Nutrition has the function that organisms obtain nutrition from the outside world to maintain life. Nutrition is a science that studies the effects of food on living things. Nutritionists interpret nutrition as the interaction and balance between nutrients and other substances in food and health and diseases, as well as the process of ingestion, digestion, absorption, transportation, utilization and excretion of substances. In its development process, nutrition not only includes the changes of food entering the body, such as participating in biochemical reactions and combining with tissues and cells; It also includes instructing people how to choose food to ensure the normal growth, development and reproduction of the body. Therefore, nutrition has not only biological significance, but also social and economic significance. From low to high, from single-celled organisms to higher animals and plants, from living in water to living on land, the living environment is different and the ecology is different. In this way, the required nutrients and the way of taking nutrients are also different. Nutrients needed by living things are composed of many elements, including hydrogen, oxygen, nitrogen and carbon. These are the main elements that make up the protein of organisms and store energy. In addition, there are a small amount of sulfur, phosphorus, calcium, magnesium, potassium, sodium, chlorine and various trace elements. Some trace elements are only trace elements in organisms. Plants and microorganisms containing chlorophyll and purple pigment can directly absorb these inorganic compounds from the outside through roots, leaves or cell membranes, and use the energy of sunlight to synthesize organic substances needed for their own growth and other life activities, such as protein, lipids and carbohydrates (sugars). Organisms with this way of nutrition are called autotrophs or inorganic vegetative organisms. Other organisms (such as animals) cannot directly use external inorganic substances to synthesize organic substances needed for their own lives, and must obtain nutrition from autotrophs or other similar organisms. Through the metabolic process, the ingested substances are transformed into organic substances such as protein, lipids and carbohydrates. The creature with this way of nutrition is called heterotrophs. Nutrients are essential food ingredients to maintain normal life activities. The study of nutrients in modern nutrition is mainly aimed at the nutritional needs of people and livestock. Nutrients are divided into five categories: protein, lipids, carbohydrates, vitamins and minerals. Protein human tissues and cells are mainly composed of protein, and body fluids also contain protein. The nutritional function of protein lies in its various amino acids. There are more than 20 kinds of amino acids that make up food protein, some of which cannot be synthesized by humans and animals and must be obtained from food. These amino acids are called "essential amino acids", namely methionine, lysine, tryptophan, threonine, valine, phenylalanine, leucine and isoleucine. In addition, the growth of young children needs histidine, and birds such as chickens also need arginine and glycine. Amino acids other than these essential amino acids are called "nonessential amino acids" because they can be synthesized in vivo. The types and contents of amino acids in protein are different. Some protein lacks certain essential amino acids, such as gelatin protein without tryptophan and corn gluten without lysine. Therefore, evaluating the nutritional value of a food protein mainly depends on whether the amount of various essential amino acids it contains can meet the needs of the body. When it is insufficient, the body cannot effectively synthesize protein, and other amino acids can only produce sugar (glycogen heterogenesis) through deamination metabolism and provide heat energy as fuel. Therefore, the amino acid pattern of food protein is the key to determine its quality. At present, the nutritional value of food in protein is evaluated based on the whole egg essential amino acid model, or the essential amino acid model in human milk, or the hypothetical model based on the amount of essential amino acids in human body. This is the so-called chemical evaluation method of nutritional value in protein. In addition, there is a biological evaluation method, which evaluates the nutrition according to the utilization rate of food protein in the body. Commonly used are "protein Physiological Value" (abbreviated as BV, that is, the percentage of nitrogen retained and absorbed in the body), "Net Protein Utilization Rate" (abbreviated as NPu, that is, the percentage of nitrogen retained and absorbed in the body, that is, BV× protein digestibility), or "protein Efficiency Ratio" (abbreviated as PER, that is, the weight gain per gram of protein intake). Lipids include neutral fats and lipids. The former mainly supplies energy, while the latter has important physiological functions. The basic composition of lipids is fatty acids, which can be divided into essential fatty acids and non-essential fatty acids. There are three kinds of essential fatty acids, namely linoleic acid, linolenic acid and arachidonic acid. The biological activities of these three essential fatty acids are different, among which arachidonic acid is the largest, linoleic acid is the second, and linolenic acid is the most. Long-chain fatty acids of carbon 0 and carbon 22 play a special role in the development and function of brain and retina. When animals lack essential fatty acids, they grow slowly and have skin symptoms (depilation, eczema and dermatitis, scaly skin, etc.). It is reported that children will have the same symptoms when they are short of essential fatty acids. However, it is difficult for adult animals and people to have symptoms of deficiency because a large amount of linoleic acid is stored in the body. Lack of essential fatty acids can change the composition of phospholipid fatty acids in cell membrane, thus affecting the function of membrane; It can also reduce the synthesis of prostaglandin. The precursors of prostaglandin are 18 carbon and 20 carbon polyunsaturated fatty acids. It is suggested that the ratio of trienoic acid to tetraenoic acid in blood should be used as an indicator of whether essential fatty acids are lacking. This is due to the competition of enzyme system in fatty acid metabolism. When linoleic acid is deficient, the amount of arachidonic acid produced by the extension and desaturation of linoleic acid carbon chain decreases, and the metabolism of oleic acid, another group of fatty acids, increases, producing a large amount of eicosatrienoic acid, so the ratio of trienoic acid to tetraenoic acid in blood increases. The