Food intolerance: symptoms, causes, tests and what to do

The NHS says people with a food intolerance usually get symptoms a few hours after eating the food or ingredient. The common symptoms are diarrhoea, bloating, farting and tummy pain. There are lots of other possible symptoms, including headache, feeling tired or exhausted, feeling sick, constipation, joint pain or rashes. Symptoms can last a few hours or days.

For lactose intolerance specifically, the NHS says symptoms can start a few minutes or a few hours after having food or drink containing lactose, and can include tummy pain or discomfort, bloating, tummy rumbling, diarrhoea or constipation, and feeling or being sick. Headaches, tiredness and joint or muscle pain can also occur.

Allergy UK describes a food intolerance as a reaction that does not involve the immune system. It can happen when the body has difficulty digesting or processing a particular food or a substance within it, such as lactose in lactose intolerance. Unlike a food allergy, it does not cause anaphylaxis.

The NHS says you can be intolerant to any type of food or ingredient, and lists gluten, histamine, caffeine, alcohol, sulphites, salicylates, monosodium glutamate (MSG) and benzoates among the examples. This is a list of substances people report trouble with, not a list of diagnoses; most people react to only one or two.

The two are easy to confuse because both can follow a meal. The key differences, from the NHS and Allergy UK, are in the mechanism, the timing and the risk.

The NHS says you may be referred to a dietitian if a GP thinks you have a food intolerance. Tests may include a special diet where you avoid the suspected food to see if symptoms improve, followed after a few weeks by eating it again to see if symptoms return, blood tests, and a breath test if you might be lactose intolerant. You may also be asked to keep a food and symptoms diary.

For lactose intolerance, the NHS describes a lactose elimination diet, blood tests and a hydrogen breath test, where hydrogen gas in the breath is measured to find out how well you digest lactose. If symptoms are severe and do not get better, a gastroscopy with a small tissue sample may be needed.

The NHS is clear that some tests sold to the public that claim to diagnose food intolerance are not recommended. There is limited evidence that they give accurate results, and they may suggest you should avoid multiple foods, which can be harmful.

The NHS says to try to avoid or reduce eating the food you are intolerant to, including foods where you are intolerant to any of the ingredients. It adds that you should not cut foods out of your or your child’s diet without the advice of a GP or dietitian, because you or your child could miss out on vitamins and minerals, which are especially important for a child’s growth and development.

For lactose intolerance the NHS says symptoms can often be prevented by eating smaller portions of foods that contain lactose or avoiding them completely, and that taking a lactase supplement before food or drink containing lactose may reduce or prevent symptoms for some people. Allergy UK adds that some people with intolerance may tolerate a small amount of the food without symptoms.

Some bowel conditions cause similar symptoms and are worth ruling out if symptoms keep returning.

• Coeliac disease: the NHS describes an autoimmune condition triggered by gluten in wheat, barley and rye that damages the small bowel. It can cause diarrhoea, stomach aches, bloating, tiredness and weight loss, and needs a gluten-free diet; test before cutting out gluten.

• Irritable bowel syndrome: the NHS says it is a common condition that can cause stomach cramps, bloating and diarrhoea and is usually lifelong, but lifestyle changes and medicine help.

• Lactose intolerance caused by another condition: the NHS notes it can be linked to coeliac disease, inflammatory bowel disease, bowel infections or bowel surgery, and treating the cause should help.

The NHS says to see a GP if you or your child have food intolerance symptoms that keep coming back. For lactose intolerance it adds seeing a GP if you have changes in your poo that are not usual for you, such as looser poo, pooing more often or constipation for 3 weeks, blood in your poo for 3 weeks, tummy bloating and discomfort for 3 weeks, or weight loss without trying.

Call 999 if your lips, mouth, throat or tongue suddenly swell, you are breathing very fast or struggling to breathe, your throat feels tight, your skin, tongue or lips turn blue, grey or pale, you become very confused or drowsy, or someone faints. These are signs of a serious allergic reaction rather than intolerance and need immediate treatment in hospital.

On the NHS a GP can examine you, arrange blood tests and refer you to a dietitian or gastroenterology service. Dietitians guide elimination and reintroduction safely, so that nutrition is protected.

Private dietitians and clinics offer similar assessment, often faster. Whatever route you choose, the NHS advises being wary of home intolerance tests with limited evidence, and a supervised diet remains the usual way to confirm a suspected trigger.