Lactose intolerance refers to the symptoms of diarrhea caused by the difficulty in digesting lactose in human milk or cow's milk due to a deficiency of lactase in the body, also known as lactase deficiency syndrome. It is primarily seen in infants and young children who consume milk as their main food, with the highest incidence in China among children aged seven to eight years old. In cases of lactase deficiency, it is advisable to avoid using cow's milk or human milk. Instead, soy milk or sour milk can be used as substitutes, as they do not contain lactose, thus avoiding the phenomena of lactose intolerance.