Causes of lactose intolerance include lactase deficiency, insufficient secretion and reduced activity. 1. Lactase deficiency: mostly congenital and relatively rare, symptoms can be relieved by supplementing lactase preparation or probiotics. 2. Insufficient lactase secretion: If a large amount of lactose is consumed, far exceeding the secretion capacity of the intestinal mucosa, it can lead to insufficient lactase secretion, causing lactose intolerance, which can be recovered after dietary adjustment. 3. Reduced lactase activity: Intestinal infections can cause damage to the intestinal mucosa and change the intestinal environment, which can lead to reduced lactase activity and lactose intolerance, which can be recovered after removing the original disease. There are many other causes of lactose intolerance, it is recommended to go to the regular hospital in time, after clarifying the cause of the disease, under the guidance of the doctor to carry out targeted treatment, in order to reduce the adverse effects of the disease.