What is a food intolerance? Everything you need to know

What is a food intolerance? Everything you need to know
Photo of Kate Hilton RD
Written by Kate Hilton RD Published 12 June 2023
Sonia Khan, Pharmacist
Medically reviewed by Sonia Khan, Pharmacist Last reviewed 4 September 2026

A food intolerance is difficulty digesting a particular food or ingredient, which causes symptoms such as bloating, stomach pain, wind, diarrhoea and tiredness. It is not the same as a food allergy. An intolerance does not trigger the immune reaction that makes allergies dangerous, symptoms usually depend on how much you eat, and they tend to appear hours after eating rather than minutes.

The most common intolerances by far involve sugars and carbohydrates the gut struggles to break down. Lactose is the clearest example. The NIH estimates that around 68% of the world’s population has lactose malabsorption, though far fewer people get symptoms from it.

This guide explains the common types, the symptoms, what causes an intolerance, and how one is identified and managed.

What is a food intolerance?

When you are intolerant to a food, your gut cannot fully digest or absorb one of its components. Whatever is not absorbed travels on to the large intestine, where it draws in water and is fermented by your gut bacteria, producing gas. That combination of extra water and gas is what causes the bloating, cramps, wind and loose stools.

This mechanism explains the two features that mark an intolerance out. Symptoms are dose-dependent. Many people with lactose intolerance can manage milk in tea but not a milkshake. And symptoms are delayed, because the food has to reach the large intestine before fermentation starts, which typically takes anywhere from 30 minutes to several hours.

Intolerances are common, and self-diagnosis is more common still. A 2023 review in Nutrients notes that most reported food intolerances are self-identified rather than confirmed by testing, and that many people restrict foods unnecessarily as a result.

Food intolerance vs food allergy

A food allergy is an immune reaction, usually rapid, that can affect breathing and circulation and can be life-threatening. A food intolerance is a digestion problem. It can make you feel dreadful, but it does not cause anaphylaxis, and eating a small amount of the food is uncomfortable rather than dangerous. There is more to the difference between a food allergy and a food intolerance, including how allergies are tested in the UK.

Three practical differences separate them.

  • Speed. Allergic reactions usually start within minutes, while intolerance symptoms build over hours
  • Dose. A trace of an allergen can trigger an allergy, but intolerance symptoms scale with the amount eaten
  • Symptoms. Swelling of the lips or throat, difficulty breathing and widespread hives point to allergy and need emergency care (call 999). Bloating, cramps and diarrhoea point to intolerance

Common food intolerances

Lactose intolerance

People with lactose intolerance have difficulty digesting lactose, the sugar in milk and milk-based products. Most of the world's adults naturally produce less lactase, the enzyme that digests lactose, than they did as children, so this is the best understood and most validated intolerance. A 2021 review puts its worldwide prevalence between 57% and 65%. Common trigger foods include milk, ice cream and cream, though many people tolerate hard cheese and yoghurt, which contain less lactose.

Gluten intolerance

Some people who do not have coeliac disease still get gut symptoms after eating wheat, barley or rye, a condition researchers call gluten intolerance (non-celiac gluten sensitivity). The research here is unsettled. In a double-blind trial of 59 people who believed they were gluten sensitive, fructans (a fermentable fibre found in wheat alongside gluten) caused more bloating than gluten itself, and gluten scored no worse than placebo. So wheat may be the problem while gluten is not. There is no blood test for this condition, and clinical reviews recommend ruling out coeliac disease first, then a structured trial of removing and reintroducing wheat. Do not cut out gluten before your GP has tested for coeliac disease, because the test only works while you are still eating it.

Fructose intolerance

Fructose is a sugar found in fruit, honey and many sweeteners. Some people absorb it poorly, and large amounts, particularly from apples, pears, fruit juice and high-fructose syrups, can cause bloating and diarrhoea. This everyday fructose malabsorption is different from hereditary fructose intolerance, a rare inherited condition affecting around 1 in 20,000 people (a 2024 clinical guideline) that is diagnosed medically and managed with a strict fructose-free diet. If fructose seems to be a trigger, the amount usually matters more than the food itself.

Histamine intolerance

Histamine occurs naturally in aged and fermented foods such as cheese, cured meats, wine and beer. Some people appear to break it down slowly and react with flushing, headaches, hives or gut symptoms. The evidence base here is younger than for lactose. There is currently no validated test, and the 2023 review above lists histamine intolerance among the conditions that still need better research and are often over-diagnosed. If you suspect it, a short, supervised low-histamine trial with reintroduction is the recognised way to check.

Food additives and chemicals

Some individuals are sensitive to certain food additives, most typically sulphites, which are used to prevent spoilage and maintain colour in foods and drinks. Trigger foods may include dried fruits, processed meats, wine and beer. Sulphite sensitivity can also behave like an allergy in some people, particularly those with asthma, so reactions involving wheezing need medical advice rather than a food diary.

