In addition to surgery, radiation and chemotherapy, tumor treatment with nutritional support is also one of the important aspects. Tumor itself is a kind of wasting disease, and patients generally suffer from malnutrition. Weight loss is not only the main symptom of tumor patients, but also the cause of deteriorating prognosis. Tumor patients treated by various modalities will develop different degrees of anorexia, nausea, vomiting and swallowing difficulties, which can also lead to deterioration of nutritional status, which in turn affects the effect of chemotherapy and radiotherapy, therefore, nutritional support for malnourished patients is necessary. Nutritional support is important to improve the quality of life and physical status of oncology patients. Nutritional support is indicated for patients receiving aggressive anti-tumor therapy with malnutrition problems or expected prolonged inability to digest or absorb nutrients. Nutritional support is usually not recommended as palliative care for patients with end-stage cancer. The impact of malignancy on the nutritional status of patients is related to the type, site, size, and stage of the tumor. Compared with non-digestive tract tumors and early stage tumors, digestive tract tumors and intermediate and advanced stage tumors have a greater impact on the nutritional status of the body. The incidence of weight loss and malnutrition is 9% in breast cancer patients and up to 80% in esophageal cancer patients. As the tumor load increases, the impact on the body also increases, and a group of progressive nutritional deterioration syndrome characterized by anorexia and tissue depletion may occur, which is usually called cancerous cachexia. About 33% to 75% of cancer patients have anorexia. Among patients in the progressive stage, anorexia is as high as 80%, including about 60% of patients with gastric cancer. Among the patients with different types of tumors, the most prone to anorexia is patients with gastric cancer, accounting for about 45% of patients with cancerous anorexia. Patients with cancer accompanied by weight loss have a worse prognosis compared to those with stable weight. Tumor patients with malnutrition have significantly poorer prognosis compared with those with good nutritional status. Factors leading to malnutrition and cancerous cachexia 1. Local effect of tumor The local effect of tumor on nutrition varies according to the location of tumor. Gastrointestinal tumor may cause malabsorption, abdominal distension and even obstruction, for example, pharyngeal and esophageal tumors may cause swallowing difficulty and pain due to local swelling, while gastric and intestinal cancers may cause partial or complete gastrointestinal obstruction or bleeding, resulting in abdominal pain, abdominal distension, blood loss, etc., causing reduced intake or anemia, etc. 2.Tumor changes metabolism Anorexia is the common manifestation of most patients with progressive tumor. The neuroendocrine center of appetite control is located in the lateral hypothalamus. In the tumor-loaded state, some factors change the input and output stimulation of this area, leading to change in taste perception and loss of appetite. It is also possible that the negative physiological experience of the center in response to the tumor is integrated with olfaction and gustation. Among the three major abnormalities of nutrient metabolism caused by tumors, the most prominent one is the abnormalities of glucose metabolism. The main manifestation is the enhanced lactic acid-glucose cycle, followed by increased glucose production in tumor tissues. Glucose production is significantly increased in the middle and late stages, especially in those with weight loss. Despite the accelerated glucose renewal, the body’s ability to utilize glucose was poor, and it is assumed that a large amount of glucose produced with increased glucose renewal may be acquired and consumed by the tumor. In addition, a large amount of lactic acid produced by anaerobic digestion of glucose may lead to lactic acid accumulation or decreased clearance in the body, followed by nausea and anorexia. Secondly, abnormal fat metabolism, which is characterized by increased fat mobilization and loss of body fat, is one of the typical features of cancerous cachexia. This may be due to decreased intake and impaired utilization, increased catecholamine secretion, insulin resistance, production and release of lipolytic factors by tumors or other tissues. Weight loss occurs when both lipolysis and fatty acid oxidation rates are increased. Since fatty acid is the main energy substance available to the body in the tumor-bearing state and one of the important substances required for tumor growth, even the administration of exogenous fat cannot completely inhibit the continuous fat catabolism and oxidation in cancer patients. The third is the abnormal protein metabolism, which is characterized by an increase in the total protein renewal rate of the body, but ultimately characterized by a greater protein breakdown than synthesis. The endogenous nitrogen loss is firstly reflected in skeletal