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Digestive Disorders > Intolerances

Recognizing the symptoms of lactose intolerance

Dr. med. André Sommer

Dr. med. André Sommer

Lactose intolerance can cause a wide variety of symptoms. Contrary to popular belief, lactose intolerance is not an allergy, but a malfunction of the small intestine. This means that milk sugar (lactose) is not broken down. The unconverted milk sugar, instead of being reabsorbed by the intestinal wall, moves further into the large intestine and supplies gas-producing bacteria with energy. These bacteria then cause a variety of symptoms in the digestive system.

What are specific symptoms of lactose intolerance?

As a rule, lactose intolerance manifests itself through the following symptoms:

  • Flatulence
  • Abdominal pain and abdominal cramps
  • Diarrhea
  • Stool with mucus
  • Bloating
  • Nausea and vomiting

Click here for the lactose intolerance self-test.

The overloaded intestine often emits loud noises, which are perceived as intestinal rumbling. In addition to these clear symptoms, the body often also reacts with symptoms that are difficult to associate with digestion, such as headaches, dizziness and sleep disorders. Some patients also complain of back pain related to their digestive problems. A non-functioning digestive system puts a strain on the entire body, which is why difficulty concentrating, fatigue and palpitations may be observed.

The most common symptoms of lactose intolerance in detail

1. Flatulence

Flatulence is the main symptom of lactose intolerance. It comes about through strong gas development (hydrogen and CO2).

2. Stomach pain

If these gases cannot be released, the intestinal wall may expand. This causes abdominal pain and abdominal cramps. This can be very uncomfortable, especially in the upper small intestine. Upper abdominal pain and lower abdominal pain are common.

3. Diarrhea

With an intolerance, the intestinal bacteria produce short-chain fatty acids in particular. These cause more water to be drawn into the intestine. This causes diarrhea, which can sometimes be very watery.

4. Mucus in the stool

If the fat content of the stool increases due to a lactose intolerance, slimy and greasy stools result. Fatty acids from the intestinal bacteria are also to blame for this.

5. Feeling of fullness

The increased gas formation can lead to a feeling of fullness, which can be very uncomfortable and reduce appetite.

6. Nausea and vomiting

Many sufferers feel bad after eating lactose-containing foods. This can also lead to vomiting.

How can you recognize lactose intolerance?

Identifying food intolerances such as lactose intolerance is not easy. Many of the symptoms associated with lactose intolerance, such as bloating, abdominal pain and diarrhea, also occur in other diseases of the gastrointestinal tract. The digestive system is also extremely sensitive and its trouble-free functioning depends on countless factors. Since our diet is usually made up of a wide variety of components, it can be difficult to determine the cause of indigestion. If, for example, a glass of milk with cookies was identified as a trigger for stomach ache, the lactose of the milk, the gluten of the wheat or the added fructose of the purchased cookies may be the decisive factor. Perhaps the cookies also contain peanuts and an undetected nut allergy causes the symptoms that arise. Keeping a food diary offers good insights, but can be extremely involved. This link (click) offers a simple self test that quickly provides an initial assessment of the personal risk of suffering from lactose intolerance.

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What diagnostic options are there?

The most secure way is to go to a specialist because only this person has the appropriate methods to diagnose lactose intolerance without a doubt and to rule out other diseases of the gastrointestinal tract. The medical diagnosis of lactose intolerance is associated with a certain amount of effort, since only the breath test and taking a small intestine tissue sample provide reliable results. With the so-called H2 breath test, the hydrogen content of the patient's breathing air is measured over a longer period of time. The basis of this test is the fact that the undigested milk sugar molecules in the large intestine are broken down by bacteria that produce hydrogen, among other things. The measurement of gas production in the fasted patient and after consuming a lactose solution thus provides information about how well the lactose has been metabolized.

Under certain conditions, if several intolerances are suspected, the removal and examination of a tissue sample from the small intestine is necessary to determine the lactase content of the intestinal wall. This removal can only be done endoscopically and is a strenuous procedure for the patient.

