Lactose is normally broken down by lactase, a digestive enzyme, in the small intestine so that it can be absorbed into the bloodstream. People with lactose intolerance lack this enzyme or do not produce it in sufficient amounts. The lactose reaches the large intestine intact, where it is broken down by intestinal bacteria. Fermentation gases are generated in the process, which lead to increased intestinal motility and symptoms that may include a feeling of fullness, bloating, abdominal pain, diarrhoea or nausea.

Primary lactose intolerance, the most common form, is caused by a genetic lack of the enzyme, which grows more pronounced throughout life. The other type, secondary lactose intolerance, is acquired as a consequence of chronic inflammation of the intestinal mucosa, such as with coeliac disease or Crohn’s disease. If the underlying disease is successfully treated, the lactose intolerance may regress.

Lactose intolerance is diagnosed with a breath test. To prevent symptoms, allergic individuals must either avoid lactose entirely or reduce the amount of lactose they ingest.

Fructose is a monosaccharide present in foods such as fruit. Two different types of fructose intolerance can be distinguished: the primary, genetic lack of an enzyme (hereditary fructose intolerance) and an acquired intestinal transport defect (fructose malabsorption).

In primary fructose intolerance, the sugar cannot be properly broken down due to an enzyme deficiency. This can result in hypoglycaemia, profuse sweating, vomiting, diarrhoea and abdominal pain. This form of fructose intolerance is rare. Fructose malabsoprtion is much more common. Due a missing transport protein, the sugar is not taken up properly by the intestine and is instead fermented. The fermentation products (gases) lead to a feeling of fullness, bloating, cramps and, in many cases, diarrhoea. This form of fructose intolerance can be diagnosed with a breath test.

People with primary fructose intolerance must avoid fructose throughout their entire life in order to prevent serious consequences such as liver damage. Acquired fructose malabsorption does not lead to any long-term consequences. It can often be improved with a specific diet, allowing patients to eventually tolerate small amounts of fructose again.

Histamine is a biogenic amine found in animal and plant tissues. It is formed by bacteria and is therefore found in higher amounts in fermented, ripened or stale foods. Histamine is also produced in the human body and stored primarily in the mast cells. It plays a key role in allergic reactions. Histamine also performs many tasks in the human body. It promotes the formation of gastric acid, lowers blood pressure, and is involved in the sleep-wake cycle, appetite control and learning ability as a neurotransmitter.
When consumed in very large amounts, histamine triggers symptoms in all people, some of which can be very serious. For this reason, the body has a protective mechanism against this substance, the enzyme diamine oxidase, which is produced in the intestinal mucosa and breaks down histamine.

In people with histamine intolerance, histamine is not properly broken down in the intestine. The reasons behind this are not yet fully understood. The assumption is that the patient either does not produce enough diamine oxidase, or that the diamine oxidase is in an inactive form. In any case, people with histamine intolerance react to smaller amounts of histamine in food than healthy people, and experience symptoms such as rhinitis, eczema, hives, diarrhoea or asthma.

Since there are currently no reliable laboratory reference ranges for diagnosing histamine intolerance, doctors work together with patients to determine their individual threshold. This can be done by modifying the diet in three phases. The patient’s personal tolerance is determined during the first two phases. Once the entire process has been completed, any required dietary changes can be implemented.