Food Intolerance Testing: IgG, Elimination Diets, and the Evidence

Adverse reactions to food are incredibly common, yet the terminology surrounding them is frequently misused. Patients often tell me they are "allergic" to dairy because it causes bloating, or "intolerant" to peanuts when they actually carry an EpiPen. In clinical medicine, distinguishing between a true food allergy and a food intolerance is not just a matter of semantics; it dictates the diagnostic approach, the severity of the risk, and the management strategy.

The confusion stems from the fact that these conditions involve very different physiological pathways and produce vastly different symptoms — and importantly, the testing landscape for each is also very different.

IgE Food Allergy: The Immediate Threat

A true food allergy is an immediate, hypersensitive immune response mediated by Immunoglobulin E (IgE) antibodies. When a person with an IgE allergy consumes the offending food (common culprits include peanuts, shellfish, eggs, and dairy), their immune system perceives the protein as a deadly invader.

The IgE antibodies trigger mast cells to release massive amounts of histamine and other inflammatory chemicals into the bloodstream almost instantly. The reaction occurs within minutes to a maximum of two hours.

Symptoms are acute and often severe, including:

• Hives, intense itching, or eczema.
• Swelling of the lips, face, tongue, and throat (angioedema).
• Wheezing, shortness of breath, or a drop in blood pressure.
• Anaphylaxis: A life-threatening, systemic reaction requiring immediate emergency medical intervention (adrenaline/epinephrine).

IgE allergies are diagnosed via skin prick testing or specific IgE blood tests under the supervision of an NHS allergist or immunologist. If you suspect a true allergy — particularly one involving any breathing difficulty, facial swelling, or anaphylaxis — you should always seek formal allergy assessment through your GP rather than self-investigating.

Food Intolerance: A Different Mechanism Entirely

Food intolerance is fundamentally different from a food allergy. It does not involve the IgE pathway, is not life-threatening, and the underlying mechanism is often non-immune (for example, lactose intolerance is caused by a deficiency of the lactase enzyme, not an immune reaction). True intolerances such as lactose intolerance, fructose malabsorption, and histamine intolerance can cause significant gastrointestinal distress, but they do not trigger anaphylaxis.

Symptoms of intolerance are typically delayed and chronic rather than acute, and frequently include:

• Gastrointestinal: Bloating, cramping, excessive gas, diarrhea, or constipation (often overlapping with IBS).
• Neurological: Brain fog, chronic headaches, or migraines.
• Dermatological: Persistent eczema or unexplained rashes.
• Systemic: Joint pain, chronic fatigue, and a general feeling of malaise.

The Truth About IgG Food Intolerance Testing

You may have come across IgG food intolerance tests advertised online. These tests measure Immunoglobulin G antibody reactivity against a panel of common foods. It is essential to be transparent about what current clinical evidence says about this type of testing.

IgG food intolerance tests are not endorsed as a diagnostic tool by the major UK and international allergy bodies, including the British Society for Allergy and Clinical Immunology (BSACI), the European Academy of Allergy and Clinical Immunology (EAACI), and NICE. The clinical position of these organisations is that the presence of IgG antibodies to food is a normal sign of immune exposure to that food — not evidence of intolerance or sensitivity. A high IgG reading often simply means you eat that food regularly.

For this reason, IgG tests should not be used to diagnose food intolerance, and any results should never be interpreted in isolation. The only validated approach for identifying a true food trigger remains a structured elimination and reintroduction diet, ideally supervised by a registered dietitian. Some patients use IgG results as a starting point to decide which foods to trial in an elimination diet, but the diagnostic value comes from the elimination and reintroduction itself — not the antibody test.

The Right Clinical Pathway

If you are experiencing chronic gut symptoms, the priority should be to rule out treatable medical conditions before assuming food sensitivity is the cause. A structured workup typically includes screening for Coeliac disease, inflammatory bowel disease, lactose intolerance, and Helicobacter pylori infection — all of which can mimic the symptoms of food intolerance and all of which have evidence-based treatments.

Once these conditions are excluded, working with your GP or a registered dietitian on a structured elimination diet (such as the low-FODMAP diet for IBS-type symptoms) is the most evidence-based way to identify your specific food triggers.

Medical References

1. National Institute for Health and Care Excellence (NICE). (2011). Food allergy in under 19s: assessment and management. Clinical guideline [CG116].
2. Stapel, S. O., et al. (2008). Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy, 63(7), 793-796.
3. British Dietetic Association. (2019). Food Allergy and Food Intolerance: Food Fact Sheet.
4. Turnbull, J. L., Adams, H. N., & Gorard, D. A. (2015). Review article: the diagnosis and management of food allergy and food intolerances. Alimentary Pharmacology & Therapeutics, 41(1), 3-25.