The final digested products of the protein in the digestive tract are often small peptides rather than free amino acids. The small peptides can be completely absorbed and enter the blood circulation in the form of dipeptides and tripeptides. Small peptides play an important role in protein nutrition. In order to achieve the best production performance of animals, a certain number of small peptides must be required. The authors present a review of the domestic and foreign research and application of small peptides in pig nutrition. With the deepening of protein and amino acid nutrition research, people have gradually realized the importance of peptide nutrition. Aga r (1953) observed that the intestine was able to transport diglycine intact. Naey (1959) and Smith (1960) first proposed the exact evidence that the peptides could be completely translocated. The simplest peptide is a dipeptide consisting of 2 amino acids containing 1 peptide bond. Peptides containing 3, 4 and 5 amino acids are called tri-, tetra- and pentapeptides, respectively. Straight-chain peptides formed by peptide bonds from 2--10 amino acids are called oligopeptides or small peptides. Recent studies have shown that the use of proteins in the gut by pigs is not restricted to free amino acids but is mostly absorbed in the form of small peptides consisting of 2 to 3 amino acids. Le Guowei (1997) reported that small peptides can completely enter the systemic circulation through intestinal mucosal cells. The idea that such peptides are fully absorbed gradually gains attention, and peptide research has followed. 1. Absorption mechanism of small peptides in pigs Drockcoopdeng et al. (1962) found the presence of specific peptides in plasma, such as peptides containing amino acids. Chen et al. (1962) and Adibi (1978) detected proteins with small size peptides and amino acids using the electromagnetic probe. These provide further evidence for the small peptide absorption mechanism. The protein in the diet is degraded into free amino acids and small peptides by the action of a series of enzymes in the digestive tract of pigs. The small peptides in the diet are affected by aminopeptidase A and aminopeptidase N in the brush margin of the pig intestinal small intestine. Free amino acids and small peptides are absorbed by pigs. Small peptides and amino acids have completely different absorption mechanisms. Current research results show that there are at least the following three absorption mechanisms: 1 The absorption of free amino acids is an active transport process that actively relies on Na+ pumps, while Matthrws (1987) and Vin-cenzini et al. (1989) found that the absorption of small peptides is a major The energy-consuming transport process depends on the H+ or Ca2+ ion concentration conductance. This mode of transport is inhibited in the absence of oxygen or the addition of metabolic inhibitors. 2 Absorption of most small peptides requires an acidic environment, and 1 molecule of peptide requires 2 H+, ie, this absorption mechanism has a pH-dependent non-consumable Na+/H+ exchange transport system. Daniel et al. (1994) reported that the driving force for small peptide transport is derived from the electrochemical gradient of protons. The power generated by the transport of protons into the cell drives the small peptides to move inside the cells, so that small peptides enter the cells in a diffuse form, causing cytoplasm. The PH decreases, activating the Na+/H+ channel, H+ is released from the cells, and the intracellular pH returns to the original water. In the absence of H+, the absorption of small peptides depends on the concentration of the substrate outside the membrane. When the concentration of extracellular H+ is higher than in the cell, it is transported back to the substrate by the codon system. 3 Glutathione (GSH) transport system. Vincenzini (1989) reported that the transmembrane transport of GSH is related to the concentration gradients of sodium, potassium, lithium, calcium, and manganese ions, but not to the hydrogen ion concentration. Since GSH has antioxidative effects in biofilms, the GSH transport system may have special physiological significance. 2. Nutritional and physiological effects of small peptides on pigs 2.1 Promotion of protein synthesis Boza et al. (1995), Infante et al. (1992), Monchi and Rerat (1993) Pullain et al. (1991) showed that the peptides were used as nitrogen sources. At the time, the overall protein deposition was higher than the corresponding amino acid diet or the whole protein diet. Pierzynowski et al. (1997) reported that small peptides in the blood circulation can directly participate in the synthesis of tissue proteins, in addition, the liver, kidneys, skin, and other tissues can also fully utilize small peptides. The kidney is the main site for digesting and absorbing peptides and reabsorbing amino acids. Rerat et al. (1998) reported that plasma insulin concentrations were higher in perfused pigs' duodenum than in perfused free amino acids. One of the functions of insulin is to participate in protein chain elongation and increase protein synthesis. Wang et al. (1994) observed that the two and three methionine peptides promote the incorporation of 3H-Leu into tissue proteins more than methionine. Protein deposition is the result of a combination of protein synthesis and degradation in pigs. Increased nitrogen deposition means that the protein synthesis rate increases or the degradation rate decreases. 