Lactose in milk and dairy products is decomposed into glucose and galactose by lactase in intestinal mucosal epithelial cells and absorbed by human body. If the human body lacks lactase, after drinking milk, lactose cannot be decomposed and accumulated in the intestine, which increases the osmotic pressure of the intestinal cavity and prevents water from being absorbed; At the same time, stimulating the intestine and increasing intestinal peristalsis can induce diarrhea. A large amount of lactose reaches the large intestine, which is fermented by bacteria to produce a large amount of organic acids and gases, causing abdominal pain, bloating and vomiting. Lactose in feces increases and lactose appears in urine, which is called lactose intolerance. Babies or young animals generally do not lack lactase, so lactose intolerance does not occur. However, due to the lack of lactase, some adults may have lactose intolerance when drinking milk or eating some dairy products. After milk is made into sour dairy products, lactose can be decomposed by lactic acid bacteria to produce lactic acid, so lactose intolerance can be avoided.