muscle consumption, followed by visceral protein. When comparing malnourished tumor patients in a starvation state (10 days) with malnourished patients with benign disease and healthy patients, the overall protein turnover rate was 32% and 35% higher in tumor patients than in the latter two patients, respectively. The overall protein renewal rate of patients with different types of tumors increased differently, such as about 50% or more in patients with small cell sarcoma, 50% to 70% in patients with lung cancer and colorectal cancer, but some patients with other tumors had a normal protein renewal rate. The treatment of malignant tumor involves surgery, chemotherapy, radiotherapy, immunotherapy and biological therapy, etc. These treatments can affect the nutritional status of patients from different aspects in addition to the tumor. For example, pre-treatment examinations, preparation and treatment of the GI tract before and after surgery often limit the use and function of the GI tract. Accelerated catabolism due to surgical and anesthetic stress. During chemotherapy, chemical agents act on centrally relevant receptors or localized areas (e.g., the gastrointestinal tract), producing symptoms such as mucositis, tongue inflammation, pharyngitis, and nausea and vomiting, which ultimately affect appetite. Mucositis of the pharynx and esophagus and radiation enteritis caused by radiotherapy to the throat, chest and abdomen affect the digestion and absorption of food. In addition, fear, anxiety and hopelessness about the tumor, as well as cancer pain, can suppress appetite. Sleep disorders also affect the nutritional status and immunity of patients. Nutritional support therapy and its treatment mode? About 31% to 87% of malignant tumor patients have nutritional deficiency, especially in patients with digestive system or head and neck tumors. Inadequate nutrition often leads to increased incidence of postoperative complications, adverse effects of radiotherapy and depression, and in severe cases, increased mortality. Appropriate parenteral and enteral nutritional support can help improve the quality of life and prolong the survival of these patients. Nutritional support methods for cancer patients include oral, tube feeding, and intravenous (parenteral nutrition therapy). Nutrients used for nutritional support in cancer patients include energy (monosaccharides, fat emulsions), nitrogen sources (proteins, amino acids), vitamins, electrolytes, trace elements, insulin, water and some specific amino acids such as glutamine and arginine. It is generally considered that whole blood, plasma, and serum albumin are not suitable for use in gastrointestinal nutrition therapy. Gastrointestinal hyper-nutritional therapy should not be used for those chemotherapy cancer patients who are not malnourished, but proper nutritional support can enhance the tolerance of chemotherapy patients and reduce the side effects of chemotherapy drugs, and some components of nutritional support may have synergistic effects with chemotherapy. 3. Do radiotherapy patients need nutritional support therapy? Various adverse effects during radiotherapy and/or chemotherapy can affect the nutritional intake and absorption of patients, thus affecting the nutritional status of patients. Nutritional support therapy is generally not recommended routinely for patients during radiotherapy, but for patients with head and neck cancer or esophageal cancer, dietary guidance should be given to increase intake during radiotherapy or radiochemotherapy. The relationship between nutritional support and oncologic chemotherapy involves two issues: whether nutritional support can reduce the adverse effects of chemotherapy, and whether nutritional support can enhance the immediate efficacy of chemotherapy or prolong the survival of patients. Although there are several clinical studies with small samples exploring the advantages and disadvantages of nutritional support during chemotherapy, it is difficult to explain the problem because of the small sample size, the complexity of the patient population, and the inconsistency of the patients receiving chemotherapy regimens and nutritional support regimens, so nutritional support is not recommended to be routinely given during chemotherapy. However, in patients who are already undernourished or at nutritional risk, appropriate nutritional support should be given. Clinical data show that patients receiving high nutrition therapy gain weight, most of them recover or improve their immune function and enhance the effect of chemotherapy and radiotherapy, and some patients who were unable to take larger doses of anti-cancer drugs or radiotherapy due to malnutrition can improve their physical condition and tolerate anti-cancer treatment after high nutrition support therapy. Therefore, this kind of high nutrition support therapy has become an auxiliary means to improve the anti-cancer treatment. Do patients with non-terminal malignant tumors need nutritional support therapy? So far, there is no clinical study to confirm the effect of nutritional support on tumor growth. Currently, it