Lactose intolerance: symptoms, causes, tests and diet

The NHS says symptoms of lactose intolerance can start a few minutes, or a few hours, after having food or drink containing lactose. Common symptoms include tummy pain or discomfort, bloating, farting, tummy rumbling, diarrhoea or constipation, and feeling sick or being sick.

The NHS adds that you may also have symptoms not related to the digestive system, such as headaches, tiredness and joint or muscle pain. The British Dietetic Association (BDA) lists the common ones as diarrhoea, bloating, wind, nausea, tummy rumbling and pain, and advises seeing your GP to check for other causes before changing your diet.

Lactose is a sugar found in animal milk (dairy products), including milk from cows, goats and sheep. The NHS lists milk, butter, cheese, cream, yoghurt and ice cream among the dairy products, and notes that many processed foods can contain lactose, including some muesli and breakfast cereals, baked foods such as bread, crackers, cakes, biscuits and pastry, sauces and salad dressings, and diet and protein shakes.

Lactose intolerance is when eating these foods causes symptoms. The NHS says the symptoms can be prevented by eating smaller portions of foods that contain lactose or avoiding them completely.

The NHS says the most common cause is when the body does not make enough of an enzyme called lactase, which helps you digest lactose. It can start at any age and can also be caused by conditions such as inflammatory bowel disease or coeliac disease, bowel infections, bowel surgery or an injury to the bowel.

Some premature babies are unable to digest lactose because their bowel has not developed enough, but the NHS says this usually gets better as the baby gets older. Some people do not make any lactase at all, but this is very rare.

The NHS says a food intolerance is different from a food allergy. Allergies can cause symptoms such as difficulty breathing, itchy skin, and swelling of the lips, face and eyes, and a food allergy can be life threatening. Lactose intolerance is a digestive problem rather than an immune reaction.

The NHS says to call 999 if your lips, mouth, throat or tongue suddenly become swollen, you are breathing very fast or struggling to breathe, your throat feels tight, your skin, tongue or lips turn blue, grey or pale, you suddenly become very confused, drowsy or dizzy, someone faints and cannot be woken up, or a child is limp, floppy or not responding normally. These can be signs of anaphylaxis.

The NHS says that if a GP thinks you have lactose intolerance, you may be asked to follow a lactose elimination diet, where you stop eating foods containing lactose to see if your symptoms improve, have blood tests, or do a hydrogen breath test, where hydrogen gas in your breath is measured to find out how well you digest lactose.

If your symptoms are severe and do not get better, the NHS says you may need a gastroscopy, where a long, thin, flexible tube is passed into your mouth and down into your stomach, and a very small sample of cells may be taken from your small intestine to be tested.

The NHS says that for most people, symptoms can be prevented by reducing the amount of food you eat that contains lactose or by avoiding these foods completely. It stresses that it is important not to cut foods out of your diet or your child's diet without the advice of a GP or a dietitian.

The BDA suggests trying to replace your usual dairy products with lactose-free alternatives for 2 to 4 weeks. You can choose milk products which have had the lactose removed, which still contain calcium, or completely dairy-free products made from soya, oat, rice, coconut or nuts, making sure they are calcium fortified. If symptoms improve, you can retry milk and dairy products by building up the amount and frequency to find out what you can tolerate, as the level of tolerance varies from person to person.

In the long term the BDA says to make sure your calcium intake is adequate along with vitamin D and that your diet is well balanced, and that you can ask your GP to refer you to a dietitian.

The NHS says taking a lactase supplement before having food or drink containing lactose may reduce or prevent symptoms for some people. In some people lactose intolerance is caused by another condition, such as coeliac disease, and treating the underlying cause should help.

The NHS says some premature babies cannot digest lactose because their bowel has not developed enough, but this usually gets better as the baby gets older. It also says not to cut foods out of your child's diet without advice from a GP or dietitian.

The NHS says some symptoms of lactose intolerance are common, but to get medical help if they keep happening or do not go away. See a GP if your symptoms keep coming back and often happen after eating, you have noticed changes in your poo that are not usual for you (such as looser poo, pooing more often or constipation) for 3 weeks, you have had blood in your poo for 3 weeks, you have had tummy bloating and discomfort for 3 weeks, or you have been losing weight without trying.

On the NHS, a GP can suggest an elimination diet and arrange blood tests or a breath test, and can refer you to a dietitian. The BDA recommends asking your GP for a dietitian referral for further support.

Private breath tests and food intolerance panels are widely sold. The tests the NHS describes are an elimination diet, blood tests and the hydrogen breath test, so ask what a private test measures, and share results with your GP so that other causes such as coeliac disease are not missed.