DIGESTION AND REACTIONS TO FOOD

Food allergy or food intolerance? Why does the body react to food?

Bloating, stomach pain, diarrhea, rash, or discomfort after eating? A reaction does not automatically mean an allergy. Here’s how we distinguish between allergies, intolerances, and other food reactions—and work out what may be worth investigating further.

Paulina Borkowska
Clinical Nutritionist and functional medicine practitioner with over 12 years of experience in Poland and Norway.
12 min read

“Is it an allergy, or is it just my stomach?”

Some of the questions we hear most often in the clinic:

“Milk gives me stomach pain and makes me bloated—could I be allergic?”
“Bread gives me stomach pain—is it the gluten?”
“I get a rash after eating—is it an intolerance?”
“I used to tolerate everything. Now I react to more and more foods. Why?”

These are real, valid questions—and the answer is rarely simple. Very similar symptoms can come from entirely different mechanisms in the body: the immune system, digestion, the intestinal lining, or the nervous system can all play a role, individually or together.

That’s why we ask a slightly different question than “what am I allergic to?”:

When your body reacts to food—what is it actually reacting to?

This article won’t diagnose you based on a single symptom. It will help you understand the mechanisms involved, what questions may be relevant to ask next, and who should investigate what.

First, let’s clear up the terminology

“Allergy” and “intolerance” are often used interchangeably in everyday conversation—but they are not the same, and they are not investigated in the same way.

The table provides a simplified overview. It is not a substitute for an individual medical assessment, and neither symptoms nor how quickly a reaction occurs are enough on their own to make a diagnosis.

Food allergy—when the immune system reacts

With a true food allergy, the immune system recognizes a protein in food as a threat. In IgE-mediated reactions, the body produces specific antibodies against this protein. On subsequent exposure, substances including histamine are released, which can cause symptoms such as rash, swelling, itching in the mouth/throat, digestive problems, or difficulty breathing, usually soon after eating.

The reaction is not limited to the digestive system. Because the immune system is involved throughout the body, an allergic reaction can potentially affect the skin, airways, and circulation at the same time. That’s why a thorough medical history and allergy assessment—with a skin prick test and/or specific IgE measurement, interpreted by a doctor or allergist—are essential to confirm or rule out a suspected allergy.

Food intolerance—when the problem isn’t a classic allergy

“Food intolerance” is a catch-all term covering several different mechanisms that do not necessarily involve the immune system. The best-known—and most illustrative—example is lactose intolerance.

Lactose is the sugar in milk. For the body to absorb it, the enzyme lactase must break it down in the small intestine. If lactase activity is lower than needed, some of the lactose remains undigested in the intestine. There, it can draw in fluid and be fermented by gut bacteria, which can cause bloating, gas, stomach pain, and loose stools—typically 30 minutes to a few hours after consumption.

A hydrogen breath test is one of the tools used to assess lactose malabsorption, and should be interpreted by a doctor or other qualified healthcare professional alongside the symptoms.

Lactose-free does not mean dairy-free

This is one of the most practical distinctions to understand—and one we most often have to explain in the clinic.

Milk contains two completely different components that the body can react to, and they have nothing to do with each other:

LACTOSE—the milk sugar. The problem here is a digestive enzyme (lactase) that is not active enough. This is an intolerance, not an allergy.

MILK PROTEIN—including casein and whey proteins. A reaction to milk protein is an immune response—in other words, a true allergy.

In other words, a lactose-free milk drink is still milk. It still contains milk protein. It is safe for someone with lactose intolerance, but not for someone with a milk protein allergy.

Why have I suddenly stopped tolerating milk?

Some people have tolerated dairy products all their lives, then find that their tolerance disappears. One possible—but by no means the only—explanation is secondary lactose intolerance.

Lactase is produced by cells on the outer surface of the intestinal villi in the small intestine. If this lining is damaged—for example, by untreated celiac disease or other intestinal inflammation—lactase production can decrease, even in someone who previously tolerated milk well. If the underlying cause is treated, the intestinal lining may recover in some people, and lactose tolerance may improve over time. This does not happen automatically for everyone, and the timeline varies from person to person.

This is a principle we see time and again in PureNordic’s approach:

Sometimes food is the problem. Other times, food tells us that the digestive system needs a closer look.

Gluten—the key distinctions to know

Gluten is a big topic in its own right and deserves a full article of its own . Here, we’ll simply clarify the three terms that are often confused:

Celiac disease—an autoimmune condition in which gluten triggers an immune reaction that damages the lining of the small intestine. Requires a strict, lifelong gluten-free diet.

Wheat allergy—a true IgE-mediated allergy to proteins in wheat, separate from celiac disease.

Non-celiac gluten/wheat sensitivity—symptoms associated with consuming gluten or wheat in people who have been ruled out for both celiac disease and wheat allergy.

When you react to many different foods

Some people experience discomfort after eating a wide range of seemingly unrelated foods—red wine, aged cheese, tomatoes, chocolate, fermented foods. This pattern can have several explanations, and histamine is one of the topics that may sometimes be worth investigating further.

