The site of absorption of nutrients is generally the small intestine, and the site of absorption of bile salts and vitamin B12 is also the small intestine, specifically at the end of the ileum. Bile salts are secreted by bile and have the function of decomposing and absorbing fat, and promoting the absorption of vitamin A, vitamin D, vitamin E and vitamin K together with fat, and stimulating gastrointestinal motility. The bile salts entering the small intestine are absorbed into the blood by the ileal mucosa and return to the liver through blood circulation to form bile. Vitamin B12 is widely found in animal foods, such as meat, eggs and milk, and needs to be absorbed under the action of intestinal secretions to enter the blood and bone marrow to promote the development and maturation of red blood cells to avoid pernicious anemia, and to increase the utilization of folic acid to promote fat and protein metabolism and other effects. If the body is healthy, generally do not need to deliberately supplement these two substances, usually pay attention to the protection of the gastrointestinal tract, the main role of the small intestine is to absorb nutrients, if the intestinal flora dysbiosis or abnormal function, the body will be difficult to obtain nutrients, resulting in a state of sub-health or disease.