Food allergies and intolerances: what they are and how they differ

What is a food allergy?

A food allergy is a reaction of the immune system to a protein in a food. For everyone else, that protein is harmless. In the best-known form, the body makes IgE antibodies against it. The next time the protein arrives, those antibodies trigger the release of histamine and other substances, and symptoms appear.

Three features set it apart from everything else.

• The reaction is almost always to proteins. Milk, egg, fish, shellfish, tree nuts, peanuts, pulses, wheat, fruit and vegetables can trigger a reaction to one of their proteins.
• A very small amount is enough. A crumb, a trace or the oil the food was fried in can set it off.
• It is fast. Symptoms almost always appear within the first two hours, and often within minutes.

What you notice ranges from an itchy mouth, hives or swollen lips to vomiting, abdominal pain, cough, difficulty breathing and a drop in blood pressure. We speak of anaphylaxis when the skin is affected together with breathing, blood pressure or the digestive system. We also do when breathlessness, a closing throat or a drop in blood pressure appear suddenly, even if the skin is fine. It is an emergency [5].

IgE-mediated food allergy affects up to 10% of the population, mainly younger people [6].

What is a food intolerance?

An intolerance is not caused by the immune system. It is a problem of digestion or absorption, and the most common mechanism comes in two versions.

• An enzyme is missing or low. This happens with lactose. If the lactase in the gut cannot keep up, lactose is not broken down, reaches the colon and is fermented there by bacteria.
• The transport system becomes saturated. This happens with fructose and sorbitol. The gut absorbs them up to a point, and what is left over ferments in the same way.

By either route the result is similar: gas, bloating, rumbling, pain and sometimes diarrhoea, a few hours after eating. It is very unpleasant and shapes your day. But it does not put your life at risk, and a trace does not set it off. It depends on how much.

That “how much” is what changes the life of the person who has it. According to the European Food Safety Authority (EFSA), there is no single lactose threshold, because tolerance varies widely between people [1]. Even so, the great majority of people who do not digest lactose well tolerate 12 g at a time with few or no symptoms [1]. That is roughly what a large glass of milk of about 250 ml contains. How much you tolerate and where lactose hides is explained in the lactose-intolerance guide.

The same applies to fructose and sorbitol, with different figures. You have them food by food in the fructose-intolerance guide and in the sorbitol-intolerance guide.

There is one exception worth knowing. Hereditary fructose intolerance is an inborn error of metabolism, and it is not about amounts. It usually shows up in childhood, when fruit is first introduced, although it is sometimes diagnosed in adulthood. It is managed by a metabolic disease unit. If this is your case, what follows about thresholds does not apply to you. See “Allergies when introducing foods to babies”.

How do an allergy and an intolerance differ?

The difference fits in one sentence. In an allergy, the problem is what your body reacts against; in an intolerance, it is how much your body tolerates.

You can have both at the same time, and it is not unusual. Someone with an LTP allergy may also digest lactose poorly. The job then is to make sure that, once the two lists are put together, what is left is still a complete diet.

Do the IgG tests sold in pharmacies work?

No. It is the question we get most often in consultations, and the answer saves years of unnecessary dieting.

IgG tests for foods do not diagnose anything. They are sold in pharmacies and private centres and return a list of twenty or two hundred flagged foods. Many people cut them all out. European guidelines on diagnosing food allergy recommend not using IgG or its subclasses to diagnose it, because there is no evidence to support them [4].

The harm is documented [4]. Unnecessary restrictions, a poorer diet, expense and a worse quality of life. And the opposite risk: treating as safe a food that does harm you.

IgG to a food, at most, shows that you have eaten it. It is a normal response of the body.

Neither do bioresonance, hair analysis, blood analysis under the microscope or kinesiology diagnose anything. At Alimmenta we do not use or interpret them. If you arrive with one, the first thing we do is set it aside.

DAO deficiency is often mentioned alongside migraine and the histamine in food. We have not found any scientific society guideline that recognises it as a diagnosis with a validated serum DAO test. That is why we do not treat it as one more intolerance. The link between food and migraine does have substance, and the food and migraine guide covers it.

Finally, some symptoms are neither allergy nor intolerance. A large, very fatty, very fast or very late meal gives similar symptoms without any IgE or enzyme behind it. If you have irritable bowel, the map is different. The low-FODMAP diet guide explains it: a temporary diet in which reintroduction allows you to judge the result.

