Food Intolerance vs. Allergy: GI Symptoms and Testing

When your stomach churns after eating dairy, or you get bloated after a slice of pizza, it’s easy to assume it’s a food allergy. But here’s the truth: food intolerance and food allergy are not the same thing - and confusing them can lead to unnecessary fear, wrong diets, or even dangerous mistakes.

What Happens in Your Body?

A food allergy is your immune system going into overdrive. It sees a harmless protein - like the one in peanuts or eggs - as a threat. In response, it releases IgE antibodies, which trigger mast cells to dump histamine and other chemicals into your bloodstream. This can happen within minutes. Symptoms aren’t just stomach-related: you might break out in hives, feel your throat close up, or have trouble breathing. In severe cases, it’s anaphylaxis - a life-threatening emergency that needs epinephrine right away.

Food intolerance is different. No immune system involvement. No antibodies. Just your digestive system struggling. The most common cause? Enzyme deficiency. Lactose intolerance, for example, happens because your body doesn’t make enough lactase to break down milk sugar. That undigested lactose moves into your colon, where bacteria ferment it. The result? Bloating, gas, cramps, and diarrhea - usually starting 30 minutes to two hours after eating.

The difference isn’t just academic. If you have a peanut allergy, even a crumb can send you to the ER. If you’re lactose intolerant, you might handle a splash of cream in your coffee just fine.

GI Symptoms: How to Tell Them Apart

Both can cause stomach trouble, but the pattern tells the story.

• Food allergy GI symptoms: Sudden vomiting, severe abdominal pain, diarrhea. These come fast - often within minutes to two hours. They rarely show up alone. You’ll likely also see swelling, itching, wheezing, or a drop in blood pressure.
• Food intolerance GI symptoms: Gradual bloating, excessive gas, cramping, loose stools. These build over time. You might feel fine right after eating, then start to feel off an hour later. Symptoms usually depend on how much you ate. A small amount? Maybe nothing. A whole bowl of ice cream? Trouble.

Lactose intolerance is the classic example. About 65% of the world’s population has some degree of lactase deficiency. In Australia, that’s roughly 1 in 5 adults. You don’t need to avoid all dairy - just find your personal threshold. Most people can tolerate up to 12 grams of lactose daily (about a cup of milk) without symptoms.

Testing: What Actually Works

There’s a lot of misinformation out there. You’ve probably seen ads for blood tests that claim to detect “food sensitivities” by measuring IgG antibodies. Don’t waste your money. The American Academy of Allergy, Asthma & Immunology says these tests are not scientifically valid. Sensitivity? Below 30%. Specificity? Under 45%. They give false positives and send people on unnecessary diets.

Here’s what real doctors use:

For Food Allergies

• Skin prick test: A tiny drop of allergen is placed on your skin, then lightly pricked. A raised bump (wheal) larger than 3mm compared to a control suggests allergy. It’s fast, cheap, and widely available.
• Specific IgE blood test: Measures the level of allergy antibodies in your blood. Results are in kU/L. A value above 0.35 kU/L is considered positive - but even this isn’t foolproof. Up to 50-90% of positive results need confirmation with an oral food challenge.
• Oral food challenge: The gold standard. Done under medical supervision. You eat tiny, increasing amounts of the suspected food while being monitored. If you react, you get treated immediately. If not, you’re cleared. This is the only way to be 100% sure.
• Component-resolved diagnostics: For complex cases like peanut allergy, this test looks at specific proteins (like Ara h 2). If Ara h 2 levels are above 0.23 kU/L, there’s a 95% chance you have a true peanut allergy.

For Food Intolerances

• Hydrogen breath test: Used for lactose and fructose intolerance. You drink a sugar solution, then breathe into a tube every 15-30 minutes. If hydrogen levels rise by 20 ppm or more above baseline, it means bacteria in your colon are fermenting undigested sugar. This test is 95% sensitive for lactose malabsorption.
• Celiac disease testing: If gluten makes you sick, celiac disease must be ruled out. First, a blood test for tissue transglutaminase IgA antibodies. A level above 10 U/mL is highly suggestive. Then, an endoscopy with a biopsy of the small intestine. Damage classified as Marsh 3 confirms celiac.
• Elimination and challenge diet: For non-celiac gluten sensitivity or other food intolerances, this is the only reliable method. You remove the suspect food for 2-6 weeks. If symptoms disappear, you slowly reintroduce it. If they come back, you’ve found your trigger. No blood test can diagnose this.

What You Shouldn’t Do

Don’t self-diagnose. A 2023 study in Clinical Gastroenterology and Hepatology found that 80% of people who think they have a food intolerance actually have something else - like IBS (45%), IBD (12%), or functional dyspepsia (23%).

Don’t cut out entire food groups without testing. Avoiding dairy because you think you’re intolerant? You might be missing calcium and vitamin D. Going gluten-free without celiac? You could be eating more processed, low-nutrient foods.

Don’t rely on unproven tests. IgG blood tests, hair analysis, applied kinesiology - none of these are backed by science. They’re expensive and misleading.

Managing Your Condition

If you have a food allergy:

• Avoid the allergen completely. Read every label. The FDA requires the top 9 allergens (milk, eggs, fish, shellfish, tree nuts, peanuts, wheat, soy, sesame) to be clearly listed.
• Carry two epinephrine auto-injectors (like EpiPen) at all times. Prescription costs range from $550 to $750 without insurance.
• Wear a medical alert bracelet.
• Teach family, coworkers, and school staff how to use the injector. Anaphylaxis can happen fast - even if past reactions were mild.

If you have a food intolerance:

• Find your tolerance level. Most lactose-intolerant people can handle 12g of lactose per day. That’s one cup of milk, or a small serving of yogurt.
• Use enzyme supplements. Lactase pills taken before eating dairy can help many people.
• Choose low-lactose or lactose-free dairy. Hard cheeses, butter, and Greek yogurt often have less lactose.
• For sulfite sensitivity, avoid wine, dried fruit, and processed potatoes. The FDA requires sulfites above 10ppm to be labeled.
• Work with a dietitian. You don’t need to eliminate everything - just manage your intake.

When to See a Doctor

See a specialist if:

• Your symptoms are getting worse or happening more often.
• You’re losing weight or having blood in your stool.
• You’ve tried eliminating foods but still feel unwell.
• You’ve had a reaction that involved breathing trouble, swelling, or dizziness - even once.

These aren’t just “bad digestion.” They could be signs of celiac disease, IBD, or a hidden allergy. Early diagnosis changes everything.

What’s New in Research

Science is catching up. A 2024 study in Nature Communications found specific blood metabolites that can distinguish non-celiac gluten sensitivity from IBS with 89% accuracy. That’s huge - it means we might soon have a blood test for something we’ve only diagnosed by elimination.

Organizations like FARE are funding 17 clinical trials right now, exploring better ways to predict reactions using basophil activation tests and improved biomarkers.

The goal? Stop guessing. Start knowing.