Lactose intolerance: Learn More – Causes and diagnosis of lactose intolerance (2024)

Some people have digestive problems after drinking milk or eating products that contain lactose (the sugar in milk). They only tolerate very small amounts of lactose because their bowel has trouble breaking it down.

Lactose is mainly found in dairy products. Human breast milk also contains lactose. Plant-based dairy alternatives like soy or oat milk don't contain any lactose.

Lactose is made up of two sugars: glucose and galactose. Our small intestine produces an enzyme called lactase. This enzyme breaks down the lactose into its two parts so that they can be absorbed by the bowel.

How does lactose intolerance develop?

Babies' digestive systems are designed to survive on breast milk alone. In order to digest the milk, they produce large amounts of lactase. When a child is weaned off breast milk, their digestive system gradually adapts to digest and process other foods. When this happens, some people no longer produce enough of the enzyme lactase to break down the lactose in the foods and drinks that they consume. As a result, they don't tolerate lactose-containing products as well as other people do. This genetic condition is referred to as primary lactase deficiency.

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

What causes the symptoms of lactose intolerance?

When lactose enters the small intestine, it is broken down by an enzyme called lactase. Only then can the separate building blocks of the lactose (glucose and galactose) be absorbed by the small intestine. If there's enough lactase, only a small amount of lactose will reach the colon (large intestine).

If bigger amounts of lactose do enter the colon, they are broken down and converted by bacteria. This fermentation process increases the production of gases, and more liquid passes into the colon. This is what causes flatulence (bloating and “wind”) and diarrhea.

Lactose intolerance: Learn More – Causes and diagnosis of lactose intolerance (1)

Digestion in a lactose-intolerant person

As well as the amount of lactase available, there are some other factors that affect the severity of the symptoms. Some people produce hardly any lactase but have a fast-working colon. They can handle fairly large amounts of lactose quite well because the lactose doesn't stay in their colon for very long. The longer it stays in the colon, the more likely it is to cause problems. The amount and type of bacteria in the colon (called intestinal flora) might play a role too.

Another factor is the type of food a person eats and what they drink, because that determines how long lactose stays in the stomach or small intestine. Food that has a lot of fat in it can slow down your digestion. This gives the small intestine more time to produce enough lactase to break down the lactose, often resulting in less severe symptoms.

What else could cause these symptoms?

Bloating, feeling full, pain and diarrhea are also symptoms of a condition called irritable bowel syndrome (IBS). This disease is very common as well, especially over the age of 35 years. In irritable bowel syndrome the colon is very sensitive and lactose may be less well tolerated. Emotional stress can play a role too.

Digestion problems might also be caused by sensitivities to foods other than dairy products, like fructose.

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 test involves avoiding anything that contains lactose for roughly four weeks. If your symptoms improve, it is a sign that you could have lactose intolerance. You also keep a log of what you ate and any symptoms you had. Then a tolerance test is done. This test checks whether the symptoms return after you drink a lactose solution.

  • Hydrogen breath test: This involves measuring the amount of hydrogen in your breath before and after drinking a lactose solution. When lactose reaches the colon and is broken down by bacteria, hydrogen is made as a byproduct. The hydrogen is released into the bloodstream through the walls of the bowel 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. Smoking, chewing gum, poor oral hygiene and other factors can influence the results of the test.

  • 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're 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.

The hydrogen breath test and lactose tolerance test measure whether your body can break down and absorb the lactose properly. They don't provide any information about your symptoms. So it's important to look out for symptoms like diarrhea and cramps as well. You can only be certain that you're lactose-intolerant if the lactose solution also causes typical symptoms.


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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.

Lactose intolerance: Learn More – Causes and diagnosis of lactose intolerance (2024)
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