Are there symptoms related to the skin associated with lactose intolerance?

Symptoms of lactose intolerance usually do not include changes in the complexion, since these are largely confined to digestive system complaints. Skin reactions that have been observed in connection with the consumption of dairy products are more likely be due to asuspected milk allergy which is based on a completely different mechanism:

In the case of an allergy, the immune system actually interprets harmless substances as pathogens and triggers the body to alert, in order to then start the natural defense mechanisms. A milk allergy, in contrast to a lactose intolerance, causes the entire body to be in turmoil; the symptoms of an allergy are not limited to the digestive tract, but often go hand in hand with skin reactions and breathing difficulties.

Is it possible to suddenly suffer from lactose intolerance?

Your digestive system and your tolerance of food are not set in stone. In the course of life, diet, the composition of the intestinal flora and the efficiency of digestion change. It therefore does not appear unreasonable that food intolerances such as lactose intolerance change over the course of life or can only occur in adulthood. If there is a suspicion of lactose intolerance, a four-week complete elimination of lactose has proven itself, through which the intestine normally calms down again. After this time, you can slowly start to include individual lactose-containing products in your diet to carefully observe how your body reacts.

What symptoms occur in adults with lactose intolerance?

Adults can be affected by the complete range of symptoms of lactose intolerance. Most of them are symptoms that directly affect the digestive tract. These include abdominal pain, abdominal cramps and bowel winds as well as a feeling of fullness, nausea, vomiting and diarrhea. In addition to these gastrointestinal symptoms, there are a number of other symptoms especially of adults not directly associated with lactose intolerance, since depending on your personal constitution, life situation and age, they are more likely to be linked to other causes. These symptoms of lactose intolerance include back pain, headache, fatigue, tiredness and poor concentration, but also sleep disorders, dizziness and even a rapid heartbeat.

What symptoms do children with lactose intolerance experience?

Basically, the symptoms of lactose intolerance in children do not differ from those in adults: that is abdominal pain, bloating, flatulence, nausea, vomiting and diarrhea. In children, however, it is much more difficult to diagnose lactose intolerance. On the one hand, it is difficult for children to articulate their discomfort, and on the other hand, some examinations cannot yet be carried out under certain age limits. If children are suspected of lactose intolerance, a medically assisted change in diet is often the only way to counteract the problem.

Is fatigue one of the symptoms of lactose intolerance?

Many patients who suffer from lactose intolerance also report chronic fatigue. People undoubtedly feel tired if, for example, they suffer from insomnia due to indigestion or if their body has to invest too much energy in the digestion. In addition, most people react with fatigue after eating large meals - a natural reaction to digest at rest. Fatigue can therefore be a symptom of lactose intolerance, but the cause of the gastrointestinal tract is likely to be found elsewhere.

How long do the symptoms of lactose intolerance last?

Digestion, speed and responsiveness are different for everyone. Because here, in addition to genetics, the bacterial environment of the intestinal flora can have an impact. In some patients, the symptoms are weak and persist. In other patients, there is a quick, violent reaction of short duration shortly after the meal. Unfortunately there is no rule of thumb for symptoms - individual experiences always have to be considered.

When do the symptoms of lactose intolerance appear?

Many patients with lactose intolerance report suffering from flatulence, abdominal cramps or diarrhea as early as a quarter of an hour after consuming milk or milk products - the reaction usually takes no longer than two hours.

D. L. Swagerty, A. D. Walling, R. M. Klein (2002). Lactose Intolerance. American Familiy Physician. 65:9. 1845–1850. Downloaded on 22.06.2018 from

F. J. Simoons (1970). Primary adult lactose intolerance and the milking habit: A problem in biologic and cultural interrelations. The American Journal of Digestive Diseases. 15:8. 695–710. Downloaded on 22.06.2018 from

H. G. Desai, A. V. Chitre, K. N. Jeejeebhoy (1967): Lactose loading. Gastroenterologia. 108:4. 177–188. Downloaded on 22.06.2018 from

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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