Stomach cancer, small intestine malabsorption syndrome, Crohn’s and intestinal cancer will cause the human body to become thinner and thinner. Firstly, stomach cancer is a malignant disease, which not only affects the function of the stomach, but also consumes a lot of energy in the human body, so rapid weight loss will occur after the disease. Secondly, the nutrition in food is mainly absorbed through the small intestine. Once the function of small intestine is damaged, the nutrition in food cannot be absorbed, resulting in insufficient nutrition in the body, which will lead to a thin body in the long run. Crohn’s disease is a disease of unknown etiology and complex symptoms, which can seriously affect the absorption function of the small intestine and also affect the shaping of feces in the intestine, and after the disease, patients not only become thinner, but also have symptoms such as bloating and diarrhea. Finally, intestinal cancer will affect the function of the intestine and violate the immunity of the body, which will lead to changes in bowel habits and gradual loss of weight after the disease.