Milk is a food of high nutritional value, and has medical effects such as preventing and treating night blindness, protecting gastric mucosa, lowering blood cholesterol, and nourishing the five internal organs, benefiting fatigue and strain, nourishing the heart and lungs, benefiting the skin, and moistening hair. Therefore, it is one of the best foods for infant feeding, normal health care, and patient nourishment. However, some people (mostly adults) experience bowel sounds and diarrhea after drinking milk. Not only do they fail to obtain any of the nutrients from the milk, but they also expel all the food they have eaten. This is known as milk diarrhea.
It turns out that the sugar in human, cow's, or goat's milk is lactose. It is composed of one molecule of glucose and one molecule of galactose. Digesting lactose requires a special enzyme—lactase. Infants' stomachs are rich in this enzyme, so they digest lactose with ease. As people age, this enzyme gradually decreases, and by adulthood, there is little to none of it left. When milk is consumed again, there is not enough lactase to digest the lactose. The lactose reaches the intestines, where it is broken down and fermented by intestinal bacteria, producing a large amount of carbon dioxide gas. This causes the intestines to expand and stimulates intestinal peristalsis, strengthening contractions and resulting in bowel sounds and diarrhea.
There is another reason. Some people are allergic to milk protein. Normally, people digest and break down proteins before absorbing them, which eliminates the specificity of animal protein. However, some individuals absorb small amounts of milk protein directly into the body without being digested and broken down. For humans, this is a foreign protein. The body produces a rejection reaction to this foreign protein, which manifests as an allergic reaction. The intestines become edematous due to the allergy, a large amount of fluid accumulates in the intestines, and intestinal peristalsis is strengthened to expel the source of the allergy, thereby causing bowel sounds and diarrhea.
Milk diarrhea in adults is not a disease, so it does not require treatment. However, milk should be avoided.