What are the symptoms of a food intolerance?

What are the symptoms of a food intolerance?
Kate Hilton, Registered Dietitian
Written by Kate Hilton, RD Published 3 July 2023
Sonia Khan, Pharmacist
Medically reviewed by Sonia Khan, Pharmacist Last reviewed 4 September 2026

The most common symptoms of a food intolerance are bloating, wind, stomach pain, diarrhoea and feeling sick after eating. Some people also notice headaches, tiredness or a dip in mood. Symptoms usually appear hours after eating rather than straight away, and how strong they are often depends on how much of the food you ate.

The Food Standards Agency estimates that around 2.4 million people in the UK live with a diagnosed food allergy, and many more report reacting to foods without a formal diagnosis. A food intolerance is not the same as an allergy. It happens when the body has difficulty digesting a certain food or ingredient, and it does not involve the immune reaction that makes allergies dangerous.

This guide covers the common symptoms, the ones that need medical attention, and what you can do about them.

Food intolerance symptoms

Symptoms vary from person to person. Some people react to one food with a single symptom. Others have several at once, and the same food can affect two people very differently.

The most common symptoms of a food intolerance:

  1. Bloating: Bloating is the feeling of fullness after eating, caused by gas building up in your gut, which can also lead to flatulence and belching
  2. Gas: Excessive wind often comes together with bloating and stomach ache
  3. Stomach Pain: Stomach pain and cramps in the abdominal region
  4. Diarrhoea: Postprandial Diarrhoea can cause loose and watery bowel movements within 30 minutes to 2 hours after eating
  5. Nausea: Feeling sick after eating, often triggered more by some foods than others
  6. Indigestion: Discomfort, heartburn, or feeling overly full after eating
  7. Tiredness and fatigue: Feeling tired after eating can be normal, but tiredness that reliably follows one particular food can point to an intolerance

The best understood example is lactose intolerance. When lactose is not broken down in the small intestine, it draws water into the bowel and is fermented by gut bacteria, producing gas. That combination causes the bloating, pain and diarrhoea, which is why the symptoms build over the hours after a meal rather than appearing instantly. A 2021 review estimates that most of the world's adults produce less lactase, the enzyme that digests lactose, than they did as children, though far fewer notice symptoms.

Symptoms to look out for

The symptoms below are not typical of a food intolerance and need medical advice. See your GP if you have any of the following:

  • Persistent and severe abdominal pain which gets worse
  • Vomiting or being unable to keep food down, particularly if this lasts for longer than 24 hours
  • Unexplained weight loss
  • Blood in your stools, or bleeding from your bottom
  • A change in your bowel habit lasting more than three weeks, difficulty swallowing, or new persistent gut symptoms if you are over 50
  • You should call 999 immediately if you display any signs of an allergic reaction, such as a skin rash, difficulty breathing, swelling of the lips, tongue, or throat (anaphylaxis)

A food allergy involves the immune system and can be serious and life-threatening. A food intolerance does not trigger that kind of immune reaction. Its symptoms are uncomfortable, sometimes miserable, but not dangerous in themselves. Understanding food allergy vs food intolerance, including how allergy is tested in the UK, helps you take the right next step.

Coping with food intolerance symptoms

There are a number of things you can try to help reduce the symptoms of a food intolerance:

  • Keep a food and symptom diary: Write down what you eat and how you feel over two to four weeks. Because intolerance symptoms are delayed and dose-dependent, a diary is often the quickest way to spot a pattern
  • Remove and reintroduce suspect foods, with support: Once a food is under suspicion, removing it for a few weeks and then reintroducing it shows whether it is really the cause. Do this with guidance from your GP or a registered dietitian rather than on your own, especially for whole food groups like dairy or wheat. Cutting food groups long term without support can leave you short of nutrients
  • Portion control: Intolerance symptoms usually depend on the amount eaten, so smaller portions of a problem food may sit fine where a large one does not
  • Eat more slowly: Eating quickly means swallowing more air, which adds to bloating and wind. Chew your food properly and slow down a little
  • Take stress seriously: Many people find their gut symptoms flare during stressful periods, because the gut and brain are closely connected. If that pattern fits you, tackling the stress alongside the diet is worthwhile
  • Speak to a professional: A registered dietitian or nutritionist can help you replace problem foods without losing out nutritionally, and a doctor can prescribe treatments for individual symptoms such as indigestion or sickness where needed

Testing for a food intolerance

No single test can diagnose a food intolerance on its own. A 2022 clinical review describes diagnosis as resting on your history and symptoms, supported by targeted tests, with a structured process of removing and reintroducing the suspect food. In practice that means the approach above, ideally with professional support.

For lactose intolerance there is an established medical test. A hydrogen breath test measures the gas produced when undigested lactose is fermented in the bowel, and research rates it as accurate, non-invasive and inexpensive.

Blood tests look at how your immune system responds to individual foods. The Feel Gut food intolerance blood test measures all 4 food specific IgG subtypes (Immunoglobulins G) against 130 or 270 foods, depending on the test you choose, and shows which foods your body is reacting to most strongly. Results like these work best as a starting point: a shortlist of foods to take into the diary-and-reintroduction process, not a diagnosis on their own.

Can you suddenly become intolerant to foods?

Yes, food intolerances can develop at any age. The most common reason is a natural fall in digestive enzymes: in most of the world's adults, lactase production declines after early childhood, which is why lactose intolerance so often appears in adulthood, as a 2022 review describes. A stomach bug or other gut condition can also temporarily reduce your ability to digest lactose. Genetics, hormonal changes and underlying health conditions all play a part too.

Can a food intolerance cause headaches or migraines?

Some people find that particular foods or drinks set off their headaches or migraines, though the research is mixed. A review of diet and headache found the strongest evidence for caffeine withdrawal as a trigger, with weaker evidence that foods containing histamine, alcohol and gluten may precipitate headaches in some people. For dairy specifically, a 2024 review concluded the evidence is mixed, with some studies showing a link and others none. Keeping a food diary remains the most practical way to work out whether a food is triggering your headaches.

Can a food intolerance affect mood or mental health?

Some foods can affect mood more than others. Caffeine is the clearest example: a 2022 analysis of ten studies found that high doses, roughly five cups of coffee, increased anxiety in healthy adults and could trigger panic attacks in people prone to them. Some people also notice symptoms of anxiety after eating foods that cause a sharp rise and fall in blood sugar.

Do food intolerances cause fatigue or low energy levels?

A food coma is the everyday name for fatigue or low energy after eating; the medical term is postprandial somnolence. Feeling tired or lethargic after eating is very common and usually has an ordinary explanation, such as a large meal or one heavy in refined carbohydrates. In some cases it can be a sign of a food intolerance, particularly if one specific food seems to set off your symptoms every time you eat it.

Can food intolerance symptoms be delayed?

Yes, most food intolerance symptoms are delayed, and it is unusual for any symptom to appear immediately after eating. This is one of the clearest differences from an allergy, where reactions tend to be rapid. Some symptoms arrive faster than others: diarrhoea can begin within 30 minutes of eating, while headaches and tiredness may take considerably longer.

Can food intolerances worsen over time?

They can change over time in either direction. Digestive enzyme levels shift naturally as we age, which may cause new intolerances or make existing ones more noticeable. Stress, sleep, overall diet and gut health can all influence how strongly you react to a problem food from one period of life to the next.

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