Normal metabolism and good nutritional status of the body is an important guarantee for the maintenance of life activities. Any metabolic disorders or poor nutrition can affect the function of tissues and organs, and further deterioration can lead to organ failure. The nutritional status of the organism is closely related to the morbidity and mortality rate. Many critical illnesses in the field of surgery have different degrees of malnutrition, which is often difficult to be treated successfully if active measures are not taken to correct it. Based on the adequate understanding of body metabolism, the establishment of effective input pathways and the successive production and application of various nutritional preparations that conform to physiology and have little side effects, clinical nutritional support therapy has achieved outstanding results in recent times and saved the lives of many critically ill patients. Nutritional support therapy is one of the major developments in clinical medicine in the 20th century, and has become an indispensable and important element in the treatment of critically ill patients. In order to implement nutrition support therapy in a reasonable way, we should first fully understand the normal metabolism of the body and the metabolic changes caused by starvation and trauma. So that nutrition support treatment measures can be adapted to the patient’s metabolic state, which is both effective and less likely to have complications. The current nutrition support methods can be divided into enteral nutrition and parenteral nutrition. The basic nutrition metabolism of human body: the metabolism of the organism involves a wide range of aspects. From the perspective of nutritional therapy, the most important aspects are protein metabolism and energy metabolism. (Some of the amino acids in NEAA are synthesized at a very low rate in the body and need to be replenished when the body needs more. They are called conditionally essential amino acids, such as arginine, glutamine, histidine, tyrosine and cysteine. The synthesis of NEAA in the body can be affected by reduced intake and insufficient sources of EAA when the body is ill. Therefore, from the perspective of clinical nutrition, NEAA should be placed in the same important position as EAA. Glutamine (Gin) is abundant in tissues, and it is the main energy substance for small intestinal mucosa, lymphocytes and pancreatic alveolar cells, providing substrate for anabolism and promoting cell proliferation. glutamine is also involved in the synthesis of the antioxidant glutathione. The lack of Gln in the body can lead to atrophy of the small intestine and pancreas, reduced intestinal barrier function and bacterial translocation, etc. Lack of Gin in skeletal muscle can reduce the rate of protein synthesis, and Gin deficiency can also lead to fatty liver. Gin deficiency can easily occur during trauma and stress. Currently, Gln is not only regarded as a conditionally essential amino acid, but also as a drug with special effects. The special role of arginine has also received attention. Arginine stimulates the release of insulin and growth hormone, thus promoting protein synthesis. Arginine is also a good source of energy for lymphocytes, macrophages, and cells involved in wound healing, among others. Branched-chain amino acids (BCAAs) belong to the range of EAAs, including leucine, isoleucine and valine, which can compete with aromatic amino acids to cross the blood-brain barrier and facilitate the correction of imbalances in the amino acid profile of the brain in hepatic encephalopathy. Under stress, BCAA becomes the energy substance for muscles, and its supplementation will facilitate metabolism. Protein synthesis is influenced by many factors, among which the input of amino acids and the enhanced action of insulin and growth hormone can significantly promote protein synthesis. Protein catabolism is also influenced by many factors, including glucagon, corticosteroids, adrenaline, etc. Many cytokines, such as interleukin-1 and 6 (1L-1, IL-6) and tumor necrosis factor (TNF), are stimulators of proteolysis. The daily protein conversion rate is 3% (250-300g/d), and the amount of nitrogen excreted through the feces is only lg/d. The absorbed amino acids are mainly used for protein synthesis, about 250g/d. Among the daily synthesized proteins, there are 50g of muscle protein, 20g of plasma protein (including albumin, globulin and fibrinogen, etc.), 8g of hemoglobin and 8g of leukocyte protein. Hemoglobin 8g and leukocytes 20g etc. The provision of calories is extremely important for protein synthesis, and normal protein synthesis is possible only when calories are adequately guaranteed. The protein (amino acid) requirement of normal body is 0.8~1.0g/(kg?d), which is equivalent