is advocated that nutritional support should be given to patients with expected survival >3 months, if there is nutritional risk or nutritional deficiency, in conjunction with the clinical situation. The main purpose of nutritional support is to supplement the difference between actual and projected intake to maintain or improve the nutritional status of the patient. Standard nutritional formulas are currently available for oncology patients without the need for oncology-specific formulas. Randomized controlled studies have confirmed that megestrol can promote appetite, increase food intake and body weight, and improve nutritional indicators and subjective mood in patients with advanced tumors, so progestin is recommended for cachectic patients. Recommendation: 1. So far, there is no evidence that nutritional support has any effect on the tumor growth of such patients, so cancer has no effect on the choice of parenteral enteral nutritional support for such patients. 2. For patients with cachexia, progestin is recommended to stimulate appetite and improve their quality of life. 3.The patient’s nutritional preparation can be standard formula. 4. Indications for starting nutritional support: ① Nutritional risk or nutritional deficiency already exists, or the patient is expected to be unable to eat for >7 days; ② The expected oral intake is <60% of the expected energy consumption and the duration is >10 days; ③ For patients with recent weight loss >5% due to insufficient nutritional intake, the presence or absence of indications can be judged in conjunction with the actual clinical situation. V. Do patients with end-stage malignant tumor need nutritional support therapy? The treatment of patients with end-stage malignancy should be aimed at ensuring quality of life and relieving symptoms, among which quality of life is the most important element in nutritional support assessment. Near the end of life, most patients need very little food and water to reduce hunger and thirst and to prevent mental confusion due to dehydration, and even very little water can help prevent mental confusion due to dehydration. At this point, it is no longer important to maintain the patient’s nutritional status, and excessive nutritional therapy can instead increase the patient’s metabolic burden and affect his or her quality of life. If the patient is close to death, the dying process should not be prolonged. Does nutritional support therapy promote tumor growth? There are concerns whether high nutritional support for cancer patients will promote tumor growth and disease progression. At present, there is no definite conclusion from the research in this area. It is found that after high nutritional supplementation, although tumor cells accelerate proliferation, their sensitivity to chemotherapeutic drugs increases, and the effect of nutritional support on tumor cell proliferation can be used to enhance the effect of chemotherapeutic drugs, thus improving the therapeutic effect. And some specific amino acids (glutamine) and their derivatives may be both nutrients and chemotherapeutic agents themselves. High nutritional support for cancer patients as an adjuvant therapy should be applied to chemotherapy drugs as early as possible after starting nutritional therapy to get the best therapeutic effect. For the nutritional support therapy for general cancer patients, how to choose the nutrients to cooperate with has been proved that the ability of tumor tissues and normal tissues to utilize exogenous nutrients is different, in order to make the nutritional therapy to improve the nutritional status of patients without promoting the growth of tumor. The results of the study show that the use of emulsified fat instead of glucose plus a sufficient amount of amino acids is the most ideal, which can make the patient get the best nutritional supplement without promoting tumor growth significantly, because tumor tissues are very sensitive to the supply of exogenous glucose and amino acids, and tumor energy intake comes from the anaerobic decomposition of sugar and the use of amino acids, which are the products of protein decomposition in the patient’s body, for anabolism. Replacing glucose with fat emulsion to provide energy does not increase the energy material available to tumor tissues, and it can also provide sufficient heat energy to patients, thus reducing protein catabolism, which has wide clinical significance for the nutritional treatment of tumor patients. In conclusion, high nutritional support for tumor patients has an important role in improving patients’ general condition, enhancing immune function and improving the effect of anti-tumor therapy. Although nutritional support therapy has not been seen to promote tumor growth clinically, however it is still a potential danger. Therefore, nutritional support should be accompanied with anti-cancer therapy as early as possible. For those advanced tumor patients who cannot tolerate anti-cancer treatment, the simple use of high nutrition supplementation is detrimental to patients and may shorten their life span, so caution is needed.