Causes and diagnosis of lactose intolerance

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Some people have digestive problems after drinking milk or eating dairy products. They only tolerate very small amounts of lactose because their bowel has trouble breaking it down.

Lactose is the main carbohydrate in milk produced by cows and other animals. Human breast milk also contains lactose. It is not present in vegetable products like soy milk. Lactose consists of two sugars: glucose and galactose. An enzyme in our small intestine called lactase quickly breaks down the lactose into its two parts. Only after the two sugars have been separated can they be absorbed by our bowel.

How does lactose intolerance develop?

Babies' digestive systems are designed to survive on breast milk alone. In order to digest the milk, babies produce a large amount of the enzyme lactase. When a child is weaned off breast milk, their digestive system gradually adapts to other foods. Their body then produces considerably less lactase, and can therefore only break down smaller amounts of lactose. This is referred to as primary lactase deficiency. 

Due to genetic differences, some people's bodies produce much less lactase than others, so they don't tolerate milk as well as other people. The amount of lactase normally produced also varies between ethnic groups. For example, Northern European adults produce more lactase than adults from parts of Asia and Africa, where traditionally only very small amounts of dairy products are eaten.

Some people's bodies produce less lactase due to a bowel problem. This is called secondary lactase deficiency. It may be caused by things like gluten intolerance (celiac disease) or chronic inflammations in the membranes lining the small intestine, such as Crohn's disease.

What causes the symptoms of lactose intolerance?

When lactose enters the small intestine it is quickly broken down by the enzyme called lactase. Only then can the separate building blocks of the lactose (glucose and galactose) be absorbed by the small intestine. If that happens quickly and completely, only a small amount of lactose or no lactose at all enters the large intestine (colon). 

Illustration: The digestive system - as described in the article

The digestive system

But if large amounts of lactose do enter the colon, they are broken down and converted by . This increases the production of gases, and more liquid passes into the large intestine. The colon can absorb some, but not all, of these extra gases and liquids. This is what causes flatulence (bloating and “wind”) and diarrhea.

The longer the lactose stays in the large intestine, the more it is broken down, which can cause discomfort. Some people may not have much lactase, but their large intestine works more efficiently. Then larger amounts of lactose are eliminated faster and don't cause too much trouble. The amount and type of in the large intestine (called intestinal flora) might play a role here.

What is in the food and the amount of time it stays in the stomach or the small intestine also affects the symptoms. Food that has high amounts of fat in it can slow down your digestion. Then the lactase has more time to break down the lactose, and the symptoms are often less severe as a result.

What else could cause these symptoms?

Bloating, feeling full, pain and diarrhea are also symptoms of a condition called irritable bowel syndrome (IBS). IBS is very common too, especially amongst people over the age of 35. In the large intestine is sensitive and lactose may be less well tolerated.

These symptoms might also be caused by sensitivities to foods other than dairy products, like fructose. Emotional stress can play a role too. 

How is lactose intolerance diagnosed?

Finding the cause of a food intolerance is not easy. There are various ways to diagnose lactose intolerance.

  • Elimination diet: This involves avoiding anything that contains lactose. If your symptoms improve, it is taken as an initial sign that you have lactose intolerance. You also keep a log of what you ate and any symptoms you had. Then a tolerance test is done under medical supervision. This test checks whether the symptoms return after you drink a lactose solution.
  • Hydrogen breath test: If the tolerance test results are inconclusive, the amount of hydrogen in your breath is measured before and after drinking a lactose solution. When lactose reaches the large intestine and is broken down by , hydrogen is produced as a byproduct. The hydrogen is released into the bloodstream through the walls of the intestine and ends up being breathed out once it reaches the lungs. So people with lactose intolerance generally have more hydrogen in their breath. The test solution for adults contains 50 mg of lactose in 0.4 liters of fluid and is taken on an empty stomach. The breath test is repeated every 15 to 30 minutes for at least two or three hours. But smoking, chewing gum, poor oral hygiene and other factors can influence the results of the test. That is why it is important to check whether the lactose solution also causes typical lactose intolerance symptoms like bloating and diarrhea. If not, then higher hydrogen levels are likely to be caused by something else.
  • Lactose tolerance test: This test measures blood sugar levels after drinking a lactose solution. If your blood sugar doesn't rise significantly, it's a sign that you are lactose intolerant. This is because the body can't absorb the sugar building blocks if the lactose isn't broken down. The test solution for adults contains about 50 grams of lactose and is also taken on an empty stomach. Blood sugar levels are measured regularly over the next two hours. It is important to watch out for symptoms like diarrhea or cramps during this test as well.

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IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Updated on November 29, 2018
Next planned update: 2021


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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