Oil in diarrhea is caused by indigestion, intestinal infections, small intestine malfunction, chronic pancreatitis, and other factors. Under normal circumstances, feces should not contain fatty substances, but when oil in the feces occurs, it is mostly due to diseases of the intestinal system. When the body has indigestion, a lot of food will be excreted in a semi-digested state, so the excreted stool will be accompanied by fats and oils. When there is an infection in the intestinal system and a large amount of fatty foods are consumed, some of the fatty substances are excreted in the stool because the intestine is unable to absorb the nutrients from the food in time. The small intestine is mainly responsible for absorbing nutrients from food, and if the function of the small intestine is impaired, the absorption of nutrients will be affected, and the feces will be oily. Fatty foods need to be digested by the pancreatic juice before they can be absorbed by the body. Some people have chronic pancreatitis, which affects the secretion of pancreatic fluid, and because of the lack of pancreatic fluid in food, fatty substances are excreted along with food residues.