Diarrhea in infants, especially rotavirus infected diarrhea, can cause destruction of the intestinal mucosa, resulting in secondary lactose intolerance, which in turn can aggravate diarrhea, leading to a vicious cycle of prolonged diarrhea. Lactose deprivation diet is an important treatment for secondary lactose intolerance. Diarrhea in infants is a common clinical disease in pediatrics. The loss of nutrients and intestinal dysfunction in the body during diarrhea can easily lead to various complications and affect the healthy growth of infants. I. Disorders of fluid balance Fluid balance is an important condition for maintaining normal physiological functions. Since infants are relatively immature in fluid regulation, diarrhea is more likely to cause disorders of water-electrolyte and acid-base balance. Fluid imbalance is very dangerous and should be detected and corrected in time. Dehydration is the most frequent fluid imbalance. In severe cases, electrolytes are lost along with body fluids, the most common one being hypokalemia. Hypokalemia is the most common cause of hypokalemia, which can lead to muscle weakness, cardiac arrhythmias and renal impairment. In addition, calcium (magnesium) ions are also lost from the stool during diarrhea, and hypocalcemia occurs. Metabolic acidosis is also a complication of severe diarrhea, which is caused by the loss of large amounts of alkaline substances due to diarrhea and the accumulation of acid in the body and impaired acid excretion. Infants may show symptoms such as depression, deep and rapid breathing and exhalation of acetone-smelling gas. Second, malnutrition Delayed or chronic diarrhea can lead to malnutrition. In addition to the loss of a large amount of nutrients during diarrhea, long-term diarrhea will destroy the normal digestive and absorption environment of the infant’s intestines, resulting in impaired absorption of nutrients, thus causing malnutrition, which in turn causes atrophy and degeneration of the gastrointestinal mucosa, further affecting the digestion and absorption of nutrients, forming a vicious circle. Therefore, we should actively look for the causes of the prolonged diarrhea, treat the causes, and pay attention to dietary adjustment to ensure sufficient calories and nutrients. Third, intussusception Infant diarrhea is one of the main contributing factors to the occurrence of intussusception. When diarrhea, intestinal peristalsis is disturbed, and when low potassium is present, it causes a weakening of intestinal smooth muscle dynamics, especially viral diarrhea can cause edema and congestion of the intestinal wall, ileal lymph node hyperplasia and local intestinal wall thickening, leading to the occurrence of intussusception. Intussusception is a life-threatening emergency, clinicians should be highly alert to the sudden appearance of crying, irritability, abdominal distension, vomiting, etc. in children with diarrhea and ultrasound to make a clear diagnosis. Secondary lactose intolerance Infant diarrhea, especially rotavirus infected diarrhea, can cause destruction of the intestinal mucosa, resulting in secondary lactose intolerance, which in turn can aggravate the diarrhea, leading to a vicious cycle of prolonged diarrhea. A lactose-free diet is important for the treatment of secondary lactose intolerance. Studies have shown that lactose-free formula in combination with other treatments can significantly reduce diarrhea symptoms and shorten the duration of diarrhea. Therefore, children with diarrheal disease should be tested for secondary lactose intolerance as early as possible and lactose-free formula should be used as early as possible.