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Digestive Disorders > Intolerances > Lactose Intolerance

Lactose intolerance: The essential questions answered

Dr. med. André Sommer

Dr. med. André Sommer

Lactose intolerance, also known as milk sugar intolerance, is when the milk sugar from food in the intestine cannot be digested due to a deficiency in the enzyme lactase. The milk sugar then remains undigested in the intestine, resulting in diarrhea, abdominal pain, and gas.

Lactose intolerance is often seen in late adolescence and adulthood. Those affected should follow a lactose-free or low-lactose diet to alleviate symptoms. If necessary, the enzyme lactase can be replaced in tablet form.

Top things to know about lactose intolerance

  • Lactose intolerance means that those affected cannot digest milk sugar.
    Symptoms include diarrhea, flatulence and abdominal pain after eating milk and dairy products
  • The cause is a deficiency or the complete absence of the enzyme lactase in the intestine
  • Lactose intolerance usually only develops in the course of life, so it is extremely rare in babies and toddlers
  • The therapy for lactose intolerance is a low-lactose or lactose-free diet
  • The enzyme lactase can be replaced in tablet form.

What does lactose intolerance mean?

Milk sugar (lactose) occurs in almost all milk products that have not matured due to bacterial colonization. Lactose is a double sugar (disaccharide) and consists of two compound single sugars: Glucose and galactose. Multiple sugars such as lactose must first be broken down into simple sugar during the digestion process in order to be absorbed:

  • Lactose is first broken down into glucose and galactose by the lactase found on the small intestinal mucosa.
  • Glucose and galactose are then absorbed into the body through the small intestine cells.

Lactose intolerance occurs when there is little or no lactase in the small intestine. Here’S what happens:

  1. The lactose is not broken down.
  2. As a double sugar, the lactose cannot be absorbed into the body via the small intestinal mucosa and then reaches the colon undigested.
  3. Lactose binds water and the contents of the colon become more fluid and voluminous.
  4. The bacteria in the colon also metabolize the lactose, which leads to gas formation.

What are typical lactose intolerance symptoms?

If lactose-containing foods are consumed, people with lactose intolerance will experience symptoms. These arise because the sugar lactose is incompletely absorbed in the intestine, and intestinal bacteria then ferment the lactose. These fermentation processes produce certain gases, such as hydrogen (H2) and carbon dioxide (CO2), which create the feeling of gasiness in the stomach. Carbon dioxide in particular promotes flatulence.

In addition to lactate, the bacteria also produce short-chain fatty acids such as propionic acid, formic acid, acetic acid, valeric acid and butyric acid. These fatty acids can promote diarrhea because they attract water.

Lactose intolerance, similar to the fermentation of other fermentable carbohydrates (FODMAPs) leads to typical symptoms. These can be very similar to irritable bowel symptoms. Signs of lactose intolerance include

  • Abdominal pain and intestinal cramps
  • Flatulence, bloated stomach
  • Diarrhea
  • Constipation (less common)
  • Intestinal cramps
  • Nausea and vomiting

Skin problems are also symptoms of lactose intolerance. They can appear in the form of redness on the face, itching, blemishes, or acne. Some people also report other nonspecific signs of lactose intolerance such as restlessness, dizziness, depressed mood, difficulty concentrating, sweating, headaches, and tiredness.

Who is affected by lactose intolerance?

Lactose intolerance is particularly widespread in East Asia and Africa, where around nine out of ten people there suffer from lactose intolerance. In Europe, about every tenth person is affected by lactose intolerance, with lactose being better tolerated in northern Europe than in the south.

Individual tolerance of milk differs also between adults: Lactose intolerance is therefore not an absolute thing. Rather, the tolerance of milk can change as we age.

Lactose intolerance can also affect children. Typical symptoms are flatulence, abdominal cramps, and diarrhea that occur after eating dairy products. In rare cases, this intolerance is innate. Some children also suffer from temporary lactose intolerance. Lactose intolerance in babies is also often caused by a not yet fully developed intestinal mucosa.

Why are so many people lactose intolerant?

There are more adults with lactose intolerance worldwide than adults who tolerate lactose. The enzyme lactase, which is important for the breakdown of lactose, is formed less and less as people age. This form of lactose intolerance is also called primary lactase deficiency. It is the most common cause of lactose intolerance and can be explained via our ancestors who didn’t keep any animals such as cows, goats, or sheep that produce milk.

Dairy farming is still relatively new compared to human history. The drinking of milk and thus the intake of milk sugar was therefore reserved for babies and toddlers who were breast-fed with breast milk. Almost all infants produce the enzyme lactase, which is important for the digestion of milk, in sufficient quantities. Lactase was completely superfluous to our ancestors after weaning and was therefore more or less not formed in all adults. It was only with the spread of dairy farming that the genes for milk sugar tolerance, particularly in Northern Europe, became established. Lactose intolerance is a kind of primal condition in adults that is still present in most people around the world.

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Can certain diseases cause lactose intolerance?

Some diseases can lead to lactose intolerance. When the body suddenly develops a lactose intolerance due to an illness, this is known secondary lactose intolerance. Diseases that can cause lactose intolerance include:

  • Inflammatory bowel disease (such as Crohn's disease and ulcerative colitis)
  • Intestinal infections (e.g. by enteropathogenic E. coli)
  • Gluten intolerance (celiac disease)
  • Parasites (e.g. giardiasis)
  • Damage and inflammation of the intestinal mucosa (e.g. after radiation or chemotherapy)
  • Alcohol abuse
  • Bacterial overgrowth in the small intestine
  • Whipple's disease

With secondary lactase deficiency, the small intestine villi are usually damaged. The small intestine villi, which form a carpet on the inner intestinal wall, are the place where the lactase is formed. That’s why lactose intolerance can also occur after certain operations on the stomach or intestines.

