Physiological functions of L-carnitine 2.1 Involved in the transport and β-oxidation of long-chain fatty acids   The most important function of L-carnitine is to act as a carrier in the form of acylcarnitine to transport long-chain fats from the outside of the mitochondria to the inside of the membrane, to promote the β-oxidation of fatty acids, to generate energy, to reduce the content of serum cholesterol and triglyceride, and to improve the tolerance of the organism. The whole process is catalyzed by two isoenzymes, carnitine acyltransferase I and carnitine acyltransferase II. The speed of β-oxidation depends largely on the amount of available acylcarnitines, and a high concentration of acylcarnitines increases the speed of fat oxidation and improves the utilization of fat energy. Recently, it has been experimentally proved that L-carnitine also has a certain effect on the oxidation of medium-chain fatty acids.   2.2 Regulation of acyl CoA/mercapto CoA ratio in mitochondria   The ratio of hexanoyl CoA to mercapto CoA in the mitochondrial matrix affects the activity of pyruvate kinase, and thus the normal energy metabolism in animals. Meanwhile, some branched-chain acyl groups are metabolites of leucine, isoleucine and valine, and the timely transportation of branched-chain acyl groups facilitates the normal metabolism of these amino acids. L-carnitine can participate in the transportation of branched-chain amino acid metabolites, thus promoting the normal metabolism of branched-chain amino acids. This process is catalyzed by carnitine acyl coenzyme A to complete the circle.       2.3 Possesses acylation detoxification effect   Mitochondrial fatty acid β-oxidation metabolism can produce some toxic ester acyl CoA, such as propionyl CoA and methylmalonic acid monoacyl CoA and propionic acid. These substances inhibit some key enzyme activities in mitochondrial metabolism and cause toxicity. After L-carnitine treatment, a large amount of free L-carnitine in the cell mitochondria can combine with these toxic acyls to form L-carnitine complexes, which can release their toxic effects. At the same time, L-carnitine can also promote the oxidation of acetoacetic acid, which plays a role in the elimination and utilization of ketone bodies.       2.4 Maintaining membrane stability L-carnitine removes excess long-chain fatty acyl groups from the mitochondrial membrane and maintains membrane stability. L-carnitine is also an antioxidant that traps free radicals by preventing the formation of iron chelates. When the primary antioxidant defense barrier cannot ensure the complete scavenging of free radicals, L-carnitine, as a carrier of long-chain fatty acyl groups, participates in the deacylation of membrane phospholipids in the process of membrane repair by deacylation a reacylation, which is conducive to the timely repair of the membrane.   2.5 Influence on amino acid metabolism The raw material for L-carnitine synthesis must depend on muscle protein decomposition to provide methionine and lysine, so it plays a negative feedback regulation on the metabolism of methionine and valine. The addition of L-carnitine can reduce the synthesis of endogenous L-carnitine, reduce the decomposition of protein in the body, and have the effect of saving lysine and methionine. The metabolites of leucine, isoleucine and valine in the mitochondria are some branched-chain acyls, and L-carnitine acts as a carrier to transport the branched-chain acyls out in time, which is conducive to the normal metabolism of these amino acids.   2.6 Other functional effects   L-carnitine can prevent the toxicity produced by excessive ammonia in the animal body, improve the immunity of the animal as well as the ability to resist disease and stress. Prof. Borum of the University of Florida pointed out that L-carnitine also has the function of storing energy and transferring energy from one organ cell to another. At the same time, L-carnitine also indirectly participates in the role of gluconeogenesis and regulation of ketogenesis, effectively reduces the concentration of lactic acid in the blood after exercise and participates in the maturation process of spermatozoa. In addition, L-carnitine can regulate gastrointestinal function, promote the secretion of digestive juices and increase the activity of digestive enzymes, thus improving appetite and feeding.   