Lactose intoleranceE73.9

Lactose intolerance, in which one component of milk, lactose (= milk sugar), causes intolerances. The degree of severity varies greatly from individual to individual and depends on whether the lactase (lactose-cleaving enzyme also known as lactase-phlorizine-hydrolase, a β-d-galactosidase) is completely absent or still has a residual function.

• Neonatal lactase deficiency: Hereditary, very rare metabolic disease with clinical symptoms already during breastfeeding.
• Primary acquired (physiological) lactase deficiency: manifestation when the amount of lactase produced decreases during life. This form is widespread worldwide and usually manifests itself in adulthood.

Worldwide, but regionally varying distribution. The majority of the world's population (approx. 90%) can no longer fully utilise milk sugar after infancy. In Asian countries, for example, most people lack lactase, the enzyme that breaks down milk sugar. This is why you will not find milk or cheese products on the menu in these regions. In Central Europe about 10-20% of the average population suffer from lactose intolerance; in Northern Europe about 2%; black population up to 80%.

Primary lactase deficiency is caused by mutations on the long arm of chromosome 2 (2q21) in the regulator of the lactase gene.

The secondary (acquired) lactase deficiency occurs in celiac disease and other diseases of the small intestine.

Absence or insufficient production of the digestive enzyme lactase (found in the small intestine). Note: Lactase is necessary to break down lactose into its individual components (glucose + galactose). If lactose is not broken down, larger quantities reach the lower parts of the intestine. There they serve the bacteria as a nutrient substrate. Result: fermentation of the lactose with the formation of large quantities of gases (_{CO2 +} H2) and organic acids (short-chain fatty acids).

Anamnesis (nutrition diary) with the clinical relation between lactose intake and clinical symptoms.

Oral milk sugar load test (lactose load test) with 50 g milk sugar. If the lactose is not sufficiently absorbed in the small intestine, this can be detected by two methods:

• Blood sugar test: After administration of 50 g of dissolved lactose in 400 ml of water, there is no or too little increase in blood sugar (blood sugar increase < 20 mg/100 ml of blood). Measurements after 30, 60, 90 and 120 min.
• H2 breath test: If there is a relevant lactase deficiency, lactose passes through the small intestine and is then broken down by bacteria in the colon, producing H2, which is resorbed by 10-20% and excreted through the lungs.
  An increase in H2 >20ppm at 50 g lactose is considered pathological.
  

Food allergy; food intolerance (see also histamine intolerance); fructose intolerance (with analogous clinical symptoms); FODMAP intolerance;

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