Lactose intolerance in newborns is a set of digestive symptoms caused by the inability to completely digest and break down lactose in breast milk or cow’s milk due to lactase deficiency. Lactase deficiency is common in newborns; however, clinical symptoms are not always present. If lactase deficiency leads to impaired lactose absorption and clinical symptoms such as diarrhea, bloating and abdominal pain, it is called “lactose intolerance”. When lactase deficiency or activity is reduced, a large amount of lactose is retained in the intestinal lumen, and intracellular water and small molecules enter the intestinal lumen due to osmosis, resulting in an increase in fluid volume and diarrhea. When undigested lactose enters the colon with intestinal peristalsis, it can be decomposed and metabolized by intestinal flora to produce lactic acid, acetic acid and hydrogen gas, and the gas produced can lead to abdominal distension, abdominal pain and increased exhaustion, further aggravating the symptoms. Diarrhea caused by lactose intolerance is often dilute watery stools with foam and sour odor, and stools are often laden with milk lumps, and diarrhea can occur more than ten times a day. Diarrhea can also damage the intestinal mucosa cells, further reducing the secretion of lactase, causing a vicious cycle and aggravating the disease. In addition, lactose intolerant newborns have a higher incidence of anemia and osteoporosis than normal children due to prolonged vomiting and diarrhea, which affects their physical development and low weight and length. Severe diarrhea can also cause dehydration and imbalance of acid-base balance.