Food intolerances: which tests make sense and which ones will you just pay for unnecessarily?

IgG testy potravinových intolerancií

Bloating after eating, feeling of heaviness, foggy head, fatigue, abdominal pain or irregular bowel movements. When such problems recur, it is natural to look for their cause.

And this is where the very lucrative test market begins.

All it takes is a blood test, and within a few days, you'll have a colorful chart of dozens or hundreds of foods divided into "good" and "bad" foods. Other tests promise to analyze your gut microbiome from a stool sample and tell you which bacteria you're missing, what you should eat, and what needs to be fixed. The problem is that just because you can measure something doesn't mean the result can diagnose the cause of your problems .

The most typical example is the IgG blood tests sold as a diagnostic for food intolerances. If there is one practical piece of information to take away from this article, it is this:

IgG food intolerance panels have no proven value in diagnosing food intolerance. From the perspective of finding the cause of digestive problems, they are generally a waste of money.

And it is very important not to confuse them with IgE tests. IgE and IgG are not two versions of the same test.

IgE vs. IgG: Similar letters, completely different meaning

When someone says "food blood test," you always have to ask: what exactly is being measured in the blood?

A classic food allergy can be mediated by IgE antibodies. The reaction is typically rapid – for example, hives, itching, swelling of the lips or tongue, vomiting, wheezing, or difficulty breathing. In the most severe cases, anaphylaxis can occur.

If there is a reasonable suspicion of such an allergy, an allergist may use skin prick tests or a blood test for specific IgE antibodies.

However, even a positive IgE does not automatically mean a clinical allergy. The result must be interpreted in conjunction with what actually happens to the patient after consuming a particular food .

IgG: does not indicate food intolerance

A completely different situation occurs with IgG or IgG4 tests.

Food-specific IgG antibodies may be a normal consequence of our immune system regularly encountering a given food. Their presence alone does not mean that the food is harmful to us. In fact, it may be a manifestation of immunological contact and tolerance rather than evidence of an adverse reaction.

Therefore, a person may have high IgG levels against foods they regularly consume.

And here arises the fundamental problem of commercial "intolerance tests".

The patient receives a result with a lot of red boxes and gets the impression that the laboratory has objectively identified the foods causing their problems. In fact, the test may not show the cause of the problems at all.

The IgG panel is not a food allergy test and is not a validated food intolerance test. Therefore, professional allergy societies do not recommend the use of food-specific IgG/IgG4 tests for the diagnosis of food intolerance or allergy.

So if someone offers you a panel of dozens or hundreds of foods with the promise that they will find out "what your body does not tolerate" from IgG antibodies, great skepticism is in order.

Why does a person sometimes feel better after a diet based on the IgG test?

This is a very good question. And it is the experience of "the test helped me" that keeps these tests alive. Imagine a person who regularly consumes pastries, dairy products, sweets, convenience foods, and a host of other foods. The test will mark ten or twenty items for him, and he will start consistently skipping them.

His diet will change dramatically. He may limit lactose. He may reduce his intake of some FODMAP carbohydrates. He may stop eating large portions or ultra-processed foods. He may start eating more regularly. And he may feel better.

However, this does not yet prove that the IgG test correctly identified the problem foods.

Improvement after an extensive elimination diet and the diagnostic accuracy of a test are two different things.

Allergy and intolerance are not synonymous

Food allergy and food intolerance represent different mechanisms.

IgE-mediated allergy is a specific reaction of the immune system. Even a small amount of allergen can cause a significant reaction in a sensitive person, and in some cases the allergy can be life-threatening.

Intolerance is often a problem with the processing of a certain food component. A typical example is lactose intolerance, which is caused by insufficient activity of the lactase enzyme.

The dose is also important here . A person may tolerate a small amount and experience bloating, abdominal pain, or diarrhea after a larger amount. However, even this division does not explain all reactions to foods.

Not everything is an allergy or intolerance

Celiac disease is an autoimmune disease. In genetically predisposed individuals, gluten triggers an immune reaction that damages the lining of the small intestine. It is not a classic IgE allergy or a simple food intolerance.

If celiac disease is suspected, there is a standardized diagnosis based on the relevant antibodies and, depending on the situation, on endoscopy with biopsy of the small intestine.

It is very important not to arbitrarily start a gluten-free diet before the diagnosis is complete.

If a patient does not consume gluten for a longer period of time, the test results may be distorted, making the confirmation or exclusion of celiac disease unnecessarily complicated.

There is also non-celiac sensitivity to wheat or gluten. In some patients , the problem may not be the gluten proteins themselves, but rather, for example, the fructans contained in wheat.

Another example is FODMAPs. – a group of fermentable carbohydrates that can cause bloating, pain and stool changes in sensitive people. Increased intestinal sensitivity, typical for irritable bowel syndrome, for example, also plays a significant role here.

