1.The purpose of nutritional support for critically ill patients is to provide energy and nutritional substrates needed for cellular metabolism, maintain the structure and function of tissues and organs; regulate metabolic disorders through the pharmacological effects of nutrients, regulate immune function, enhance the body’s ability to resist disease, and thus influence the development and regression of disease, which is the overall goal of nutritional support for critically ill patients. It should be noted that nutritional support does not completely prevent and reverse the catabolic state and altered body composition of critically ill patients under severe stress. Patients are poorly preserved for supplemented protein. However, reasonable nutritional support can reduce the catabolism and increase the synthesis of net proteins, improve the potential and occurred malnutrition state and prevent its complications. Severely ill patients often have a combination of metabolic disorders and malnutrition that require nutritional support. Nutritional support for critically ill patients should be started as early as possible. Nutritional support for critically ill patients should be adequate to take into account the tolerance of the damaged organs. If the gastrointestinal tract function exists (or partially exists), but the critically ill patients cannot eat normally by mouth, enteral nutrition should be given priority, and parenteral nutrition should be considered only when enteral nutrition cannot be implemented. 2.Nutritional support pathway and selection principle As long as the anatomy and function of gastrointestinal tract allows and can be used safely, enteral nutrition support should be actively used. If the gastrointestinal tract cannot be used or applied insufficiently for any reason, parenteral nutrition or combined application of enteral nutrition should be considered. 3.Enteral nutrition route selection and nutrition tube placement The route of enteral nutrition can be performed by nasogastric tube, nasojejunostomy, percutaneous endoscopic gastrostomy, percutaneous endoscopic jejunostomy, intraoperative gastro/jejunostomy, or transenteric fistula according to the patient’s condition. For critically ill patients who do not tolerate transgastric nutrition or have high risk of reflux and aspiration, transjejunal nutrition is preferred. (1) Transnasogastric tube route: It is often used for patients with normal gastrointestinal function, non-comatose and patients who can be transitioned to oral diet by short time tube feeding. The advantages are simple and easy to implement. Disadvantages are increased incidence of reflux, aspiration, sinusitis, and upper respiratory tract infection. (2) Transnasal jejunal tube feeding: The advantage is that because the catheter enters the duodenum or jejunum through the pylorus, the incidence of reflux and aspiration is reduced, and the patient’s tolerance to enteral nutrition is increased. However, the osmolarity of the nutrition solution should not be too high at the beginning of feeding. (3) Percutaneous endoscopic gastrostomy (PEG): PEG refers to a percutaneous gastrostomy under the guidance of fiberoptic gastroscope to place the nutrition tube into the gastric lumen. The advantage is that the nasal tube is removed, reducing the complications of infection in the nasopharynx and upper respiratory tract, and the nutrient tube can be left in place for a long time. It is suitable for critically ill patients who cannot eat for a long time such as coma and esophageal obstruction, but have good gastric emptying. (4) Percutaneous endoscopic jejunostomy (PEJ): PEJ is performed under the endoscopic guidance of percutaneous gastrostomy, and under the endoscopic guidance, the nutrition tube is placed into the upper jejunum, which can perform gastric decompression while jejunal nutrition, and can be left for a long time. The advantages of this procedure are that in addition to reducing the complications of nasopharyngeal and upper respiratory tract infections, the risk of reflux and aspiration is reduced, and gastroduodenal decompression is feasible at the same time as feeding. It is particularly suitable for critically ill patients who need gastroduodenal decompression such as those with risk of aspiration, gastric dysmotility, and duodenal depression.