to 0.15g/(kgd) of nitrogen. Stress, trauma, protein requirements are increased, up to 1,2-1.5g / (ks? d) (about 0.2 ~ 0.25g / (kgd) nitrogen). (B) energy reserves and needs The body’s energy reserves include glycogen, protein and fat. The content of glycogen is limited, supplying only about 3765.6H (900kcal), which only accounts for about 1 sum of the normal needs of a day. There is no storage of protein in the body, are the components of organs and tissues, if protein is consumed as an energy source (starvation or stress), it will certainly make the organ function impaired. Obviously, protein cannot be considered as an energy source. Body fat is the largest energy store in the body, with a storage capacity of about 15 kg. Fat is consumed for energy during starvation, which has little effect on the function of tissues and organs. However, while consuming fat, a certain amount of protein is also oxidized for energy supply. The body’s energy needs, can be calculated according to Harris-Benedict formula for basic energy expenditure (basicenergyexpenditure, BEE): Male, 陛BEE (kcal) two 66.5 + 13.7XW + 5.0XH a 6.8XA female, 陛BEE (kcal) two 655.1 + 9.56XW + 1.85XH a 4.68XA female, 陛BEE (kcal) two 655.1 + 9.56XW + 1.0XH a 6.8XA female. W – weight (kg) H – height (cm) A – age (years) The actual resting energy expenditure (REE) of the patient can be measured with the application of modern metabolic instruments. The REE value should be 110% of the BEE. The results of the metabolometer suggest that REE values are about 10% lower than the BEE values of the H-B formula. For this reason, a correction should be made accordingly when applying the H-B formula, i.e. the calculated BEE value is deducted by 10% to be the actual REE value of the patient. In addition, a simple method of estimating caloric needs is that the body requires 7,531-8,368 kI (1,800-2,000 kcal) of calories per day. The basic daily requirement in kilograms of body weight is 104.6ld (25kcal). The source of calories for the body: 15% from amino acids, 85% from carbohydrates and fats. At the time of nutritional support, the amino acids supplied are used as raw materials for protein synthesis, and the ratio of non-protein calories (kcal) to nitrogen (g) is 100-150:1 (1kcal II 4.1868k1). (iii) Assessment of nutritional status The assessment of the patient’s nutritional status can not only discern the degree of malnutrition, but also be an objective indicator of the effect of nutritional support treatment. 1, anthropometric measurements Body weight changes can reflect the nutritional status, but should exclude factors such as dehydration or edema. Body weight below 15% of the standard weight indicates the presence of malnutrition. Triceps skin wrinkle thickness is an indicator of body fat storage, and upper arm circumference measurement can reflect the condition of whole body muscles and fat. If the above measurement value is lower than 10% of the standard value, it indicates the existence of malnutrition. 2.Trimethylhistidine measurement Trimethylhistidine is the final breakdown product of myofibrillar protein and myosin, which is no longer used by anabolism. The measurement of trimethylhistidine excretion in urine can reflect the amount of protein catabolism in the body. The greater the value, the greater the catabolism in the body, and the negative nitrogen balance is obvious. 3, visceral protein measurement including serum albumin (albumin), transferrin and prealbumin concentration measurement. It is an important index for nutrition assessment. In malnutrition, the measurement value is decreased to varying degrees. The half-life of albumin is longer (20 days), while the half-life of transferrin and pre-albumin are shorter, 8 days and 2 days respectively, and the latter can often reflect the short-term changes in nutritional status. 4.Lymphocyte count Peripheral blood lymphocyte count can reflect the immune status of the body. A count of <1,500 often indicates malnutrition. 5.Nitrogen balance test In the absence of digestive tract and other additional body fluid loss (such as digestive tract fistula or extensive burns, etc.), the body protein decomposition is basically excreted from the urine in the form of urea. Therefore, the amount of urea nitrogen in urine is measured (note that 24-hour urine should be collected precisely and measured), and the amount of nitrogen excreted is determined by adding a constant of 2 to 3 g (indicating nitrogenous substances excreted as non-urea nitrogen and nitrogen excreted via feces and skin). The human nitrogen amount is then the nitrogen content of the amino acid solution that is administered intravenously. From this, it can be measured whether the patient is in a positive or negative nitrogen balance and guide nutritional support treatment.