I. Assessment of nutritional status of gastrointestinal tumor patients Gastrointestinal tumor patients mainly present with protein-energy deficiency type of malnutrition. The rough indicators of nutritional status are recent weight loss (less than 10% of the normal standard), reduction of lean tissues (such as muscle tissue) and reduction of subcutaneous fat; further examination is the measurement of serum proteins (such as albumin, transferrin and prealbumin), of which albumin is an important indicator for nutritional evaluation, and transferrin and prealbumin are more sensitive because of their short half-life; total lymphocyte count is also an indicator of nutritional status. Total lymphocyte count is also an indicator reflecting the nutritional status of the patient. Total lymphocyte count is also an index reflecting the nutritional status of patients. The rational application of the above indexes can basically reflect the nutritional status of patients, so as to assess the nutritional status and provide a basis for nutritional support treatment. The purpose of nutritional support for patients with gastrointestinal tumors is to reverse malnutrition, reduce cachexia, and reduce related complications and mortality. This goal can only be partially achieved at present, because malnutrition in gastrointestinal tumor is not caused by hunger alone, it is caused by multiple reasons. When malnutrition occurs in patients with gastrointestinal tract tumors, early nutritional support should be given. When the intestinal function of the patient can meet two-thirds of energy and nitrogen needs of the body, nutritional supplement can be given completely through intestinal route, and the specific methods are transoral, nasal feeding, gastrostomy and jejunostomy input. There are also various nutritional preparations to choose from, such as homogenized meals, whole protein preparations, peptide preparations, elemental meals, etc. Choose according to the patient’s gastrointestinal tract adaptation and nutritional needs. If the enteral nutrition cannot meet the energy and nitrogen needs of the body, it can be supplemented by parenteral nutrition. The nutritional support program is to give the patient 2545kJ/(kg-d) of calories and 124(kg-d) of amino acids, with fat accounting for 35-50% of the total calories. Due to the obvious abnormalities of glucose metabolism in oncology patients, the proportion of fat to total calories can be increased appropriately. Clinically, a metabolic cart can also be used to measure the three major nutrients of patients to calculate the nitrogen balance and help clinicians understand the exact metabolic situation of tumor patients. According to the results of metabolic cart measurement, individualized nutritional support is more ideal. After artificial nutritional support for malnourished patients with gastrointestinal tumors, many nutritional indicators have limited improvement. In this case, the better outcome of artificial nutritional support is only to prevent further development of malnutrition. If the patient’s weight loss is caused by too little food and the tumor growth is not fast, there may be time and opportunity to see the effect of long-term nutritional support. C. Parenteral nutrition and tumor growth TPN has been widely accepted since its application in the 1960s. With the recognition of the consequences of malnutrition in tumor hosts, the development of TPN has been promoted. Although ‘TPN can significantly improve the nutritional status and immune defense function of the organism and increase the tolerance of tumor patients to treatment, the application of TPN may stimulate tumor growth and metastasis, therefore, the use of TPN is still debated. Torosian et al. found that Wistar rats receiving TPN showed a significant increase in tumor cells, tumor size, and weight, and only 2 h after TPN, a significant increase in the percentage of S phase or a significant increase in the percentage of DNA synthesis and the percentage of tumor cells was found.Torosian et al. further compared TPN with Lobund rats on a deproteinized diet, and the results showed that TPN significantly increased the growth of primary tumors and metastasis in the lungs. Most clinical studies also suggest that TPN may promote tumor growth. baron et al. used flow cytometry (FCM) techniques to analyze changes in tumor cell kinetics. They found that patients with head and neck tumors receiving TPN had active tumor cell proliferation and a significantly higher percentage of hyperdiploid cells in the cell cycle compared to pre-TPN, whereas patients on a normal oral diet did not have this change. In conclusion, the role of exogenous nutrients in stimulating tumor growth and altering cell cycle dynamics is still under debate. However, most animal experiments and clinical studies have shown that nutritional support significantly stimulates tumor growth and metastasis by mechanisms that are not well understood and may be related to the different calories in TPN. The increased rate of tumor growth with increased calories may also be related to the source of calories in TPN and the composition of amino acids. Although tumor growth is associated with several factors, it has been found through animal experiments that animals receiving TPN, Acivian and insulin gained weight without tumor growth and remained at their original size. Therefore, with the continuous research on TPN and tumor metabolism, we believe that it is possible to find some kind of nutrients that only benefit host nutrition without stimulating tumor growth in the near future. Application of nutritional support in different clinical stages (a) Perioperative nutritional support The effect of perioperative nutritional support is evaluated differently. Some studies have reported that nutritional support for malnourished tumor patients from TPN 7-10 days before surgery until the postoperative stage when oral feeding is possible can improve the nutritional status of patients and the tolerance of chemotherapy in early postoperative period. The recommended energy intake is 1.5 times the basal metabolic rate and the caloric to nitrogen ratio is l50:l. Early postoperative enteral nutrition in patients with good nutritional status, using preparations with the addition of the immunonutrients φ3-PUFA, arginine, and nucleotides, shortens the postoperative hospital stay and reduces costs compared to patients on standard diets and intravenous nutrition. Benefit of glutamine administration to these patients has also been reported. (2) Nutritional support for patients undergoing radiotherapy and chemotherapy for tumors Adjuvant nutritional support for patients undergoing chemotherapy for gastrointestinal tract tumors has not yet shown obvious clinical effects. Before chemotherapy, if the patient is already significantly malnourished or if chemotherapy will seriously affect the patient’s feeding and is expected to last for more than one year, certain enteral or parenteral nutritional support can be given to the patient at the same time as chemotherapy and radiotherapy. 1. Home TPN and EN application When intestinal obstruction occurs in patients with gastrointestinal tract tumors, TPN and EN are necessary to maintain do life. Home TPN is a reasonable option for patients with progressive tumors if the following criteria are met. For patients with gastrointestinal tumors who cannot swallow and have chronic intestinal obstruction, survival is expected to exceed 2 months, and most malnutrition in such patients is caused by starvation rather than by tumor progression. 2. New nutritional substrates The possible new nutritional substrates for tumor patients are glutamine, arginine and ω-3 fatty acids. Glutamine is an essential nutrient for intestinal mucosa cells, which can improve the functional state of intestinal mucosa. Clinical use of glutamine has certain effect, and there is no information showing that glutamine will promote human tumor growth, and oral intake of glutamine will not affect the effect of chemotherapy drugs. EN preparations containing arginine, omega-3 fatty acids, and nucleotides are beneficial for patients undergoing surgery for gastrointestinal tumors. The effect is even better if preoperative administration of EN containing immune substances is started. The source of this effect is the prevention of infection, not the reversal of the patient’s cachectic state. Recent studies have shown that the use of nutritional preparations of ω-1 fatty acids in patients with progressive pancreatic cancer leads to weight gain and improvement in a series of biochemical indicators, the mechanism of which may be a reduction in the synthesis of IL-l, TNF. In conclusion, malnutrition is often secondary to some serious disease, and gastrointestinal tumors are one of the common ones. Malignant diseases of the gastrointestinal tract and malnutrition are often a vicious circle. Patients with gastrointestinal tumors often have different degrees of gastrointestinal insufficiency, which in turn reduces nutritional intake and further worsens the systemic condition of tumor patients, who may not be able to receive other oncological treatment measures. Therefore, as long as nutritional support can provide an opportunity for comprehensive treatment of patients and can improve their physical condition, nutritional support should be performed.