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Lactose intolerance

Page last reviewed: 13/07/2011

Lactose is a natural sugar that is found in milk. Lactose intolerance occurs when the body is unable to break down lactose and cannot absorb it into the blood. This can cause symptoms such as:

  • a bloated stomach
  • flatulence (wind)
  • diarrhoea


Lactose is found in the milk of mammals, including cows, goats, and sheep, and it is also added to many foods. Foods that contain lactose include:

  • dairy products, such as cream, cheese, and yoghurt
  • biscuits and chocolate
  • some breakfast cereals

As well as being a good source of energy, lactose helps the body to absorb a number of minerals, such as calcium and magnesium.

Lactose intolerance

Lactose intolerance occurs when the body cannot break down and absorb lactose. It is usually caused by the lack of an enzyme, called lactase, which is produced by the small intestine. Enzymes are proteins that cause chemical reactions to occur.

If there is not enough lactase, the lactose cannot be absorbed and it passes into the colon (the large intestine), where it begins to cause symptoms.

How common is lactose intolerance?

It is estimated that around 5% of adults have lactose intolerance.

It is more common among people of some ethnic origins, and particularly people who do not traditionally have milk as part of their diet. Lactose intolerance is thought to occur in:

  • between 50-80% of people of Hispanic, south Indian, black or Ashkenazi Jewish ethnicity
  • almost 100% of people of American Indian, or Asian, ethnicity 


Lactose intolerance can range from mild to severe, depending on how much lactose a person has in their diet and how well their body can digest it.

Lactose intolerance cannot be prevented. However, the condition's symptoms can be reduced by avoiding certain foods that contain lactose. For example, it may still be possible to eat cheese and yoghurt, but not to drink milk.

For more severe lactose intolerance, it is possible to take a lactase substitute to help digestion. It may also be necessary to seek dietary advice to avoid becoming deficient in the other nutrients, such as calcium, that dairy products usually provide.

Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Enzymes are proteins that speed up and control chemical reactions, such as digestion, in the body.

Page last reviewed: 13/07/2011

Lactose intolerance is usually the result of a lactase deficiency. Lactase is an enzyme (a protein that causes a chemical reaction to occur) that is normally produced in your small intestine. If you have a lactase deficiency it means that your body does not produce enough lactase.

Digesting lactose

After eating or drinking something that contains lactose, it passes down your oesophagus (gullet) and into your stomach, where it is digested. The digested food then passes into your small intestine. 

The lactase in your small intestine should break down the lactose into glucose and galactose (another type of sugar), which are then absorbed into your bloodstream. If there is not enough lactase, the unabsorbed lactose moves through your digestive system to your colon (your large intestine).

Bacteria in the colon ferment (react to) the lactose, producing fatty acids and gases such as carbon dioxide, hydrogen and methane. The reaction of the lactose in the colon, and the resulting acids and gases that are produced, cause the symptoms of lactose intolerance.

The main types of lactase deficiency are outlined below.

Primary lactase deficiency

Primary lactase deficiency is the most common cause of lactose intolerance. This type of lactase deficiency is genetically inherited (it runs in families) and usually develops between the ages of two and 20.

Primary lactase deficiency develops when your lactase production decreases as a result of your diet being less reliant on milk and dairy products. This is usually after the age of two, when breastfeeding or bottle-feeding has stopped, although the symptoms may not be noticeable until several years later. 

Secondary lactase deficiency

Secondary lactase deficiency is a shortage of lactase that is 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 it can be caused by some medications. 

Possible causes of secondary lactase deficiency include:

  • coeliac disease, a bowel condition that is caused by an intolerance to a protein called gluten
  • gastroenteritis, an infection of the stomach and intestines
  • Crohn's disease,a chronic (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 treatment for cancer
  • long courses of antibiotics (medicines to treat infections caused by bacteria)

Certain conditions and treatments can cause a decrease in the production of lactase. Sometimes the deficiency is temporary, but if it is caused by a long-term condition it may be permanent.

It is 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.

A faulty gene (a unit of genetic material that determines your body's characteristics) results in little or no lactase being produced. The faulty gene is passed on from both your mother and father. This is known as an autosomal recessive trait.

Familial lactase deficiency

Familial lactase deficiency is similar to congenital lactase deficiency, and is also found in newborn babies. However, with this type of lactase deficiency, your body is able to produce a substantial amount of lactase, but the enzyme does not work and cannot break down the lactose.

As with congenital lactase deficiency, the faulty gene responsible for the production of lactase is passed on from both your mother and father.

Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Enzymes are proteins that speed up and control chemical reactions, such as digestion, in the body.

The intestines are the part of the digestive system between the stomach and the anus that digests and absorbs food and liquid.

Bacteria are tiny, single-celled organisms that live in the body. Some can cause illness and disease and some are good for you.

Genetic is a term that refers to genes. Genes are the characteristics inherited from a family member.

A gene is a unit of genetic material that determines your body's characteristics.

Page last reviewed: 13/07/2011

You should visit your GP if you think that you have lactose intolerance. Before visiting 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 that you seem particularly sensitive to.

