Blood Tests and Food Intolerances: Science or Snake Oil?

What food intolerance blood tests promise you

You're suffering from bloating, unexplained fatigue, and recurring stomach pains. A private laboratory offers you a blood test capable of analysing your reactions to 150 — or even 287 — different foods, from cow's milk to spices to coffee. The results arrive as a colour-coded table: certain foods are flagged as "high" or "very high", and you're advised to avoid them for three to six months.

Compelling, certainly. But is this actually science?

IgG: a very real antibody, very much misunderstood

These tests measure your IgG antibodies specific to certain foods. IgG is a perfectly real immunoglobulin, produced by your immune system. The problem is that a high level of it in response to a particular food does not mean you are intolerant to that food.

What it mostly means is that you eat that food regularly.

In immunology, the production of food-specific IgG antibodies is considered a mechanism of normal tolerance: your body learns to recognise what it regularly ingests without triggering an alarm response. A high IgG titre for milk therefore typically indicates that you consume dairy products — nothing more.

This is precisely why the World Allergy Organization, the American Academy of Allergy, Asthma & Immunology (AAAAI), the EAACI, and ANSES in France all agree on the same point: these tests have no scientific validation for diagnosing food intolerance. Not a single randomised controlled trial has demonstrated a correlation between an elevated IgG result and real clinical symptoms.

Put plainly: these tests, often sold for several hundred pounds, have no diagnostic value recognised by medicine.

So what are blood tests actually useful for?

It would be wrong to conclude that blood tests are useless when it comes to digestive complaints or chronic fatigue. Quite the opposite — but for entirely different reasons.

Certain nutritional deficiencies produce symptoms that bear a striking resemblance to food intolerances, and these can be reliably detected through a blood test:

• Vitamin D: insufficient in 40 to 80% of Europeans during winter, it can cause fatigue, muscle pain, and low mood.
• Iron and ferritin: deficiency affects 20 to 30% of women of childbearing age and leads to exhaustion, breathlessness, and even digestive complaints.
• Vitamin B12: low in 5 to 20% of people over 65, particularly in vegans or in cases of intestinal malabsorption (such as in IBD); it affects both the nervous system and digestion.
• Folate (B9): essential for cell division and frequently deficient in restrictive diets.

These tests are grounded in clear, well-documented biological mechanisms, interpreted as part of a wider assessment by your doctor. Medical authorities recommend targeted testing based on symptoms and dietary history — not a panel covering 150 foods.

When should you suspect a genuine food intolerance?

Two intolerances are medically recognised and detectable using validated tools:

• Coeliac disease: an intolerance to gluten affecting approximately 1% of the population. It is screened for by measuring anti-transglutaminase antibodies (IgA), confirmed by intestinal biopsy.
• Lactose intolerance: diagnosed via a hydrogen breath test or genetic test — not by IgG.

Irritable bowel syndrome (IBS), which affects 10 to 15% of adults, is frequently confused with multiple food intolerances. In this case, the most well-validated strategy remains a guided elimination diet — in particular the low-FODMAP protocol — carried out under the supervision of a doctor or dietitian, not based on an IgG table.

The right approach to digestive complaints

Here is what clinical guidelines recommend:

• Start with a medical consultation to rule out an underlying condition (inflammation, coeliac disease, IBD)
• Request a targeted blood panel: vitamin D, B12, iron/ferritin, folate, CRP (an inflammatory marker)
• Keep a food diary for two to four weeks to identify correlations between symptoms and foods
• Consider a guided elimination diet with a healthcare professional if necessary
• Avoid cutting out entire food groups on the sole basis of an IgG test, which risks creating genuine nutritional deficiencies

The verdict

Food IgG tests are a marketing tool, not a medical one. They exploit a legitimate gap in public understanding of how the immune system works, and can lead to unnecessarily restrictive — and potentially unbalanced — diets.

Validated blood tests — checking for deficiencies, inflammatory markers, and coeliac-specific antibodies — do have genuine clinical value, provided they are ordered and interpreted by a qualified professional.

When faced with persistent digestive symptoms, the right question is not "which foods am I intolerant to according to my IgG?" but rather: "what is actually going on in my gut?" And to answer that properly, reliable tools do exist — so let's start with the right ones.