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Lactose intolerance

Last Update: February 2, 2012.

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A bloated belly, “wind”, stomach ache and diarrhoea: not being able to digest dairy (milk) products properly can cause great discomfort – and a lot of inconvenience, too. Up to 1 in 5 adults, teenagers and children in northern Europe are affected by symptoms like these. They are often thought to be caused by lactose intolerance, but that might not always be true. Some people who are over-sensitive to milk might actually have another problem. It is important to get the diagnosis right before deciding to make major changes to your diet, especially in children, teenagers and people who need more calcium.

Lactose intolerance is not an allergy. This is an important difference. People who have a true milk allergy could react to even a tiny amount of dairy foods or milk. But people who are lactose intolerant can actually have quite a lot of these products without any problems. Read on to find out more about this and what researchers have learned about the myths and facts about lactose intolerance – including whether there are any supplements that have been scientifically proven to really help.

What is lactose and what does it do?

Lactose is a kind of sugar called a disaccharide, which means it is made up of two sugars. The two sugars in lactose are called glucose and galactose. Lactose is the main carbohydrate in cows’ milk and the milk of other animals. It is also found in human breast milk. Soy drinks and other plant-based drinks, sometimes called soy milk or plant milk, do not have any lactose in them though.

An enzyme in our bowels called lactase quickly breaks down lactose into its two component sugars. An enzyme is a protein that starts chemical reactions in the body. Glucose and galactose can only be absorbed by the bowel once they have been separated. The two sugars are then converted into energy the body can use. We can easily get enough energy from other foods, so we do not necessarily need to have a particular amount of lactose in our diets. But we need other ingredients of milk, such as calcium.

What is lactose intolerance and why does it develop?

When we are babies, our bodies are programmed to live only on milk. To process all that milk, babies produce a lot of lactase. But the digestive system is genetically programmed to shift to processing other foods when we are weaned off milk. This means that we produce much less lactase later on in life. And less lactase means that our bodies are not able to break down as much lactose.

The amount of lactase that a person produces varies because of genetic differences. That is why some children and teenagers can tolerate milk better than others. The tendency towards absorbing only a small amount of lactose (lactose malabsorption) is sometimes inherited.

The amount of lactase production that is normal also varies among ethnic groups. Northern Europeans, for example, continue to produce more lactase as adults than some other groups. This means that the limited ability to break down lactose is much more common in people who come from parts of Asia and Africa, for example.

A lot of people cannot tolerate much lactose because they have a bowel condition, such as irritable bowel syndrome in particular. In rare cases people have lactose intolerance because they are born with a rare condition which means that they do not produce enough lactase.

There is also a rare genetic condition called galactosaemia (galactosemia) where the person cannot process galactose. This condition is not the same as lactose intolerance, though.

What is the difference between lactase deficiency, lactose malabsorption and lactose intolerance?

In people with lactose intolerance, eating and drinking food products with lactose in them causes problems. This is what the different medical terms around this condition mean:

  • Lactase deficiency, non-persistence or insufficiency: This means that a person does not produce enough lactase. It is usually caused by the genetic programming we talked about above. However, lactase deficiency can also be caused by a gluten intolerance (celiac disease) or some bowel infections. Almost everyone who has lactase deficiency will not be able to break down lactose as well as other people can. Lactase deficiency is also sometimes called hypolactasia, which is Latin for “low levels of lactase”.
  • Lactose malabsorption: “Malabsorption” is when something is not being absorbed well. People who have lactose malabsorption break down and absorb less lactose than others. Lactose malabsorption, however, does not necessarily cause symptoms. Other factors – such as how quickly people’s bowels work – play a role, too. Malabsorption can have various causes. For example, it might be caused by a chronic inflammation of the bowel like Crohn’s disease or ulcerative colitis.
  • Lactose intolerance (LI): Lactose intolerance is what happens when people have lactase deficiency or malabsorption. It is called lactose intolerance if the unabsorbed lactose is causing noticeable symptoms. These symptoms are usually linked to the amount of lactose the person has consumed: small amounts do not usually cause symptoms at all, while very large amounts can cause symptoms. But LI is not the only possible cause of these symptoms.