Application of L-carnitine in food industry   3.1 Infant food   L-carnitine is a conditioned essential nutrient for infants. L-carnitine not only plays an important role in energy production and fat metabolism, but also has certain functions in maintaining infant life and promoting certain physiological processes of infant development, such as ketogenesis and nitrogen metabolism. Infant L-carnitine synthesizing capacity is only 15% of that of adults. Generally, normal level can be maintained by simple breast milk or cow's milk formula feeding, but there is almost no carnitine in soy protein-based toddler formula and grain-based weaning food, which should be nutritionally fortified. L-carnitine supplementation is required for preterm and low birth weight infants. L-carnitine has a function in maintaining infant life and promoting infant development.     3.2 Important Nutritional Supplements for Middle-aged and Elderly People L-carnitine is an important nutrient for middle-aged and old people. The energy of human heart muscle originates from the oxidation of fatty acids; L-carnitine is an important nutrient for heart muscle, which can increase the capacity of heart muscle and lead to the increase of cardiac output, therefore, it has obvious protective effect on the heart. Clinically, L-carnitine has been shown to have anti-myocardial ischemia, anti-arrhythmia, and lipid-lowering effects. With the increase of age, the L-carnitine in the body is decreasing, which leads to the decrease of myocardial cell vitality, so the elderly need to supplement L-carnitine. Foreign countries have listed it as an important substance to prevent and reduce coronary heart disease in the middle-aged and old people, and supplementation of L-carnitine can increase the capacity of heart muscle. L-carnitine supplementation can increase the capacity of heart muscle. For those who suffer from diabetes and kidney function disease, they should also supplement L-carnitine to meet the deficiency in their body. At the same time, supplementation of L-carnitine is also very effective in improving immunity and prolonging the life of immune cells.   3.3 Food for athletes   L-carnitine is also an important supplement for athletes, as it improves explosive power and enhances athletic endurance. L-carnitine determines the energy produced by muscles; the increase of L-carnitine content in muscles during exercise can promote energy conversion, maximize the body's oxygen tolerance, and promote the full play of physical performance. L-carnitine can promote both aerobic and anaerobic metabolism as well as relieve exercise fatigue. This is of great benefit to athletes in improving their sports performance. Researchers at home and abroad have also found that L-carnitine supplementation to athletes can greatly increase their maximum oxygen consumption and energy output during exercise, and significantly reduce carbon dioxide production and plasma lactate concentration. Because of this function of L-carnitine, foreign countries have commonly used L-carnitine in sports health food, and our country has little application at present.   3.4 Slimming food   Research on obese people shows that low-calorie diet and exercise can make weight reduction, but at the same time will produce some side effects. Such as: insufficient muscle energy supply produces fatigue and related cardiovascular diseases, lipid substances in the muscle fiber and liver accumulation produces obesity, fatty liver and so on. L-carnitine, on the other hand, burns excess body fat as energy and reduces body fat content to reduce body weight, which does not cause fatigue, diarrhea, and other adverse reactions produced by other diet foods.   Clinical studies have shown that L-carnitine supplementation in weight loss is effective and has no side effects on the body. Therefore, L-carnitine supplementation is an ideal way to lose weight without dieting, anorexia, fatigue or diarrhea. L-carnitine as the main ingredient of diet food is very popular in Europe and the United States, China also has a small number of similar products on the market.     Prospect of L-carnitine in food industry Prospect of L-carnitine in the food industry L-carnitine is a vitamin-like nutrient, and at the same time, it is a component of food, with good safety and stability, obvious health care effect, and has a great prospect in the food industry. In Europe and the United States developed countries, the application of L-carnitine is more extensive, add L-carnitine sports food, diet food, infant milk powder and health food variety. At present, there are fewer such products in our country, and most consumers do not know much about the function and health care effect of L-carnitine. On the other hand, the high cost of L-carnitine also limits its application in food. We should speed up the research on the method and technology of low-cost industrialized production of L-carnitine, actively carry out the research on the application of L-carnitine, strengthen the market guidance, and actively develop the domestic market. We believe that the application of L-carnitine in our food industry will become more and more extensive.