requirement of essential fatty acids for human body is about 1 ~ 2% of the daily thermal energy requirement according to its calorimeter. Carbohydrate (sugar) is the main nutrient of bioenergy. The carbohydrates in food are polysaccharide (starch) and cellulose. Monosaccharide, the degradation product of polysaccharide, can be utilized by most organisms, while cellulose can only be degraded and utilized in organisms with cellulase. When the dietary calorie intake is insufficient, human adipose tissue and protein will decompose to supplement the calorie deficiency. It is characterized by growth stagnation and weight loss. In severe cases, it will lead to death. People's eating habits are different, and the calories provided by dietary carbohydrates generally account for 45 ~ 80% of the total calorie consumption. In economically underdeveloped areas, it can reach more than 90%, because carbohydrates are the cheapest source of heat energy. If the calorie of dietary carbohydrate is too low and the calorie of fat is too high, ketosis will occur. People who lose weight often limit carbohydrates to limit calorie intake and increase labor to consume body fat. In this case, ketosis will also occur. Therefore, the heat energy from carbohydrates should not be less than 45% of the total heat energy. Fiber cannot be digested and utilized by humans and most animals. Dietary fiber includes cellulose, hemicellulose, pectin, seaweed polysaccharide and lignin. When fiber was determined in the early years, plant tissues were digested with acid and alkali, and the residue was crude fiber, and most of the other fiber components suffered losses during the determination. Now the new method can determine the various components of fiber separately. Dietary fiber is fermented by bacterial cellulase in gastrointestinal tract, and most of it can be hydrolyzed into short-chain fatty acids. Herbivores use this as energy. Epidemiology and laboratory work have proved that dietary fiber can reduce the occurrence of tumors, such as colon cancer. The reason lies in their hydrophilicity and gel-forming ability, which increases the volume of feces and facilitates excretion, thus accelerating the excretion of sterol metabolites with tumor activity and reducing the contact time with colon. Dietary fiber is also good for other diseases, such as coronary heart disease? ⒏哐 I ⑻ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Small? Protein, lipids and carbohydrates are nutrients that generate heat energy. All biological reactions must have enough heat energy. The heat energy provided by dietary protein, lipids and carbohydrates, after deducting the undigested part, is called physiological heat energy value. The physiological caloric values of protein, fat and carbohydrate per gram are 4.0, 9.0 and 4.0 kcal respectively. This is the data commonly used to calculate dietary calories. Under normal circumstances, calorie intake and consumption should be in a state of balance, that is, intake and consumption are equal, which is an energy balance. In the growth stage of organisms, substances in the body are increasing, especially protein and lipids, so there is energy storage. But when the intake exceeds the need, it is stored in the body in the form of lipid. On the contrary, when the intake is lower than necessary, it will consume its own substances, leading to emaciation. Minerals/KLOC-It was found in the middle of the 0/9th century that feeding animals with protein, fat and carbohydrate alone could not sustain their lives, so it was considered that the ashes after burning food must have physiological functions. However, animals still die after being replenished with ashes. It was not until vitamins were discovered in the early 20th century and the important role of minerals was gradually clarified that we had a more comprehensive understanding of nutrients. There are dozens of mineral elements in the human body, which are widely distributed throughout the body. At present, it has not been proved that these elements have physiological functions. Among them, a few elements have physiological functions and are called essential elements. According to its content in the body, it can be divided into macronutrients and micronutrients. The former includes calcium, phosphorus, magnesium, potassium, sodium, chlorine and sulfur. The latter includes iron, copper, zinc, manganese, molybdenum, chromium, cobalt, nickel, vanadium, tin, iodine, silicon and fluorine. Calcium, phosphorus and magnesium are the main components of bones and teeth. Magnesium is also an important component of plant chlorophyll. The physiological functions of calcium, phosphorus and magnesium are as follows: calcium and magnesium play an extremely important role in muscle fiber contraction, nerve conduction and activation of biochemical reactions, and calcium also plays an extremely important role in blood coagulation. Phosphorus is related to energy metabolism. Adenosine triphosphate (ATP) is an important compound for storing and releasing energy. Magnesium is an activating substance that produces adenosine triphosphate. Magnesium, potassium, sodium and chlorine are all important electrolytes to maintain acid-base balance and appropriate osmotic pressure of body fluids. Sulfur is an essential amino acid containing sulfur-methionine and cystine, as well as the components of thiamine, pantothenic acid, biotin and other vitamins. Sulfhydryl groups composed of sulfur and hydrogen play an important role in biological reactions. Among micronutrients, iron is an important component of hemoglobin and a carrier of oxygen. Copper and iron have synergistic effect in hemoglobin synthesis. Iodine is the main component of thyroxine. Chromium is a component of glucose tolerance factor. Cobalt is a component of vitamin B 12. It is known that zinc is an auxiliary group of more than 40 enzymes, and zinc deficiency will lead to growth stagnation and immature sexual development. Manganese and molybdenum are also components of enzymes. Fluorine is also an essential element because it can prevent dental caries. Other elements, such as nickel, vanadium, tin and silicon, have been found not to exist in animal experiments, but their mechanisms have not been clarified. Excessive intake of essential elements can also have adverse effects on the body. Vitamins are essential trace organic compounds for human body. Every vitamin has its own physiological function. Lack will lead to special diseases, and serious lack will lead to death. Divided into fat-soluble and water-soluble. Now known fat-soluble vitamins include A, D, E and Ko, while water-soluble vitamins include ascorbic acid, thiamine, riboflavin, nicotinic acid, vitamin B6, folic acid, cobalamin, biotin, pantothenic acid, choline and inositol.