Milk is a food with high nutritional value, and it 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, health maintenance for normal people, and nutritional supplementation for patients. However, some people (mostly adults) experience bowel sounds and diarrhea after drinking milk. Not only do they fail to obtain any of the nutritional components of milk, but they also expel everything else they have eaten. This is milk-induced 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 effortlessly. As people age, this enzyme gradually decreases, and by adulthood, it is almost non-existent or scarce. When they drink milk again, there is not enough lactase to digest the lactose. The lactose reaches the intestine, where it is decomposed and fermented by intestinal bacteria, producing a large amount of carbon dioxide gas. This causes the intestines to expand and stimulates intestinal motility, strengthening contractions and leading to bowel sounds and diarrhea.
There is another reason. Some people are allergic to milk protein. Normal people digest and break down proteins before absorbing them, which eliminates the specificity of animal protein. However, some people absorb small amounts of milk protein directly into the body without digestion and breakdown. For humans, this is a foreign protein. The human body produces a rejection reaction to this foreign protein, which manifests as an allergic phenomenon. The intestines become edematous due to the allergy, a large amount of fluid increases in the intestines, and intestinal motility is strengthened to expel the source of the allergy, thereby causing bowel sounds and diarrhea.
Milk-induced diarrhea in adults is not a disease, so there is no need for treatment. However, milk should be avoided.