What diseases often occur with lactose intolerance?

Certain diseases often occur with lactose intolerance. These diseases include

  • Irritable bowel syndrome
  • Fructose intolerance / fructose malabsorption
  • Milk protein allergy
  • Osteoporosis

Is there a lactose intolerance test?

Yes, lactose intolerance can be tested using various methods, including omission tests, lactose stress tests with blood glucose monitoring or hydrogen (H2) breath tests, small intestine biopsies, and genetic tests.

How does an omission test work?

This lactose intolerance test can be done simply by following a completely lactose-free diet. This is best accompanied by a symptom diary. If this improves gastrointestinal symptoms, the omission test gives a first indication of lactose intolerance.

What types of stress tests are there?

There are two forms of lactose stress tests that differ in what is used as an indicator of lactose intolerance. An H2 breath test is carried out in one test after lactose intake, and a blood sugar measurement is done in the second kind of test.

How does the lactose load test with H2 breath test work?

H2 stands for the gas hydrogen. With lactose intolerance, after consuming lactose-containing products, more hydrogen is produced in the air we breathe, which is produced by bacteria and reaches the air we breathe through the blood.

One liter of milk contains about 50 grams of milk sugar. After giving 50 grams of lactose to adults on an empty stomach, the breath test is repeated every 30 minutes up to a maximum measuring time of 120-240 minutes. In children, lactose is reduced to about 1-2 grams per kilogram of body weight. Smoking or chewing gum before the examination can give false results, as can colonization of the small intestine or antibiotic therapy.

How does a lactose load test with blood sugar measurement work?

Adults with suspected lactose intolerance are given 50 grams of lactose and correspondingly less for children. The blood sugar level is then measured after one and two hours. With a normal utilization of milk sugar, the milk sugar is broken down and increases the blood sugar level. With lactose intolerance, the blood sugar level rises less, namely less than 20 milligrams of glucose per deciliter, despite the administration of milk sugar.

How is lactose intolerance detected by a small intestine biopsy or a genetic test?

  • Small intestine biopsy A tissue sample is taken from the small intestinal mucosa. For this, a gastrointestinal and duodenal viewing must be performed. The enzyme lactase can be detected in the tissue sample. A lack of lactase in the biopsy proves lactose intolerance. The small intestine biopsy is considered the gold standard for the diagnosis of lactose intolerance. Nevertheless, it is rarely used because the examination is comparatively complex.
  • Genetic test: Genetic lactose intolerance tests are also complex. These tests, which detect certain genes in the blood or the mucosa, can distinguish primary from secondary lactose intolerance.

How can lactose intolerance be prevented?

Lactose intolerance cannot be prevented.

  • Lactase activity decreases in many people over the course of their lives; this is also the norm for many animals.
  • After an acute gastrointestinal illness, the small intestinal mucosa needs a few days to recover and to start digesting again.
  • With chronic bowel diseases, good therapy for the underlying disease is crucial: If for example you follow a gluten-free diet for celiac disease, the inflammation of the intestinal mucosa regresses. The intestinal mucosa becomes healthier and lactase can often be digested again.

Does raw milk help against lactose intolerance?

It has long been believed that the organisms contained in raw milk help us to tolerate lactose better. A recent study from 2014 suggests this theory is probably incorrect. According to the current state of knowledge, raw milk does not help against lactose intolerance.

Is lactose intolerance curable?

Yes and no. If the lactose intolerance is genetic, it is not considered curable. By changing the diet to lactose-free products, however, those affected can live relatively symptom-free.

Even in cases of secondary lactose intolerance, people are reluctant to speak of a cure if there are no signs of intolerance after appropriate treatment. The symptoms can always return. In addition, experts are discussing the extent to which lactose intolerance can be assessed as a normal condition. This evaluation means that lactose intolerance has no disease value and therefore cannot be said to have a cure.

How can lactose intolerance be treated?

Lactose intolerance may be treated by adhering to a low-lactose diet. Usually, a completely lactose-free diet does not have to be followed because there is residual lactase activity. Studies show that sufferers tolerate up to 12 g lactose during a meal and a total of 18 g lactose throughout the day.

Lactose is mainly found in milk and milk products, and the range of lactose-free products in supermarkets is constantly growing.

Can you go on holiday or eat out with lactose intolerance?

Yes – the enzyme lactase is also available in tablet or capsule form in drug stores and pharmacies. In this way, those affected can replace the lack of enzymes and prevent the typical symptoms if a lactose-free diet is not possible. The lactase tablets are therefore a suitable solution for eating in restaurants, on vacation, or at celebrations.

Can babies be lactose intolerant?

Yes, it is possible, but it is extremely rare. In this case there is congenital lactose intolerance with a lack of lactase. The babies have persistent diarrhea because they cannot digest the lactase contained in breast milk. If there are persistent digestive problems in the infant, a pediatrician should be consulted promptly to make a diagnosis and to initiate appropriate treatment. If the baby is lactose intolerant, as with adults, a lactose-free diet would likely have to be followed.

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Dr. med. André Sommer

Dr. med. André Sommer

I’m André, a medical doctor from Berlin. Together with a team of medical doctors, nutritionists and data scientists we empower people to understand digestive issues with our app Cara Care.

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