Food Intolerance vs Food Allergy: What is the Difference?

The key difference: immune system involvement

The fundamental distinction is whether the immune system is involved. Food allergy is an immune-mediated reaction — the immune system mistakenly identifies a food protein as harmful and mounts a response. Food intolerance is a non-immune reaction — typically a digestive or metabolic response to a food component. This distinction determines the symptoms, the severity, the testing approach, and the management.

Food allergy — what it looks like

• Involves the immune system (IgE-mediated or non-IgE-mediated)
• IgE-mediated (immediate): symptoms within minutes to 2 hours of eating — hives, swelling, vomiting, breathing difficulty, anaphylaxis
• Non-IgE-mediated (delayed): symptoms 2–72 hours after eating — reflux, loose stools, blood in stools, eczema flares, colic
• Can be triggered by tiny amounts of the allergen
• Can be life-threatening (anaphylaxis) in IgE-mediated allergy
• Requires strict avoidance and, for IgE-mediated allergy, an adrenaline auto-injector
• Diagnosed by allergy testing (SPT, specific IgE) or elimination/reintroduction protocol

Food intolerance — what it looks like

• Does NOT involve the immune system
• Typically causes digestive symptoms — bloating, wind, diarrhoea, stomach cramps
• Symptoms are usually dose-dependent — small amounts may be tolerated
• Onset is usually delayed (hours after eating) and symptoms are not life-threatening
• Common examples: lactose intolerance, fructose malabsorption, sensitivity to food additives
• Standard allergy tests (SPT, specific IgE) are negative
• Managed by reducing intake of the trigger food, not necessarily strict avoidance

Lactose intolerance vs cow's milk allergy

This is one of the most common points of confusion. Lactose intolerance is caused by insufficient lactase enzyme to digest lactose (milk sugar) — it is a digestive condition, not an immune reaction. It causes bloating, wind, and diarrhoea, typically in older children and adults. Primary lactose intolerance is rare in young children. Cow's milk protein allergy (CMPA) is an immune reaction to milk proteins — it can cause immediate reactions (hives, vomiting, anaphylaxis) or delayed reactions (reflux, eczema, blood in stools) and is common in infants. The two conditions require completely different management.

The problem with self-diagnosis and elimination diets

Many families self-diagnose food intolerance or allergy and eliminate foods from their child's diet without specialist assessment. This can lead to nutritional deficiencies (particularly in growing children), unnecessary restriction of a varied diet, and — paradoxically — an increased risk of developing a more severe allergy when the food is reintroduced after a period of avoidance. Specialist assessment ensures the right diagnosis and the right management.

What about IgG food testing?

IgG food tests are widely marketed as a way to identify food intolerances. They measure IgG antibodies to foods — but IgG antibodies are a normal immune response to foods that have been eaten and do not indicate allergy or intolerance. Major allergy organisations including BSACI, EAACI, and the British Dietetic Association do not recommend IgG food testing. It is not a validated diagnostic test and can lead to unnecessary and potentially harmful dietary restriction.

When to seek specialist assessment

• Your child has had an immediate reaction (hives, swelling, vomiting, breathing difficulty) after eating
• You suspect a food is causing persistent digestive symptoms, eczema, or other symptoms
• You are considering eliminating a major food group from your child's diet
• Your child has been told they have a food intolerance but you are not sure of the diagnosis
• You want clarity on whether your child has a true food allergy