Food intolerance: how to tell which food doesn't love you back

Some foods just don’t agree with you. Maybe it’s the latte that sends you running, or the pasta that leaves you bloated all afternoon. A food intolerance can be frustrating, but it’s rarely dangerous, and with some patient detective work you can usually work out what’s going on.

What is a food intolerance?

The NHS defines it as having difficulty digesting certain foods or ingredients. It’s not usually serious, but eating the food can make you feel unwell.

How is it different from a food allergy?

This is the most important distinction:

	Food intolerance	Food allergy
What’s happening	Your body can’t digest a food or ingredient well	Your immune system overreacts to a food
When symptoms start	Usually a few hours after eating	Swelling can come on suddenly
Typical symptoms	Bloating, tummy pain, wind, diarrhea, sometimes headache or tiredness	Itchy skin, wheezing, swelling of the lips, face and eyes
Can it be life-threatening?	No	Yes

Call 999 if your lips, mouth, throat or tongue suddenly swell, you’re struggling to breathe or swallow, or your skin or lips turn blue, grey or pale. These are signs of a serious allergic reaction, says the NHS.

What are the symptoms?

The NHS lists tummy pain, bloating, wind and diarrhea, plus other possible symptoms such as:

• headache
• feeling tired or exhausted
• feeling sick
• constipation
• joint pain or rashes

Symptoms usually start a few hours after eating and can last a few hours or days.

Which foods are common culprits?

The NHS says lactose intolerance is the most common. But you can be intolerant to almost any food or ingredient:

Intolerance	Where it’s found (NHS)
Lactose	Milk and dairy, like cheese and yogurt
Gluten	Wheat, rye and barley, in bread and pasta
Histamine	Wine and cheese
Caffeine	Coffee, tea, some fizzy drinks
Sulphites	Cider, beer and wine
Salicylates	Some fruits, vegetables, herbs and spices
MSG	Ripened fruits, cured meats, savory foods
Benzoates	Soft drinks, fruit juices, salad dressing, jams and pickles

NIDDK adds fructose (in fruit, fruit juice and honey) and sucrose (table sugar) as carbohydrates some people struggle to digest, and notes sugar alcohols in sugar-free sweets and gum can cause diarrhea in some people.

Gluten intolerance isn’t the same as celiac disease, which is an autoimmune condition. If you suspect gluten, see a GP before cutting it out. The NHS says keep eating gluten until you’ve been tested for celiac disease. More in celiac disease or IBS. Our lactose intolerance or IBS post covers another common mix-up.

How do doctors work it out?

The NHS says a GP may suggest:

1. A food and symptoms diary to spot what’s triggering symptoms.
2. An elimination diet: avoiding the suspected food to see if symptoms improve, then eating it again after a few weeks to see if they return.
3. A breath test if you might be lactose intolerant.

You may be referred to a dietitian. If IBS is part of the picture, Monash University’s FODMAP approach uses a similar idea: swap foods for 2 to 6 weeks, then test them back in, one group at a time, ideally with a dietitian.

Are home intolerance tests worth it?

No. The NHS says some shop-bought tests that claim to diagnose food intolerances are not recommended. There’s limited evidence they’re accurate, and they may suggest you avoid multiple foods, which can be harmful.

What does a useful log look like?

Mon 08:00  Meal: latte, croissant  tags: dairy, gluten
Mon 10:30  Poop: Type 6
Tue 08:00  Meal: black coffee, toast  tags: gluten
Wed 08:00  Meal: latte, banana  tags: dairy
Wed 10:15  Poop: Type 6
Paper note: bloated both latte mornings

Toast without milk: fine. Two lattes: two loose mornings. Dairy looks like the lead, but one week isn’t proof. Take the log to your GP before changing anything big.

Myths vs facts

Myth	Fact
Intolerance and allergy are the same	Allergy involves the immune system and can be life-threatening; intolerance doesn’t (NHS).
A home test will tell you what to avoid	The NHS doesn’t recommend shop-bought intolerance tests.
Symptoms come straight after eating	Usually a few hours later, and can last days (NHS).
Cut out anything suspicious right away	The NHS says get GP or dietitian advice first.

Working out a food intolerance

• Log meals with times
• Log each poop and its Bristol type
• Note symptoms on paper with the time they start
• Look back a few hours for tummy symptoms
• Take your log to a GP before cutting foods out
• Call 999 for any signs of a serious allergic reaction

When should I see a GP?

See a GP if symptoms of a food intolerance keep coming back, says the NHS. See one sooner if you have blood in your poop, weight loss you can’t explain, or diarrhea that won’t settle.

Where does PoopAI fit?

Log meals by photo or text with trigger tags like dairy or gluten, and each poop as Bristol type 1 to 7. Weekly reports show which tagged foods tend to come before loose or hard days. PoopAI doesn’t log symptoms like bloating, so keep those on paper alongside.

The bottom line

A food intolerance is uncomfortable but not dangerous. Log carefully, look for repeats, skip the home test kits, and work it out with a GP or dietitian rather than cutting foods on a hunch. Next: how long after eating does food affect your poop.

Editorial note: Written by The FixMyGut team using the NHS, NIDDK and Monash University sources listed below. It’s general information, not a diagnosis. Facts checked against those sources on November 5, 2026.