Symptoms of Neonatal Lactose Intolerance Neonatal lactose intolerance is a disease caused by the inability of infants to digest lactose due to a lack of lactase. If not diagnosed and treated promptly, serious complications may result. The following introduces the symptoms of lactose intolerance in newborns from different aspects. Digestive system symptoms Lactose intolerance in newborns mainly manifests as digestive system symptoms, including frequent diarrhea, constipation, abdominal pain, bloating, vomiting and crying. Lactose intolerance in newborns can also lead to malnutrition and weight loss in infants. lactose intolerance