Your GP may suggest that you try removing lactose from your diet for two weeks to see if it helps to relieve your symptoms. This will confirm that you are lactose intolerant.

To find out how much, if any, lactase your body is producing, and what might be causing your lactose intolerance, your GP may suggest some further tests.

Hydrogen breath test

A hydrogen breath test is a simple and useful test. You will be given a drink of lactose solution after you have fasted (not eaten) overnight. The concentration of hydrogen is measured in the air that you breathe out. It is measured in parts per million (ppm).

If, after about an hour, your breath contains a large amount of hydrogen (more than 20 ppm above your baseline) it is likely that you are lactose intolerant. The baseline is the amount of hydrogen that is present in your breath before drinking the lactose solution.

Lactose tolerance test

In a lactose tolerance test, you will be given a drink of lactose solution, and then a sample of blood will be taken from your arm using a needle. The blood will be tested to see how much glucose (blood sugar) it contains.

If you are 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 will be given a glass of milk (about 500ml) and afterwards, your blood sugar levels will be tested. If your blood sugar levels do not rise, you may be lactose intolerant.

Stool sample

A stool (faeces) sample may be taken from babies and young children to test for lactose intolerance. This is because large doses of lactose, such as those given in the lactose and hydrogen breath tests, are dangerous for young children.

Instead, stool samples are taken, and the amount of acid in them is measured. If the baby, or child, is lactose intolerant, there will be a high amount of fatty acid, such as acetate, present. This is created by the reaction between bacteria in the colon and the undigested lactose.

The stool sample can also be tested to determine whether there are any parasites present, such as giardia lamblia, or cryptosporidia, which can also cause stomach problems.

Small bowel biopsy

As a small bowel biopsy is an invasive surgical procedure, it is rarely used to diagnose lactose intolerance. However, it may be carried out to confirm whether or not your symptoms are being caused by another condition, such as coeliac disease (a bowel condition that is caused by intolerance to a protein called gluten).

In a small bowel biopsy, a sample of your small intestinal lining is taken using an endoscope (a thin, flexible tube with a light and a tiny cutting tool at the end). The procedure will be carried out under local anaesthetic(a painkilling medication), so it will not hurt.

The sample of intestinal lining will be tested to see how much lactase it contains. If it only contains a small amount of lactase, lactose intolerance is likely. The biopsy will also reveal whether there is another underlying cause for your lactose intolerance.

Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

The intestines are the part of the digestive system between the stomach and the anus that digests and absorbs food and liquid.

Page last reviewed: 13/07/2011

Treatment for lactose intolerance depends on how sensitive you are to foods that contain lactose. The condition can usually be controlled by monitoring your diet.

If you decide to experiment with what you can and cannot eat, make sure that you introduce new foods gradually rather than all at once. This will help you to get used to any foods that you might be sensitive to.

Missing out on the nutrients provided by products that contain lactose can lead to deficiencies in calcium, plus other important minerals. It is particularly important for young children to have certain nutrients in their diet to ensure healthy growth and development.

If you or your child is extremely sensitive to lactose, talk to your GP about your diet. You may need to have regular bone mineral density checks, or you may be referred to a dietitian (an expert in diet and nutrition). They can advise you about what foods should be included in your (or your child's) diet. 


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.

If even a small amount of milk triggers your symptoms, there are some alternatives that you can try, such as soya milk. You can now also buy milk that is made from rice, oats, potatoes and even peas.

Dairy products

Some dairy products may be easier to digest than others. Cheese, for example, usually contains less lactose than milk. In particular, fermented dairy products, such as yoghurts, are often easier to digest. 

Fermented dairy products are products that have been broken down by substances, such as yeast, bacteria, or other micro-organisms. This means that the lactose they contain will already be partially broken down, and they may be easier to digest than fresh dairy products.

Possible dairy products you could try include:

  • yoghurts, including probiotic yoghurts (that contain live bacteria)
  • probiotic milk
  • sour cream
  • cottage cheese
  • hard cheeses, such as Edam and Cheddar

It is important that you do not eliminate dairy products completely from your diet because they provide essential nutrients.

Lactase substitute

Lactose intolerance is usually caused by a deficiency of the enzyme called lactase (a protein that causes a chemical reaction to occur). A lactase substitute is available that can be taken to replace the lactase that your body cannot produce.

The lactase substitute comes in liquid form (usually as drops) that can be taken before a meal or added to milk. This can be very effective in helping your digestive system to digest the lactose in the meal. You can also take lactase pills (lactase enzyme capsules) before a meal.

Both lactase enzyme drops and capsules are available from most health foods shops.


