What’s the difference between a food allergy and an intolerance?

At one time or another, you’ve probably come across someone who is lactose intolerant and might experience some unpleasant gut symptoms if they have dairy. Maybe it’s you – food intolerances are estimated to affect up to 25% of Australians.

Meanwhile, cow’s milk allergy is one of the most common food allergies in infants and young children, affecting around one in 100 infants.

But what’s the difference between food allergies and food intolerances? While they might seem alike, there are some fundamental differences between the two.

What is an allergy?

Australia has one of the highest rates of food allergies in the world. Food allergies can develop at any age but are more common in children, affecting more than 10% of one-year-olds and 6% of children at age ten.

A food allergy happens when the body’s immune system mistakenly reacts to certain foods as if they were dangerous. The most common foods that trigger allergies include eggs, peanuts and other nuts, milk, shellfish, fish, soy and wheat.

Mild to moderate signs of food allergy include a swollen face, lips or eyes; hives or welts on your skin; or vomiting. A severe allergic reaction (called anaphylaxis) can cause trouble breathing, persistent dizziness or collapse.

What is an intolerance?

Food intolerances (sometimes called non-allergic reactions) are also reactions to food, but they don’t involve your immune system.

For example, lactose intolerance is a metabolic condition that happens when the body doesn’t produce enough lactase. This enzyme is needed to break down the lactose (a type of sugar) in dairy products.

Food intolerances can also include reactions to natural chemicals in foods (such as salicylates, found in some fruits, vegetables, herbs and spices) and problems with artificial preservatives or flavour enhancers.

Symptoms of food intolerances can include an upset stomach, headaches and fatigue, among others.

Food intolerances don’t cause life-threatening reactions (anaphylaxis) so are less dangerous than allergies in the short term, although they can cause problems in the longer term such as malnutrition.

We don’t know a lot about how common food intolerances are, but they appear to be more commonly reported than allergies. They can develop at any age.

It can be confusing

Some foods, such as peanuts and tree nuts, are more often associated with allergy. Other foods or ingredients, such as caffeine, are more often associated with intolerance.

Meanwhile, certain foods, such as cow’s milk and wheat or gluten (a protein found in wheat, rye and barley), can cause both allergic and non-allergic reactions in different people. But these reactions, even when they’re caused by the same foods, are quite different.

For example, children with a cow’s milk allergy can react to very small amounts of milk, and serious reactions (such as throat swelling or difficulty breathing) can happen within minutes. Conversely, many people with lactose intolerance can tolerate small amounts of lactose without symptoms.

There are other differences too. Cow’s milk allergy is more common in children, though many infants will grow out of this allergy during childhood.

Lactose intolerance is more common in adults, but can also sometimes be temporary. One type of lactose intolerance, secondary lactase deficiency, can be caused by damage to the gut after infection or with medication use (such as antibiotics or cancer treatment). This can go away by itself when the underlying condition resolves or the person stops using the relevant medication.

Whether an allergy or intolerance is likely to be lifelong depends on the food and the reason that the child or adult is reacting to it.

Allergies to some foods, such as milk, egg, wheat and soy, often resolve during childhood, whereas allergies to nuts, fish or shellfish, often (but not always) persist into adulthood. We don’t know much about how likely children are to grow out of different types of food intolerances.

How do you find out what’s wrong?

If you think you may have a food allergy or intolerance, see a doctor.

Allergy tests help doctors find out which foods might be causing your allergic reactions (but can’t diagnose food intolerances). There are two common types: skin prick tests and blood tests.

In a skin prick test, doctors put tiny amounts of allergens (the things that can cause allergies) on your skin and make small pricks to see if your body reacts.

A blood test checks for allergen-specific immunoglobulin E (IgE) antibodies in your blood that show if you might be allergic to a particular food.

Food intolerances can be tricky to figure out because the symptoms depend on what foods you eat and how much. To diagnose them, doctors look at your health history, and may do some tests (such as a breath test). They may ask you to keep a record of foods you eat and timing of symptoms.

A temporary elimination diet, where you stop eating certain foods, can also help to work out which foods you might be intolerant to. But this should only be done with the help of a doctor or dietitian, because eliminating particular foods can lead to nutritional deficiencies, especially in children.

Is there a cure?

There’s currently no cure for food allergies or intolerances. For allergies in particular, it’s important to strictly avoid allergens. This means reading food labels carefully and being vigilant when eating out.

However, researchers are studying a treatment called oral immunotherapy, which may help some people with food allergies become less sensitive to certain foods.

Whether you have a food allergy or intolerance, your doctor or dietitian can help you to make sure you’re eating the right foods.

Victoria Gibson, a Higher Degree by Research student and Research Officer at the School of Nursing, Midwifery and Social Work at the University of Queensland, and Rani Scott-Farmer, a Senior Research Assistant at the University of Queensland, contributed to this article.