We do not diagnose histamine intolerance based on symptoms alone—it requires a more thorough assessment. If you recognize yourself in a broad range of symptoms after eating many different foods, our full guide to histamine may be a good next step:

→ Read our article on histamine intolerance

“My test showed 25 intolerances”—which tests actually tell us something?

This is one of the most important things to understand in this entire article.

There are many different tests on the market, and they answer very different questions:

Allergy testing(skin prick test, specific IgE)—investigates whether the immune system reacts to a specific food protein through an IgE-mediated response.

Celiac disease testing(antibodies + intestinal biopsy)—investigates the autoimmune reaction to gluten while you are still eating it.

Hydrogen breath test—investigates malabsorption of substances such as lactose in the intestine.

Elimination and reintroduction under guidance—a structured clinical method for mapping the relationship between food and symptoms over time.

Commercial IgG panels—measure IgG antibodies to foods, which can be present in healthy people and do not indicate an allergy or intolerance. This is not a recognized standard method for diagnosing food allergy or food intolerance, and the results should be interpreted with great caution.

The point is not that one test is inherently “better” than another:

The right test depends on the question we are trying to answer.

Why is the list of foods I can’t tolerate getting longer and longer?

This may be the most important pattern we see in the clinic. It often starts like this: first milk. Then gluten. Then eggs. Then onions. Then fruit. Eventually, you are left with fewer and fewer foods you feel safe eating.

Is the solution always to eliminate even more foods? Not necessarily.

When more and more foods cause discomfort, it is often worth looking at the entire digestive process as a whole, rather than at each individual food separately:

Stomach → enzymes → bile → gut motility → small intestine → microbiome

Discomfort after eating does not always mean that the body is immunologically “rejecting” a particular food. Often, one or more parts of this process are not working optimally—and then many different foods can cause symptoms, not because they are inherently dangerous, but because the digestive system is overloaded.

→ Find out more about what affects your digestion
→ Learn more about IBS and a sensitive gut

Elimination should have a goal—and a plan

An elimination diet can be a useful tool—but only when it is structured:

Suspected trigger → targeted elimination → observation → controlled reintroduction (when medically safe) → assessment

The exception is conditions that require permanent elimination, such as confirmed celiac disease, where gluten must be avoided for life.

Uncontrolled, ever-expanding elimination without a plan comes at a real cost: the risk of insufficient energy and protein intake, nutrient deficiencies, an increasingly limited and less varied diet, and—not least—increasing anxiety around eating. The goal of an elimination diet is never to remove as much food as possible, but to answer a specific question and then rebuild the diet.

The PureNordic approach: Build up your diet—don’t just eliminate foods

Once one or more foods have been identified as problematic, the next step is just as important as the elimination itself: building a balanced, nutrient-rich diet around what you can actually tolerate.

Our recommendation is based on a simple, anti-inflammatory food framework—protein + vegetables + healthy fats—tailored to your individual tolerance, not a generic template.

→ See our anti-inflammatory meal plan

Why do we look at digestion before eliminating even more foods?

Digesting a meal involves a series of steps that need to work in the right order: stomach acid helps break down protein and activate digestive enzymes, enzymes from the pancreas help further break down protein, fat, and carbohydrates, and bile is needed to digest fat. If any of these steps fail, even foods you “should be able to tolerate” can cause discomfort.

This is why Digestive Support is formulated the way it is—with digestive enzymes, betaine HCl, ox bile, herbal ingredients, and Saccharomyces boulardii, which together support several of these digestive processes.

Reacting to food—what should you do now?

Immediate/allergic reaction(swelling, difficulty breathing, rash shortly after eating)
→ See a doctor or allergist. If there are signs of anaphylaxis, call 113.

Reaction to milk
→ Find out whether lactose or milk protein is the problem—the mechanisms and how to manage them are very different.

Reaction to bread/wheat
→ Rule out celiac disease first, while you are still eating gluten. Don’t remove gluten on your own before this has been clarified.

Reaction to many different foods
→ Consider the entire digestive process as a whole, not just individual foods.

The list of foods you can’t tolerate keeps getting longer
→ Don’t keep eliminating foods without a plan. Talk to a doctor or Clinical Nutritionist about a structured assessment.

Digestive problems combined with reactions to many different foods
→ Read about histamine intolerance

Recognize yourself in several of these patterns?

During a consultation, we look at your diet, symptom patterns, previous blood tests, digestion, and any previous elimination attempts—to understand which investigations are actually relevant to you and how to safely rebuild your diet.

Book a consultation →

Dietary supplements are not a substitute for a varied diet and a healthy lifestyle. If you suspect a food allergy, celiac disease, or another medical condition, are pregnant, or are taking medication, discuss dietary changes and supplement use with a doctor or other qualified healthcare professional. Individual needs vary. This article is intended for general information and is not a substitute for individual medical advice.