Lactose intolerance is usually the result of your body not producing enough lactase.

Lactase is an enzyme (a protein that causes a chemical reaction to occur) normally produced in your small intestine that's used to digest lactose.

If you have a lactase deficiency, it means your body does not produce enough lactase.

Digesting lactose

After eating or drinking something containing lactose, the digested food passes from your stomach into your small intestine.

The lactase in your small intestine should break lactose down into sugars called glucose and galactose, which are then absorbed into your bloodstream.

If there's not enough lactase, the unabsorbed lactose moves through your digestive system to your colon (large intestine).

Bacteria in the colon break down the lactose, producing fatty acids and gases like carbon dioxide, hydrogen and methane.

The breakdown of the lactose in the colon, and the resulting acids and gases that are produced, cause the symptoms of lactose intolerance, such as flatulence and bloating.

Types of lactase deficiency

Primary lactase deficiency

Primary lactase deficiency is the most common cause of lactose intolerance worldwide.

This type of lactase deficiency is caused by an inherited genetic fault that runs in families.

Primary lactase deficiency develops when your lactase production decreases as your diet becomes less reliant on milk and dairy products.

This is usually after the age of 2, when breastfeeding or bottlefeeding has stopped, although the symptoms may not be noticeable until adulthood.

Secondary lactase deficiency

Secondary lactase deficiency is a shortage of lactase caused by a problem in your small intestine.

It can occur at any age, and may be the result of another condition, surgery to your small intestine or taking certain medication.

Secondary lactase deficiency is the most common cause of lactose intolerance in the UK, particularly in babies and young children.

Possible causes of secondary lactase deficiency include:

• gastroenteritis – an infection of the stomach and intestines
• coeliac disease – a bowel condition caused by an adverse reaction to a protein called gluten
• Crohn's disease – a long-term condition that causes inflammation of the lining of the digestive system
• ulcerative colitis – a long-term condition that affects the large intestine
• chemotherapy – a cancer treatment
• long courses of antibiotics

The decrease in the production of lactase in secondary lactase deficiency is sometimes only temporary, but it may be permanent if it's caused by a long-term condition.

It's also possible to develop secondary lactase deficiency later in life, even without another condition to trigger it.

This is because your body's production of lactase naturally reduces as you get older.

Congenital lactase deficiency

Congenital lactase deficiency is a rare condition that runs in families and is found in newborn babies.

It's caused by an inherited genetic fault that means affected babies produce very little or no lactase.

The genetic mutation responsible for congenital lactase deficiency is passed on in an autosomal recessive inheritance pattern.

This means both parents must have a copy of the faulty gene to pass on the condition.

Find out more about genetic inheritance

Developmental lactase deficiency

Some babies born prematurely (before the 37th week of pregnancy) have a temporary lactose intolerance because their small intestine was not fully developed by the time they were born.

This is known as developmental lactase deficiency and usually improves as affected babies get older.

It's important to visit your GP if you think you or your child may have lactose intolerance, as the symptoms can be similar to other conditions.

Before seeing your GP, keep a diary of what you eat and drink, and what symptoms you experience.

Tell your GP if you notice any patterns or if there are any foods you seem particularly sensitive to.

Your GP may suggest trying to remove lactose from your diet for 2 weeks to see if it helps to relieve your symptoms.

This will provide further evidence of whether you're lactose intolerant.

Further testing

Other tests are not usually needed, but your GP may sometimes suggest further tests to:

• help confirm the diagnosis
• find out how much lactase (the enzyme used to digest lactose) your body is producing
• try to determine what might be causing your lactose intolerance

Some of the main tests that may be used are:

Hydrogen breath test

A hydrogen breath test is a simple way of determining if you may be lactose intolerant.

You'll be asked to avoid eating or drinking during the night before the test.

When you arrive for the test, you'll be asked to blow up a balloon-like bag. 

This sample of your breath will be tested to find out how much hydrogen is present, measured in parts per million (ppm).

You'll then be given a drink of lactose solution and your breath will be tested every 15 minutes over the next few hours to see if the level of hydrogen changes.

If your breath contains a large amount of hydrogen (more than 20ppm above your baseline) after consuming the lactose solution, it's likely that you're lactose intolerant.

This is because lactose intolerance can cause the bacteria in the colon (large intestine) to produce more hydrogen than normal.

Lactose tolerance test

In a lactose tolerance test, you'll be given a drink of lactose solution and a blood sample will be taken.

The blood will be tested to see how much blood sugar (glucose) it contains.

If you're lactose intolerant, your blood sugar levels will either rise slowly or not at all.

This is because your body is unable to break down the lactose into glucose.

Milk tolerance test

In a milk tolerance test, you'll be given a glass of milk (about 500ml) and your blood sugar levels will be tested.

If your blood sugar levels do not rise after drinking the milk, you may be lactose intolerant.

Small bowel biopsy

A small bowel biopsy is rarely used to diagnose lactose intolerance.

But it may be carried out to see if your symptoms are being caused by another condition, such as coeliac disease.

There's no cure for lactose intolerance, but most people are able to control their symptoms by making changes to their diet.

Some cases of lactose intolerance, such as those caused by gastroenteritis, are only temporary and will improve within a few days or weeks.

Other cases, such as those caused by an inherited genetic fault or a long-term underlying condition, are likely to be long-lasting.

Changing your diet

In most cases, cutting down on or avoiding food and drink containing lactose and replacing them with lactose-free alternatives is enough to control the symptoms of lactose intolerance.

The exact changes you need to make to your diet depend on how sensitive you are to lactose.

Some people are able to tolerate some lactose in their diet without any problems, whereas others experience symptoms after consuming food containing only a tiny amount of lactose.

