Food intolerance: which tests work and which do not

A food intolerance is a reaction to food that does not involve your immune system making antibodies against that food. That sounds like a technicality, and it decides which test is worth taking. The most popular tests on the Dutch market, the IgG panels covering 100 to 200 foods, measure exactly the wrong signal.

I think this is the least honest corner of the health market. Almost every page you find on the subject is selling the test it describes.

We do not sell intolerance panels. So we can simply write down what the research says.

What is a food intolerance exactly?

A food intolerance is a symptom reaction to food that does not run through your immune system. Usually an enzyme is missing, such as lactase for lactose, or your gut reacts to compounds that are poorly absorbed. The symptoms tend to stay in your abdomen: bloating, cramps, wind, or a change in bowel habit.

Dose matters. With an intolerance you can often handle a small amount fine, while a large portion causes symptoms. That differs fundamentally from an allergy, where a trace can be enough.

The Dutch Nutrition Centre, Voedingscentrum, describes lactose intolerance and coeliac disease as separate conditions with their own mechanisms. That separation matters, because they call for completely different tests.

What is the difference between food allergy and food intolerance?

In a food allergy your immune system makes IgE antibodies against a protein in the food. That reaction can start within minutes and show up outside your gut, with rash, swelling or breathing trouble. An intolerance runs without IgE, usually builds up more slowly, and generally stays limited to abdominal symptoms.

That difference drives the diagnostics. For a suspected IgE allergy there are validated tests: a skin prick test or specific IgE in blood, always alongside the clinical story.

For most intolerances no such blood test exists. There is no antibody to measure, because there is no antibody reaction.

Coeliac disease is the exception that feeds the confusion. It is not an intolerance but an autoimmune condition, and it does have a strong blood test. How that works is covered in gluten intolerance or coeliac disease.

How reliable is a food intolerance test?

That depends entirely on which test you mean. A hydrogen breath test for lactose and coeliac serology are well founded and used in hospitals. The broad IgG or IgG4 panels sold online are not: the European allergy body advised against them as a diagnostic tool back in 2008.

The table below is the core of this article. It separates what is validated from what is merely sold.

Look mainly at the last column. A test is only useful when the result changes your behaviour in a way that holds up.

Why do IgG tests produce so many hits?

Because IgG against food is a normal phenomenon. You make those antibodies against proteins you eat regularly, and you measure them just as readily in healthy people without symptoms. A panel scanning 200 foods therefore almost always returns dozens of positives.

The EAACI task force concluded in 2008 that IgG4 against food fits tolerance rather than disease, and advised against the test for food-related complaints (PMID 18489614). That position has not been reversed since.

The practical danger is in what happens next. An athlete who cuts 30 products based on such a list loses variety, often calories too, and sometimes whole food groups.

Take an 82 kilo crossfitter who drops wheat, egg, milk and yeast because they came back red. His protein intake falls, his carbohydrates fall with it, and after three weeks his training stalls. His gut may feel slightly better, but he has no idea which of the four did it.

That is the problem in one line: a list without ranking gives you no answer, only less food.

So how do you establish a food intolerance?

Through elimination and reintroduction, with guidance and a diary. You remove a suspected food for a few weeks, check whether symptoms measurably drop, then give it back in a controlled way. Only when symptoms return on reintroduction do you have a pointer worth anything.

That sounds old-fashioned next to a panel of 200 boxes. It is also the only method that puts cause and effect in the right order.

For irritable bowel symptoms a structured FODMAP process is often used, with an elimination phase and then a reintroduction phase per group. Reintroduction is where many people drop out, which is a shame, because that is where the information sits.

For suspected gluten sensitivity without coeliac disease, researchers use a double-blind challenge of around 8 grams of gluten per day, with a symptom difference of at least 30 percent between gluten and placebo (PMID 26096570). That shows how high the bar sits before you can genuinely demonstrate anything.

What I would do myself: start with two weeks of noting what you eat and how your gut responds. It costs nothing and yields more usable data than most tests you can buy.

What athletes tend to miss here

Hard training changes your gut temporarily, intolerance or not. During heavy exercise less blood reaches your intestines and permeability briefly rises, which can coincide with cramping, nausea or urgency. A systematic review calls this whole pattern exercise-induced gastrointestinal syndrome (PMID 28589631).

That matters, because the symptoms look strikingly like an intolerance. If your gut only complains on race days, the load is a better suspect than the bread.

Heat, dehydration and a high sugar intake during exercise all amplify the effect. One gel too many at kilometre 30 explains more than an IgG result for wheat.