How is a food allergy diagnosed?

It is diagnosed by an allergy specialist, and the order of the steps matters.

1. Clinical history. What you ate, how much, what happened, how long it took, how long it lasted and whether it happened again. Without this, no test means anything.
2. Skin test or specific IgE in blood, only for the foods the history points to. A panel of twenty foods ordered blindly gives positives that mean nothing, and then it takes years to undo them.
3. Controlled oral food challenge, in hospital, when the history and the tests do not agree. It is the one that confirms, and also the one that allows foods to be reintroduced into the diet.

Having IgE to a food is called being sensitised, and it is not the same as being allergic. The diagnosis comes from the clinical history together with the test, never from the test alone [4].

That is why we repeat one rule so often. Do not remove the food before you are seen, because removing it blindly erases exactly the information the allergy specialist needs.

How is an intolerance diagnosed?

It is handled by your GP or gastroenterologist. The reference test is the hydrogen breath test. You take a dose of the sugar being studied, and hydrogen is measured, as is methane, which appears in the breath over the following hours.

The European consensus on breath tests sets the doses [7]. For lactose intolerance, 25 g of lactose; to see whether it is poorly absorbed, 25 to 50 g. For fructose, 20 to 25 g [7]. For sorbitol it gives no guidance for adults, and in children it asks for the test to be clarified and standardised better [7]. That is why the sorbitol test is not validated as a diagnostic test.

The same consensus separates two things that are often confused. Malabsorption means the sugar is not absorbed, and it is what the breath test measures. Intolerance means that, in addition, it gives you symptoms. That is why the symptom record is part of the test [7]. Your doctor interprets the result figures together with your symptoms. A positive breath test without symptoms is not an intolerance, and does not justify removing the food.

There is another useful fact for avoiding unnecessary restrictions. Below 25 g of fructose there was no discomfort in healthy volunteers, even in those who absorbed fructose poorly [7]. Twenty-five grams is considerably more than a piece of fruit contains.

What changes at the supermarket once you have a diagnosis

Something different changes depending on what you have, and mixing them up makes life harder.

With an allergy, you need to check labels and avoid traces. In the European Union, 14 substances or groups that cause allergies or intolerances must always be declared and highlighted in the ingredients list [2]. They are cereals containing gluten, crustaceans, eggs, fish, peanuts, soya, milk, tree nuts, celery, mustard, sesame seeds, sulphur dioxide and sulphites, lupin and molluscs. In a restaurant they must also inform you about them [2]. “May contain” is not regulated in the same way. It is voluntary and does not say how much.

If your allergy is to LTP, there is one more layer, the cofactors. Severe reactions to LTP are not rare, and exercise or alcohol often make them more likely [3]. That is why the advice has to be individual, to avoid unnecessary exclusions and reduce those cofactors [3]. There is no safe amount of alcohol for health (WHO): the less, the better; and none during pregnancy, breastfeeding, childhood, if you are going to drive or if your medication does not allow it. If you often take anti-inflammatories, mention it to your doctor. The detail is in the LTP-allergy guide.

With an intolerance, you need to know your threshold. It is about knowing how much you tolerate, at what time of day and alongside what. Almost nobody has to remove the sugar altogether. For fructose, the fructose guide has the food-by-food table and the weekly fructose-free menu, with the week already planned. Sorbitol hides mainly in “sugar-free” products and chewing gum, and the sorbitol guide covers it.

Which mistakes do we see most often in consultations?

• The list that only grows. Every time something disagrees with you, one more food is removed and none ever comes back. Two years later six foods are left and the symptoms are still there.
• Trying at home to see if it is tolerated now. In an allergy with a risk of anaphylaxis, reintroduction is done in hospital with emergency equipment at hand.
• Confusing sensitisation with allergy. A positive IgE to a food you eat every day without a problem is not an allergy.
• Eating with fear. Sometimes fear of a reaction leads to avoiding more and more meals, restaurants or trips. This is worked on in consultation, sometimes with the psychology team as well as the nutrition team.

European guidelines on managing food allergy say the same from the other side. They recommend continuing to eat the foods that are tolerated, to keep the diet varied [6]. And they warn that avoidance diets without supervision, and kept up longer than needed, carry a nutritional risk, especially in the youngest [6]. Removing foods without need also has a cost.