What are the symptoms and what causes them?

People who have lactose intolerance notice symptoms shortly after eating or drinking a lot of dairy products or milk. The amount of lactose that causes symptoms varies from person to person. The symptoms can include:

  • A bloated belly
  • Pain in the abdomen (lower belly)
  • Extra flatulence (“gas” or “wind”)
  • Diarrhoea

When lactose reaches our small bowel (small intestine), the lactase enzyme breaks it down quickly. Only then can it be absorbed through the wall of the small bowel in the form of glucose and galactose. If this is done quickly and completely, then little or no lactose reaches the colon. The colon is the biggest part of the large bowel, which comes after the small bowel.

Intestinal tract

If a large amount of lactose reaches the colon, it is broken down by bacteria through a process called fermentation. Fermentation produces extra liquid and gases (“wind”). The colon can absorb some, but not all, of these.

The longer lactose stays in the colon, the more it will ferment, and the more problems it will cause. Some people might not produce a lot of lactase, but their colon might work very quickly. Then the lactose will be removed from the body and it will cause few noticeable problems. The type and amount of bacteria in the colon (the so-called intestinal flora) might play a role too.

What else could be causing these symptoms?

Bloating, pain, flatulence and diarrhoea are also symptoms of a condition called irritable bowel syndrome (IBS). IBS is very common as well, especially over the age of 35 years. You can read more about IBS here. IBS makes the colon hypersensitive (over-sensitive). That means that a person with IBS might not be able to tolerate as much lactose as people who do not have the syndrome. Lactose intolerance can develop in addition to IBS.

Sensitivities to foods other than dairy products could also cause these symptoms. Gluten in wheat, for example, can have this effect. Celiac disease has already been mentioned as a possible factor. Stress or other psychological factors can also cause, or contribute to, these symptoms.

How can I be certain whether or not I am lactose intolerant?

As with all food sensitivities, it is better to be sure of the problem before treating it. This is not always easy, especially since psychological factors can have an impact on how we react to foods.

Researchers commissioned by the US Agency for Healthcare Research and Quality (AHRQ) report that there are several tests used to detect lactase deficiency, lactose malabsorption and lactose intolerance. These include challenge tests, in which people are given lactose and then their breath or blood is tested. The AHRQ researchers concluded that a very careful challenge test is still the best way to find out whether symptoms are being caused by lactose intolerance. You can read about how IBS is diagnosed here.

Before a challenge test, people are often asked not to eat the suspected food (or all foods) for a certain number of hours. That is called elimination. However, simply eliminating a food or drink and seeing if there is a reduction in symptoms is not reliable enough to be sure it was really the problem.

In a lactose challenge test, after the “elimination step”, the person drinks lactose dissolved in water or tea and his or her reaction is monitored. For practical reasons, the elimination step might be skipped in small children. The AHRQ researchers concluded that a single dose of 50g of lactose (or less in certain cases) should be taken in the test for adults. 50g of lactose is the equivalent of about 4 glasses of milk.

How much lactose can someone with lactose intolerance cope with?

The AHRQ researchers found that people with lactose intolerance can usually tolerate:

  • Up to 12g of lactose at once (about 1 cup of milk)
  • Up to 24g spread out through a day (about 2 cups of milk)

This means that most people – even those who are lactose intolerant – can actually consume the daily recommended levels of dairy products without getting any symptoms if they are careful not to have it all at once.

The AHRQ researchers also reported that eating other food at the same time as drinking milk can reduce symptoms. This is because food slows down digestion in the stomach, so the lactose does not reach the small bowel in one big “hit”. The small bowel then has time to absorb more lactose before letting it through to the colon, where the problem starts.

Research also suggests that some people might be able to improve their tolerance by slowly and carefully increasing the amount of lactose in their diet. This has not been fully tested as an approach to managing lactose intolerance, though.