Your head plays a part too. Stress raises small intestinal permeability through a mast cell dependent mechanism, shown in healthy volunteers (PMID 24153250). How that plays out is covered in gut symptoms and stress.

Before you blame a food, it is worth putting your training week, your sleep and your race nerves next to your symptom diary.

Which blood values actually add something here?

Blood work does not establish an intolerance, but it can show whether long-running gut symptoms have left traces. Think of values around iron, vitamin B12 and folate, which can drop when absorption is reduced. An inflammation marker also gives context to symptoms that will not settle.

A few values often included in this context:

• Ferritin, your iron store, which drops earlier in endurance athletes anyway
• Vitamin B12 and folate, both sensitive to absorption and to a narrow diet
• CRP as a general inflammation marker
• Coeliac serology, but only if you keep eating gluten until the draw

That last condition is the mistake made most often. Go gluten free first and get tested afterwards, and the test can come back negative while you do have the condition (PMID 31568151).

A broader panel such as 360 Health gives you these values in one draw, with a doctor's assessment. It does not replace the elimination process, but it fills in the other half of the picture.

If fatigue sits alongside your gut symptoms, also look at blood testing for fatigue.

What about food allergy tests?

For a suspected food allergy there are validated tests. A skin prick test and specific IgE in blood are used in care, always alongside your symptom history. A positive result on its own is not enough, because you can carry IgE against a food and still tolerate it perfectly well.

That is called sensitisation. It is the most common misunderstanding around allergy testing.

So a result is always read against practice. Do you get symptoms when you eat it, and how quickly? Where doubt remains, a supervised challenge sometimes follows.

Note the difference from the IgG panels. IgE belongs to an allergic reaction. IgG belongs to ordinary food exposure. One letter apart, a completely different story.

How long does an elimination trial take?

Expect two to four weeks of removal, followed by a reintroduction of a few days per food. Shorter than two weeks is usually too short to see a difference. Much beyond four weeks rarely adds anything and narrows your diet for no reason.

It is dull work. It is also the only route to an answer you can trust.

Test one food at a time. Drop wheat, dairy and onion in the same week and an improvement still tells you nothing about which one did it.

Schedule the trial for a quiet period too. An elimination week during a race block or a house move produces data you cannot use.

When is it probably not an intolerance?

Some symptoms fit the intolerance story badly and belong with your GP instead. Think of blood in your stool, fever, unintended weight loss, or symptoms that wake you at night. Symptoms starting for the first time after your fifties fall into that category too.

These are not reasons to panic. They are reasons to have a doctor look with you rather than starting a diet on your own.

The same applies if you have cut many food groups in a short time. A heavily restricted diet can cause symptoms by itself, and then you are going in circles.

If coeliac disease runs in your family, say so early. It changes how likely the diagnosis is, and it makes a blood test more valuable rather than less.

The rest of this cluster

Every common question here has its own trap, and they are too specific for one article. These four go deeper on the individual pieces.

• Lactose intolerance test, on the breath test, the DNA test and why the results can disagree
• Gut flora and athletic performance, on what a microbiome test really tells you
• Leaky gut, on a real mechanism and a questionable test
• Gluten intolerance or coeliac disease, on the one blood test here that genuinely holds up

Where I would start

Not with a test, but with two weeks of notes. Write down what you eat, how your gut responds, how hard you train and how you sleep. Those four columns side by side almost always surface a suspect, and that is exactly where an elimination trial begins.

If symptoms persist, or you lose weight without meaning to, discuss it with your GP before cutting food groups on your own.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Stapel SO, Asero R, Ballmer-Weber BK, et al. Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. 2008;63(7):793-796. PMID 18489614.
• Catassi C, Elli L, Bonaz B, et al. Diagnosis of Non-Celiac Gluten Sensitivity (NCGS): The Salerno Experts' Criteria. Nutrients. 2015. PMID 26096570.
• Husby S, Koletzko S, Korponay-Szabo I, et al. European Society Paediatric Gastroenterology, Hepatology and Nutrition Guidelines for Diagnosing Coeliac Disease 2020. J Pediatr Gastroenterol Nutr. 2020;70(1):141-156. PMID 31568151.
• Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome. Aliment Pharmacol Ther. 2017. PMID 28589631.
• Vanuytsel T, van Wanrooy S, Vanheel H, et al. Psychological stress and corticotropin-releasing hormone increase intestinal permeability in humans by a mast cell-dependent mechanism. Gut. 2014;63(8):1293-1299. PMID 24153250.