Breast milk is the best natural food for babies, and sufficient breast milk can fully satisfy the growth and development of babies within six months. However, some babies have diarrhea, farting, even rotten buttocks, crying and affecting baby’s growth after eating breast milk, consider the emergence of lactose intolerance. Lactose is one of the sugars and is one of the main nutrients for baby’s growth and development. Milk contains lactose and lactase exists on the surface of the small intestine mucosa, which can break down lactose into glucose and galactose for our body to absorb and use. If lactase secretion decreases, it cannot completely digest and decompose lactose in milk, so that part of lactose is fermented by colonic flora into lactic acid, carbon dioxide, hydrogen and other gases. Lactic acid stimulates the intestinal wall, increases intestinal osmotic pressure, increases intestinal peristalsis, and the baby will have diarrhea, flatulence, colic crying, etc. If the baby does not have a lot of stools and the weight gain is normal, the mother adjusts the diet and takes oral intestinal microecological preparations. If it does not relieve and affects the baby’s weight gain, supplementation with lactase can be done before breastfeeding or changing to a special formula without lactose. Congenital lactase deficiency is rare, most of them are secondary to reduced lactase secretion, and as the baby’s gastrointestinal digestive function improves, he or she can continue to drink breast milk or other milk types. Therefore, when a newborn baby has diarrhea from drinking breast milk, it is recommended to seek medical attention and feed under the guidance of a medical professional, and not to stop using breast milk or taking medication without permission.