I react to almost everything. What does that suggest?

Reacting to a very wide and shifting range of foods is more consistent with a mediator or handling problem than with many separate allergies, because true IgE allergies are usually few, stable and specific. That is a pattern worth reporting, not a diagnosis.

Do reactions have to happen straight away?

No, and timing is a weaker clue than it is usually presented as. Immune allergy tends to be fast and reproducible. The other mechanisms are less predictable, and a delayed reaction is compatible with several explanations, including ones that have nothing to do with the last thing you ate. Record the timing, but do not let it decide the question.

Why does the same food affect me differently on different days?

That pattern points away from a fixed problem with one food and towards something with a threshold, where the total load matters more than any single item. It is also why one-off trials mislead so often: the same food tested twice can give two answers without either being wrong. Worth reporting exactly as it happens rather than smoothing it into a list.

Is it MCAS or a food intolerance?

By BeatSIBO editorial team · Updated August 6, 2026

At a glance

Mechanism
MCAS
Mediator release, not an allergen response
Food intolerance
Enzymatic or absorptive
Food list shape
MCAS
Wide and shifting
Food intolerance
Specific and stable
Confirmatory test
MCAS
Mediator measurement during a reaction
Food intolerance
Breath test for the specific sugar
IgG panels
MCAS
Not useful
Food intolerance
Not useful

MCAS compared with Food intolerance, on the dimensions that separate them
Dimension	MCAS	Food intolerance
Mechanism	Mediator release, not an allergen response	Enzymatic or absorptive
Food list shape	Wide and shifting	Specific and stable
Confirmatory test	Mediator measurement during a reaction	Breath test for the specific sugar
IgG panels	Not useful	Not useful

The one that matters: A list of trigger foods that grows and changes month to month is a different shape of problem from several separate allergies.

One word is doing three jobs here, and the reason people get stuck is that the word hides the difference.

What actually separates them

The short answer

Three mechanisms share the phrase food reaction. An IgE allergy is immune and testable. An intolerance is enzymatic or absorptive, like lactose, and has a breath test. Mediator release is mast cells responding to something that is not an allergen. Same words, three mechanisms, three different investigations, and only two of the three have accepted tests.

The pattern that carries the most information

The short answer

Reacting to a very wide and shifting list of foods is more consistent with a mediator or handling problem than with many separate allergies. True IgE allergies are usually few, stable and specific, so a list that grows and changes month to month is a different shape of problem. That is a pattern worth reporting, not a diagnosis.

We are being careful with the wording because this is exactly the observation that gets turned into a confident claim elsewhere. It narrows the field. It does not close it.

Why the popular test does not help

Broad IgG food sensitivity panels are not endorsed by major allergy bodies for diagnosing intolerance. IgG to a food reflects exposure rather than reaction, so the panel tends to flag whatever you eat most often. The result is a long list that feels like an answer and is substantially a record of your diet.

What is not settled here

Where a food-triggered mediator response ends and a food intolerance begins is not cleanly defined, and clinicians disagree. Histamine complicates it further, since some foods carry it and some mediator responses release it, so the same meal can produce the same symptom by two different routes. Nobody can currently apportion that in an individual.

Why eliminating everything makes this harder

All three mechanisms improve when you remove the food. So a broad elimination that helps tells you something among the removed foods mattered and nothing about which mechanism was involved, while making a structured investigation much harder.

Sanchez-Perez S et al., Nutrients 2021 Peer-reviewed study

It is also worth knowing how well-founded the popular lists are. Across ten published low-histamine diets, 68% of excluded foods showed no significant histamine content, and citrus was excluded by 60% of them despite none being detectable.

What to do next

Get the testable things tested. IgE allergy has established testing; lactose and fructose malabsorption have breath tests with published protocols. Excluding those narrows the field considerably before anyone reaches for a label that has no test at all.

Common questions

I react to almost everything. What does that suggest?: Reacting to a very wide and shifting range of foods is more consistent with a mediator or handling problem than with many separate allergies, because true IgE allergies are usually few, stable and specific. That is a pattern worth reporting, not a diagnosis.
Do reactions have to happen straight away?: No, and timing is a weaker clue than it is usually presented as. Immune allergy tends to be fast and reproducible. The other mechanisms are less predictable, and a delayed reaction is compatible with several explanations, including ones that have nothing to do with the last thing you ate. Record the timing, but do not let it decide the question.
Why does the same food affect me differently on different days?: That pattern points away from a fixed problem with one food and towards something with a threshold, where the total load matters more than any single item. It is also why one-off trials mislead so often: the same food tested twice can give two answers without either being wrong. Worth reporting exactly as it happens rather than smoothing it into a list.