People can react to almost any food, but the intolerances above account for the great majority of genuine cases.

Symptoms of a food intolerance

Symptoms vary from person to person and with the specific intolerance. These are the typical ones.

Most symptoms appear in the hours after eating a trigger food rather than immediately, and their strength usually tracks how much of the food you ate. Our guide to food intolerance symptoms covers the full picture, including the warning signs that point to something other than an intolerance.

What causes a food intolerance?

The most common cause is a shortage of a digestive enzyme. Lactose intolerance is the model. Lactase production falls naturally after early childhood in most of the world's population, so the deficiency is usually genetic and lifelong rather than a sign anything is wrong. Enzyme levels can also fall temporarily after a stomach bug or with other gut conditions, which is why an intolerance can appear, and sometimes disappear, at any age.

Other intolerances work differently. Fructose causes trouble when it is poorly absorbed, histamine when it is broken down too slowly, and fibres such as fructans when the small intestine cannot digest them and gut bacteria ferment them instead.

Gut dysbiosis

An imbalance in the bacteria in our gut, also known as gut dysbiosis, may change how you tolerate foods, because gut bacteria do some of the digestive work, particularly fermenting what the small intestine leaves behind. This is an active research area rather than settled science. It is one reason tolerance can shift over time, but an imbalance on its own does not explain a specific intolerance. A gut microbiome test can show whether the balance of your gut bacteria looks disrupted, which is useful context alongside, not instead of, identifying your trigger foods.

How is a food intolerance diagnosed?

No single test can diagnose a food intolerance on its own, and intolerance symptoms overlap with other gut conditions, so the process is one of narrowing down.

  • Keep a food and symptom diary. Two to four weeks of what you ate and how you felt. Because symptoms are delayed and dose-dependent, patterns beat hunches
  • Remove and reintroduce, with support. Take the suspect food out for a few weeks, then bring it back and watch what happens. This remains the recognised way to confirm an intolerance, and it is best done with a registered dietitian or your GP, especially for whole food groups
  • Medical tests where they exist. A hydrogen breath test can confirm lactose intolerance, and your GP can arrange blood tests to rule out coeliac disease and other conditions that mimic intolerance

Food Intolerance test

The Feel Gut food intolerance test analyses a small blood sample against over 200 foods and shows which ones your body is reacting to most strongly. Results like these work best as a starting point. They give you a shortlist of foods to take into the diary-and-reintroduction process above, not a diagnosis on their own.

See your GP first if your symptoms are severe, persistent, or include any of the warning signs below, because ruling out other conditions comes before intolerance testing of any kind.

How is a food intolerance treated?

Identify, reduce, and find your threshold

Once a trigger food is confirmed, most people do not need to remove it completely. Because symptoms are dose-dependent, the practical goal is finding how much you can comfortably tolerate, and many people with lactose intolerance, for example, manage small amounts spread through the day. Where a food does need removing, work with a registered dietitian to replace what it provided; cutting out food groups long term without support can leave you short of calcium, iron, fibre or B vitamins.

Enzyme supplements

For lactose intolerance specifically, lactase drops or tablets taken with dairy can reduce symptoms. A randomised trial of 60 people found they improved both breath test results and symptoms compared with placebo. Evidence for enzyme supplements aimed at other intolerances is much weaker, so treat broad "digestive enzyme" blends with scepticism and speak to a pharmacist before buying.

Can a food intolerance turn into an allergy?

No. An intolerance and an allergy work through different mechanisms, a digestion problem versus an immune reaction, and one does not become the other. The confusion usually runs the opposite way. Allergy-type symptoms such as wheezing, swelling or widespread hives were never an intolerance in the first place and need assessment by a GP or allergy clinic.

Is it possible to develop a food intolerance over time?

Yes, and it is common. Lactase levels decline naturally with age in most of the world's population, gut infections can temporarily reduce your ability to digest lactose, and changes in your gut bacteria, health and medication can all shift what you tolerate. The reverse also happens. Tolerance can improve, which is another reason to retest your own limits occasionally rather than treating an intolerance as permanent.

When to see a GP

Book a GP appointment before assuming a food intolerance if you have any of the following, because they point to conditions that need proper investigation.

  • Blood in your stools, or bleeding from your bottom
  • Unexplained weight loss
  • A change in your bowel habit lasting more than three weeks, or difficulty swallowing
  • Persistent vomiting, or severe abdominal pain that is getting worse
  • New, persistent gut symptoms if you are over 50
  • Symptoms that suggest coeliac disease (see your GP for a blood test before removing gluten from your diet)

Call 999 immediately for any signs of an allergic reaction, such as difficulty breathing or swelling of the lips, tongue or throat.

Sources

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