reaction and preparing for threat.

In some people, this may contribute to sensitisation, where the alarm system becomes easier to activate and symptoms are amplified by cues that have become associated with previous reactions. Stress, anticipation, particular environments or even the expectation of becoming unwell can then potentially influence the intensity of the response.

Importantly, that doesn’t mean every mast-cell reaction is learned, or that neuroplasticity explains MCAS itself. Mast-cell activation and nervous-system sensitisation can coexist.

And that distinction opens up a much more useful possibility.

If part of the reactivity has become reinforced through nervous-system signalling, that component isn’t necessarily fixed. Neuroplasticity works in both directions – the nervous system can learn patterns of threat, but it can also learn greater safety and regulation.

So addressing the nervous system isn’t about convincing yourself that nothing is wrong.

It’s about recognising that changing an overprotective alarm response may be one part of changing how easily the whole system is triggered.

How Is MCAS Diagnosed and Tested?

MCAS is one of those conditions where recognising yourself in the symptom list is not enough to establish the diagnosis.

You may have flushing, food reactions, palpitations, diarrhoea, headaches and sensitivity to medications. You might even feel dramatically better when you take an antihistamine.

Those are useful clues. But they don’t, on their own, prove MCAS.

Under widely used consensus criteria, diagnosis generally requires three pieces of evidence to line up:

1. 1. Recurrent episodes consistent with systemic mast-cell activation, typically involving two or more organ systems at the same time.
   2. Objective evidence that mast-cell mediators increased during an episode. The best-established marker is serum tryptase, comparing a sample taken during a reaction with your usual non-event level.
   3. Improvement with treatment that targets mast-cell mediators or mast-cell activation.

This is where testing can become frustrating, because most people being investigated for MCAS don’t already have a baseline tryptase result sitting in their pathology history.

Fortunately, you don’t need one from before your symptoms began.

A baseline sample can be taken when you’re not experiencing an acute reaction. Then, if another significant episode occurs, tryptase can be measured soon after symptoms begin and compared with that non-event level.

Sometimes it happens the other way around. Tryptase is collected during a significant reaction, and a baseline sample is taken later, once the episode has completely resolved.

And importantly, the question isn’t simply whether your tryptase falls inside or outside the laboratory reference range.

Someone can have a baseline tryptase within the normal range and still experience a meaningful rise during an episode. Clinicians therefore look at how much the level increased relative to your own baseline, using a recognised calculation commonly called the 20% + 2 rule.

Other mast-cell mediators can sometimes be assessed, including urinary histamine metabolites, prostaglandin metabolites and leukotriene E4. These can provide additional information in some cases, although testing is less standardised and they don’t replace the better-established tryptase criterion.

This is also why a DAO result, high histamine result or commercial “mast-cell panel” shouldn’t be treated as proof of MCAS.

The diagnosis comes from bringing the symptom pattern, objective evidence and response to mast-cell-directed treatment together – while also making sure another condition isn’t providing a better explanation.

MCAS isn’t diagnosed by having enough symptoms that sound like MCAS.

And when the testing doesn’t support the diagnosis, that doesn’t make the symptoms any less real. It simply means the investigation needs to stay open to what else could be creating the same pattern.

What Else Can Cause Symptoms That Look Like MCAS?

Flushing, palpitations, diarrhoea, dizziness, headaches, fatigue and food reactions aren’t exclusive to MCAS.

Depending on the presentation, similar symptoms can occur with allergies, histamine intolerance, dysautonomia and POTS, medication reactions, gastrointestinal conditions, hormonal changes, mould-related inflammatory illness and other immune or inflammatory conditions. Recurrent severe reactions may also require investigation for other mast-cell disorders, including mastocytosis.

Sometimes one diagnosis does explain a remarkable amount. Sometimes several different problems are contributing to what looks like one complex symptom picture.

The value of considering MCAS isn’t in making every symptom fit the label. It’s in establishing whether mast-cell activation genuinely explains the pattern better than the alternatives.