People usually learn to manage their lactose intolerance by reducing the amount of dairy products in their diets. This might not be a bother at all: many people do not want to eat or drink a lot of dairy products anyway. But some might find it very difficult to stick to a “special” diet, especially if the rest of their family does not have a problem with lactose.

It can be particularly hard when you are young and want to join in with everything that your friends are doing. Worrying about your diet or whether you can eat the only food available might even make you more unhappy than the symptoms of lactose intolerance themselves. Over time, though, young people learn how to find out what they can eat, without being embarrassed about it. Learning how to read food labels for yourself can make it easier to be confident about knowing what you can eat and drink without worrying about it. You will find practical tips on shopping for lactose-reduced foods here.

Although managing food hypersensitivity is a never-ending process that can be wearing, it does become normal. There are many people who are lactose intolerant and many others who have vegan diets (avoiding all animal products including dairy), so the options for non-dairy eaters are growing all the time. The variety of non-animal-based drinks, like soy drinks and rice drinks, is constantly on the increase. Such purely plant-based drinks are similar to milk but do not have any lactose in them at all. Some of these products have calcium added to them.

What options are there apart from avoiding dairy products and do supplements work?

Low-fat or skim milk does not have less lactose in it, unless it is also lactose-reduced. There are varieties of cows’ milk that are adapted, either lactose-reduced or hydrolysed. Lactose-reduced milk is produced by filtering out lactose or using a machine called a chromatograph to separate the lactose from the rest of the milk. The hydrolysed milk sold for people with lactose intolerance has the enzyme lactase added to it.

These milks are more expensive than ordinary cow’s milk and they usually still have some lactose in them. This means that, no matter what it says on the container, you still need to read the label carefully if you are trying to reduce lactose in your diet. The AHRQ researchers analysed all the trials they could find where people drank lactose-reduced or hydrolysed milk (with 0 to 2 grams of lactose) instead of normal milk (a dose of up to 12 grams). The people in these trials did not have fewer symptoms on average if they had the milk substitute with less lactose in it. In trials, groups of volunteers agree to be assigned to different treatments or to placebo (fake) treatments, so that a reliable scientific comparison of the results can be made. You can read more about why research is done this way here.

Many people take lactase supplements in the form of tablets or capsules. The idea here is that if you take artificial lactase as well as having normal dairy products, the lactase supplement will help you break down the extra lactose that your bowel cannot comfortably absorb otherwise. Although these products are often claimed to help, the AHRQ researchers said that the trials do not convincingly prove that they work.

Prebiotics, probiotics and yoghurts have also been studied. Prebiotics and probiotics are types of “friendly” bacteria in the bowel which help digest food. There are supplements based on these as well. AHRQ concluded that further research is needed to be more certain about whether or not these supplements or yoghurt can help people with lactose intolerance break down more lactose.

This means that trying to stay under the limits described above, as well as only drinking milk if you eat food at the same time, are still the best known ways to reduce the symptoms of lactose intolerance.

If I limit dairy foods in my diet, how can I make sure I get enough calcium?

Dairy products are not absolutely essential for a healthy diet. However, one of the main concerns when people stop eating and drinking dairy products, or reduce their intake, is whether or not they are still getting enough calcium – especially children, pregnant women and older people. Calcium is an important part of our diets throughout our whole lives. It is important for our bones, teeth and nails, among other things. However, calcium is not only available in dairy products. Other foods and drinks are rich in calcium too. You can read more about calcium here and use our calcium calculator here.

Author: German Institute for Quality and Efficiency in Health Care (IQWiG)


  • IQWiG health information is based on research in the international literature. We identify the most scientifically reliable knowledge currently available, particularly so-called “systematic reviews”. These summarise and analyse the results of scientific research on the benefits and harms of treatments and other health care interventions. You can read more about systematic reviews and why these can provide the most trustworthy evidence about the state of knowledge here. The authors of the major systematic reviews on which our information is based are always approached to help us ensure the medical and scientific accuracy of our products.
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