Adults who drink pure milk have diarrhea, which may be caused by improper consumption, lactase deficiency, lactose intolerance, digestive system diseases, and so on. 1. Improper consumption: In the case of fasting, the milk just removed from the refrigerator immediately drink, due to the lower temperature of the milk, the gastrointestinal mucosa caused by direct stimulation, causing gastrointestinal abnormal peristalsis, diarrhea. 2. Lactase deficiency: due to the lack of lactase, the large molecules in milk cannot be broken down into lactose, causing bloating, gastrointestinal indigestion, diarrhea and other digestive symptoms. 3. Lactose intolerance: due to the lack of lactase enzyme or because of the low level of lactase enzyme in the body, the patient can not digest the lactose in milk, lactose can not be broken down directly into the large intestine, so that the osmotic pressure in the intestines, causing a series of gastrointestinal symptoms, such as active intestinal tinnitus, nausea, vomiting, abdominal distension, diarrhea, abdominal pain and so on. 4. Digestive system diseases: if the combination of digestive diseases, such as chronic gastritis, colitis, can cause diarrhea, and eating too much milk can exacerbate the burden on the stomach and intestines, inducing diarrhea. If adults drink pure milk on the dilution, it is recommended that timely medical treatment, under the guidance of the doctor to improve the relevant examination, combined with the results of the examination to determine the cause of the disease, to give symptomatic treatment, so as not to delay the condition.