If you are unable to eat most dairy products, you may not be getting enough calcium in your daily diet. You can stock up on calcium by eating foods such as:

  • broccoli
  • cabbage
  • okra (a green vegetable, shaped like a pod that is about 5-18cm (2-7 inches) long  
  • kale (a leafy green vegetable)
  • dried fruit
  • soya drinks with added calcium
  • soya beans
  • tofu (a food product that is made from soya beans and is often used by vegetarians as a substitute for meat)
  • nuts (such as almonds, brazil nuts and sesame seeds)
  • fish containing edible bones (for example, sardines, salmon, and pilchards)

Food labelling

Some foods may be labelled "reduced lactose". However, there are currently no rules to say how much less lactose a product must contain in order to be able to display this label. You should check the list of ingredients to find out exactly how much lactose the product contains.

If a product is labelled "lactose-free", it usually means that the product does not contain natural lactose. It is also a good idea to choose products with added calcium.

Food and drink containing lactose

Food and drink products that may contain lactose include:

  • milk from mammals, including cows, goats, and sheep
  • milk products, such as whey, curd and milk powder
  • dairy products, such as cream, cheese, butter and yoghurt
  • ice cream
  • salad cream salad dressing and mayonnaise
  • biscuits 
  • chocolate
  • boiled sweets
  • cakes
  • peanut butter
  • 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

Make sure that you check the ingredients of all food and drink products carefully because milk or lactose are often hidden ingredients. The lactose contained within milk or milk ingredients will not be listed separately on the food label, so you need to check the ingredients list for milk or milk ingredients as well.

Lactose in medicines

Some prescription medicines, over-the-counter (OTC) medicines and complementary medicines may contain a small amount of lactose. While this is not usually enough to trigger the symptoms of lactose intolerance, it may be if your intolerance is severe, or if you are taking a number of different medicines.

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

Page last reviewed: 13/07/2011

Dairy products such as milk are an important part of a healthy diet. They contain calcium, protein and vitamins such as vitamins A, B12 and D. For adults, the recommended daily allowance (RDA) for calcium is 700mg.

Lactose is also important because it helps your body to absorb a number of other minerals, including magnesium and zinc. These vitamins and minerals are important for the development of strong, healthy bones.

If you are lactose intolerant, getting the RDA of important vitamins and minerals can prove difficult. This may put you at increased risk of developing the following conditions.

  • Osteopenia, a condition where you have a very low bone mineral density. If osteopenia is not treated, it can develop into osteoporosis.
  • Osteoporosis, a condition that causes your bones to become thin and weak. If you have osteoporosis, your risk of getting fractures and broken bones is increased.
  • Malnutrition occurs when the food that you eat does not give you the nutrients that are essential for a healthy functioning body. If you are malnourished, wounds can take longer to heal and you may start to feel tired or depressed.
  • Weight loss. Excessive weight loss can damage your health, and it can also lead to conditions such as osteoporosis.

Page last reviewed: 13/07/2011

The treatment for lactose intolerance is to avoid milk and dairy products or to reduce your intake to a level at which you do not experience any symptoms.

Different dairy foods contain different levels of lactose. For example, a lot of mature or hard cheeses such as cheddar and parmesan are quite low in lactose. It isn't possible to tell a food's lactose content from reading the label, so it's worth experimenting.

Try to include live yoghurt in your diet if you can, as it contains live bacteria that help digest lactose. These yoghurts also boost the levels of healthy bacteria in your gut, which again can reduce symptoms and may help you recover better from secondary lactose intolerance caused by antibiotics.

Reduced or low-lactose milks are suitable for many people with lactose intolerance. These are available from most supermarkets and health food stores.

The following foods naturally do not contain lactose:

  • all soya milks, yoghurts and some cheeses
  • all milks made from rice, oats, quinoa, almonds, hazelnuts, coconut and potato
  • all foods which carry the "dairy-free" or "suitable for vegans" signs
  • carob bars

Try to avoid anything that can irritate or damage the gut, such as excessively spicy food or alcohol.

There is no need to adopt a vegan diet when you're diagnosed with lactose intolerance. Fish, meat and eggs are all allowed on a lactose-free diet and are an important source of vitamin B12, which is only available from animal sources. If you wish to become a vegan for other reasons, speak to your GP for advice.

Some medicines and tablets can contain lactose, so if you're on regular medication or are prescribed a new medicine, it's worth checking its potential lactose content.

Page last reviewed: 13/07/2011

It is perfectly safe to breastfeed your child if you're lactose intolerant. It does not put them at greater risk of becoming lactose intolerant.

If your child is lactose intolerant, they may be able to consume small amounts without experiencing symptoms. This is quite safe, but you may need to experiment in order to establish a comfortable threshold.

In some cases, your child may not be able to tolerate any quantity of dairy food at all. If so, your doctor can refer you to a dietitian for nutritional advice. There are also lactose-free soy formulas available for babies, while soya and nut milks make good substitutes for dairy milk, so older children can drink these instead. Fish, tofu, nuts and green vegetables are also calcium-rich and lactose-free.

Your doctor or dietitian may recommend the use of lactase drops. These can be added to milk to "predigest" the lactose in it, making it safe to drink.

Remember that secondary lactose intolerance, brought on by a bout of illness such as flu or gastroenteritis, is only temporary. Avoid feeding your child dairy food during the illness, but a few days after symptoms have passed you can gradually reintroduce dairy into the diet.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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