1, malnutrition and its harm to people Malnutrition includes major nutrient deficiencies and micronutrient deficiencies in two categories. The former refers to the amount of energy and protein available in the body or insufficient absorption, such as severe hunger can lead to the occurrence of this condition; while the latter refers to some essential nutrients, such as vitamins, micronutrient deficiencies will weaken the normal functioning of the body, resulting in a variety of diseases. Malnutrition can increase the risk of infection, weaken the immune system, slow down wound healing, reduce organ function and failure, and even death in serious cases. Malnutrition caused by tumor is also called “cachexia”. It is a syndrome that exists in cancer patients with complex symptoms, including slow progress, unconscious weight loss, frequent feeling of fullness and fatigue, etc. It is quite common for patients with malignancies to experience weight loss, and more than 80% of patients with advanced disease present with varying degrees of cachexia. Patients with lymphoma, leukemia, breast cancer and certain soft tissue sarcomas have a lower chance of weight loss; while patients with colon cancer and lung cancer have a slightly higher risk of weight loss; and patients with pancreatic cancer and liver cancer can have up to 60% incidence of cachexia. According to statistics, about 4-23% of patients eventually die of cachexia. Therefore, every possible effort should be made to treat and take preventive measures to stop or delay the occurrence of cachexia. Malignant tumor has a significant impact on the nutritional status of patients, which can be divided into two aspects, one is to affect the intake and absorption of food, and the other is to affect the human metabolism. In terms of the influence on patients in food intake and absorption, it is further divided into two kinds of effects, direct and indirect. The direct effect refers to the weakening effect of the tumor on the patient’s food intake and the direct invasion of the digestive tract, such as gastric cancer and esophageal cancer. Indirect effect refers to the tumor cells interfering with the patient’s digestive function by affecting digestive factors such as digestive enzymes in diseases such as pancreatic cancer. Tumor affects human metabolism refers to the abnormal metabolic state of human body caused by tumor, for example, some reactive small molecules produced by tumor tissues in the patient’s body will transmit some signals to the organism to enhance catabolism and weaken appetite, thus leading to weight loss. It is because of the existence of this abnormal metabolism that makes the malnutrition of tumor patients particularly difficult to correct. Different from malnutrition triggered by mere starvation, patients with cachexia have difficulty in nutrient absorption due to their own situation, and relying solely on supplemental nutrition cannot reverse their cachexia process. 4. Effect of anti-tumor treatment on patients’ nutritional status There are three kinds of anti-tumor treatment methods in common use: surgery, chemotherapy and radiotherapy. Doctors usually use two or even all three of them successively or simultaneously to treat malignant tumors. However, these treatments are like weapons of mass destruction, while killing tumor cells, they inevitably affect healthy cells in human body in one way or another, causing a series of physical discomfort or side effects, such as sore throat, dry mouth, nausea, vomiting, abnormal taste, pain, diarrhea, constipation, fatigue, emotional depression. Constipation, fatigue, depression, etc. Of course, not every patient will experience all of these symptoms. However, once the side effects occur, it may affect the patient’s normal eating and digestion and absorption, and have adverse effects on the patient’s nutrition level. The degree of malnutrition also has important implications for oncology patients. Studies have shown that patients with weight loss prior to treatment have a shorter survival time than those without weight loss; in addition, patients with malignant tumors who have malnutrition conditions have a reduced effect of chemotherapy and a higher risk of readmission and prolonged hospitalization.