If you decide to experiment with what you can and cannot eat, make sure to introduce new foods gradually, rather than all at once.

This will help you get used to any foods you might be sensitive to, as well as identify any that cause problems.

Eating fewer products containing lactose, or avoiding them completely, can mean you miss out on certain vitamins and minerals in your diet and increase your risk of complications.

This means you'll need to make sure you're getting enough nutrition from either lacto-free foods or dietary supplements.

If you or your child are extremely sensitive to lactose, talk to your GP about your diet.

You may be referred to a dietitian, an expert in diet and nutrition, who can advise you about what foods should be included in your or your child's diet.

Sources of lactose

Milk

A major source of lactose in our diet is milk, including cows' milk, goats' milk and sheep's milk.

Depending on how mild or severe your lactose intolerance is, you may need to change the amount of milk in your diet.

For example:

• you may be able to have milk in your tea or coffee, but not on your cereal
• some products containing milk, such as milk chocolate, may still be acceptable in small quantities
• you may find that drinking milk as part of a meal, rather than on its own, improves how the lactose is absorbed

Dairy products

Products made with milk, such as cream, cheese, yoghurt, ice cream and butter, also contain lactose and may need to be avoided if you're lactose intolerant.

But the level of lactose in these products varies and is sometimes quite low, so you may still be able to have some of them without experiencing any problems.

It's worth experimenting with different foods to find out if there are any dairy products you can eat, as they're a good source of essential nutrients like calcium.

Other foods and drinks

As well as milk and dairy products, there are other foods and drinks that can sometimes contain lactose.

These include:

• salad cream, salad dressing and mayonnaise
• biscuits
• chocolate
• boiled sweets
• cakes
• some types of bread and other baked goods
• some breakfast cereals
• packets of mixes to make pancakes and biscuits
• packets of instant potatoes and instant soup
• some processed meats, such as sliced ham

Check the ingredients of all food and drink products carefully, as milk or lactose are often hidden ingredients.

The word "lactose" will not necessarily be listed separately on the food label, so you need to check the ingredients list for milk, whey, curds and milk products such as cheese, butter and cream.

Some ingredients may sound like they contain lactose when they do not, such as lactic acid, sodium lactate and cocoa butter.

These ingredients do not need to be avoided if you're lactose intolerant.

Medication

Some prescription medicines, over-the-counter medicines and complementary medicines may contain a small amount of lactose.

While this is not usually enough to trigger the symptoms of lactose intolerance in most people, it may cause problems if your intolerance is severe or you're taking several different medicines.

If you need to start taking a new medication, check with your GP or pharmacist in case it contains lactose.

Lactose-free foods and drinks

There are a number of alternative foods and drinks available in supermarkets to replace the milk and dairy products you need to avoid.

Food and drinks that do not usually contain lactose include:

• soya yoghurts and cheeses
• coconut-based yoghurts and cheeses
• almond milk, yoghurts and cheeses
• rice milk
• oat milk
• hazelnut milk
• foods with the "dairy-free" or "suitable for vegans" signs
• carob bars

Lactose-free dairy products

There are a number of lactose-free dairy products available to buy that are suitable for people with lactose intolerance.

These contain the same vitamins and minerals as standard dairy products, but they also have an added enzyme called lactase, which helps digest any lactose so the products do not trigger any symptoms.

Lactose-free versions of milk, yoghurt and cheese are normally available in larger supermarkets.

Getting enough calcium

If you're unable to eat most dairy products, you may not be getting enough calcium in your daily diet. 

Calcium has several important functions, including:

• helping build strong bones and teeth
• regulating muscle contractions (including heartbeat)
• ensuring blood clots normally

It's a good idea to choose lactose-free products with added calcium and ensure your diet contains alternative sources of calcium, such as:

• leafy green vegetables, such as kale, broccoli, cabbage and okra
• soya beans
• tofu
• nuts
• bread and anything made with fortified flour
• fish containing edible bones (for example, sardines, salmon and pilchards)

You can also buy combined calcium and vitamin D supplements from most pharmacists to help maintain good bone health.

It's important to check with your GP or dietitian whether you should be taking supplements, however, as taking excessively high levels of calcium can cause side effects.

Lactase substitutes

In addition to dietary changes, you may also find it useful to take liquid drops, tablets or capsules that contain lactase substitutes. These are available from most health foods shops.

Lactase substitutes replace the lactase your small intestine is not producing, which can reduce your symptoms by helping your body break down any lactose in your diet more easily.

Lactase substitutes can either be added to milk or taken just before eating a meal containing lactose.

Lactose intolerance in children

If your child is lactose intolerant, they may be able to consume small amounts of lactose without experiencing symptoms.

This is quite safe, but you may need to experiment to find out how much they can comfortably eat or drink.

If your child is unable to tolerate any lactose, your doctor may refer you to a dietitian for nutritional advice.

This is because it's important for young children to have certain nutrients in their diet to ensure healthy growth and development.

In general, the same rules about foods to try or avoid are similar for children and adults.

For babies with lactose intolerance, lactose-free formula milk is available to buy from pharmacies and supermarkets.

But soya formula is not recommended for children under 6 months because it contains hormones that may interfere with your baby's future physical and sexual development.

Breastfed babies may benefit from lactase substitute drops to help their bodies digest the lactose in breast milk.

For many children, lactose intolerance is only temporary and will improve after a few weeks.

After this point, it's safe to gradually reintroduce milk and dairy products into their diet.

Advice for breastfeeding women

It's perfectly safe to breastfeed your child if you're lactose intolerant.

It does not put them at greater risk of becoming lactose intolerant and has important health benefits for your baby.

Find out more about the benefits of breastfeeding