The reasons for diarrhea are as follows: 1. Yogurt contains a large amount of probiotics, and drinking a large amount of yogurt on an empty stomach can raise the lactic acid bacteria in the human intestinal tract, leading to dysbiosis of the intestinal flora, accelerated gastrointestinal motility, and symptoms of diarrhea. 2. Yogurt still contains lactose after fermentation, although it decomposes some of the lactose. If the patient lacks lactase and is intolerant to lactose, symptoms of bloating and diarrhea will occur. 3. If you drink a lot of cold yogurt on an empty stomach, the yogurt will stimulate the mucous membrane of the gastrointestinal tract and speed up gastrointestinal motility, which will also cause diarrhea. It is best to leave the yogurt at room temperature for half an hour before drinking it to reduce the stimulation of the gastrointestinal tract mucosa, and also note that you cannot drink expired yogurt.