So the same "I feel bloated after eating" can have many different causes.

That's why the idea of one universal blood test that can pick out the culprits from hundreds of foods is so tempting - and problematic at the same time.

So what can actually be investigated for intolerances?

Skepticism about IgG panels does not mean that all tests are useless. If lactose intolerance is suspected, a hydrogen breath test can be used. If lactose is not sufficiently broken down in the small intestine, it reaches the large intestine, where bacteria ferment it. The hydrogen produced is then captured in the exhaled air.

A similar principle is used for fructose, but the interpretation of the results can be more complicated. The absorption capacity of fructose is limited even in healthy people and the result depends, among other things, on the dose used. For some patients, it may therefore be more practical to carry out a well-conducted dietary trial and monitor the relationship between the amount of a particular substance and symptoms.

What about histamine intolerance?

Histamine intolerance is associated with a presumed imbalance between the amount of histamine and the body's ability to break it down, with the role of the enzyme diamine oxidase – DAO often being discussed.

The problem is that the symptoms attributed to histamine intolerance are very non-specific: headaches, flushing, digestive problems, heart palpitations, or skin manifestations can have many other causes.

Therefore, the determination of DAO in the blood alone cannot be considered a simple definitive test that will confirm or rule out the diagnosis.

Here too, the overall clinical picture, the exclusion of other causes and, depending on the situation, a time-limited, controlled dietary adjustment with subsequent reintroduction of foods are more important.

The microbiome: fascinating science doesn't mean a useful clinical test

A similar problem to that with IgG tests is now emerging in commercial gut microbiome testing.

Microbiome research is extremely interesting. We know that gut microorganisms are related to metabolism, the immune system, and the functioning of the digestive tract.

However, this does not mean that today we can reliably determine from a stool sample what the "ideal microbiome" of a particular person should look like and, based on one commercial result, target their bloating, fatigue, or other non-specific problems.

There are large natural differences in the composition of the microbiome between people. The microbiome also changes with diet, medications, infections, environment, and other factors.

A commercial test can therefore provide you with a large amount of data – percentages of bacteria, a diversity index or a list of microorganisms – but the fundamental question still remains:

What specifically should we do based on this result and do we have evidence that such an intervention will help the patient?

In routine clinical practice, the answer is still unclear for many commercial microbiome tests.

This doesn't mean that microbiome research has no future. It just means that research potential and current usability are not the same thing .

So if a person gets their microbiome sequenced out of curiosity, that's their decision. But if the test is marketed as a reliable way to diagnose the cause of their digestive problems and the basis for personalized treatment, expectations should be considerably more cautious.

The biggest risk isn't just wasted money

With IgG panels, it's easy to say, "It's just a test. If it doesn't work, I've lost my money." The problem is that the consequences can continue. Depending on the result, a person will eliminate milk, eggs, wheat, nuts, certain fruits, and a number of other foods. In a few months, their diet may be extremely restricted.

Such restrictions can impair the nutritional quality of the diet, complicate social life, and in some people promote unhealthy food fears or disordered eating behaviors. At the same time, it can delay diagnosis of the real cause of the problems.

So what should you do if you experience discomfort after eating?

Instead of asking, “What food will I test positive for?” it is more useful to ask: What mechanism could explain my difficulties?

If an allergy is suspected, diagnosis is in the hands of an allergist, and depending on the history, skin tests or specific IgE may be important.

In case of digestive problems, depending on the nature of the problems, it is necessary to consider, for example, celiac disease, lactose intolerance, functional digestive tract diseases, FODMAP, inflammatory or other gastroenterological diseases. Here I recommend working with a gastroenterologist.

A symptom-food diary and subsequent targeted, time-limited elimination with controlled reintroduction of the food can be very useful.

Ideally under the guidance of a qualified nutrition specialist.

It's slower than a single blood draw. You don't get a color chart with a hundred foods.

But diagnostics is not a competition to see who can measure the most parameters. A good test is one whose result reliably answers a specific question and helps decide what to do next.

What to remember from this?

If someone offers you a “food intolerance” test, always ask what exactly the test measures . IgE and IgG are not interchangeable. Targeted testing for specific IgE has its place in the diagnosis of IgE-mediated allergy. However, food IgG/IgG4 panels do not demonstrate food intolerance and should not be the basis for extensive elimination diets. IgG food intolerance tests are therefore generally a waste of money from a diagnostic perspective.

And similar caution is warranted today with commercial gut microbiome tests. They can generate a wealth of interesting data, but their ability to reliably diagnose the cause of specific digestive problems and guide individualized treatment is still limited in routine clinical practice.

Modern medicine is not about doing as many tests as possible.

It's about doing the right test on the right patient for the right reason – and knowing what the result really means.

author: Mgr. Lenka Melišková - Gut health nutritionist