2.2 Promotion of Mineral Absorption Use Found (1974) reported that metal ions located in the center of a five- or six-membered ring complex are absorbed as small peptides through small intestine villi brush lines. Li Yongfu et al. (2000) added small peptide iron and dextran iron to 1-21. day-old piglets and measured serum ferritin (SF) content at 14 days of age, in which the small peptide group was higher than the added dextran. The iron group and the control group indicate that mineral ions in the form of small peptide complexes are more easily absorbed by the body. Zhang Binli (2000) found that casein phosphopeptide (CPP) is a small peptide containing a cluster of phosphoserine, CPP in the neutral and alkaline conditions, its central site SerP and Ca, Fe, Zn plasma binding, blocking Precipitation is formed, so that the amount of dissolved Ca in the intestine is greatly increased, and the residence time of Ca in the body is effectively increased. The conjugate also releases Ca after being absorbed by poplar cells, thereby promoting the absorption and utilization of Ca. 2.3 Avoiding Absorption of Amino Acids Competition Rubino et al. (1971) observed that peptides do not affect the absorption of free amino acids. Different free amino acids have no effect on the absorption of small peptides. Pharasan et al. (1987) reported that when lysine and arginine are present in free form, the two compete for the site of absorption, free arginine has a tendency to lower the hepatic portal lysine; when lysine is a small peptide When the form exists, the former has no effect on its absorption. 2.4 Improving the immunity of pigs The small peptides can enhance the proliferation of beneficial bacteria, increase the synthesis of bacterial proteins, and increase resistance to disease. Studies have shown that small peptides are effective in stimulating and inducing the increase in the activity of the chorionic brush border enzymes of small intestine and promoting the nutritional recovery of animals. Jelle (1981, 1982) showed that tripeptides and large peptides produced by the hydrolysis of β-casein promote phagocytosis of macrophages. Storia (1994) used a segment of cDNA of porcine bone marrow to synthesize a small peptide that inhibited Gram positive and negative. Andeson (1995) isolated a segment of NK-lysine oligopeptides from pig intestine to inhibit E. coli. 2.5 Physiological regulation of small peptides Small peptides act directly as neurotransmitters, indirectly stimulating the secretion of intestinal receptor hormones or enzymes and play a role. For example, for casein peptides (7-10 amino acid residues) hydrolyzed by β-casein, the amino acid arrangement order is similar to that of endogenous opioid peptides. The pentapeptide (Tyr-Pro-Phe-Gly-Iie) and the tetrapeptide (Tyr-pro-phe-pro), which are further purified from the β-casein hydrolysate, have opioid peptide activity in vitro. Le Guowei et al. (1997) reported that the precursor of opioid peptide is also present in the pepsin hydrolysate of wheat grain protein, which can fully enter the blood circulation as a neurotransmitter and exert physiological activity. 3. Effects of small peptides on pig production performance Small peptides are an important form of the physiological role of protein nutrition. It is of great practical significance to use small peptides to improve animal performance. The results of the study indicate that adding a small amount of beta-endorphin or other peptide products to the diet can significantly improve pig production performance. Qian Lichun's (1998) trial results showed that in a certain amount of low-protein feed supplemented with an appropriate amount of small peptide-containing substances, the production level of high-protein diets can be achieved. Parisini (1989) and others added a small amount of peptides to the growing pig diet to significantly increase daily pig weight gain, protein utilization, and feed conversion. Lootekniga et al. (1994) reported that the addition of synthetic small peptides to finishing pig diets can increase feed remuneration and lean meat percentage. Scheppach et al. (1994) found that small peptides were effective in stimulating and inducing the activity of the intestinal villus brush-line enzymes, and that the enzyme activity was increased in chorionic membranes, indicating that the ability of the enzymes to hydrolyze proteins was enhanced and the body's absorption of small peptides was enhanced. Thus, small peptides have more application value for immature digestive tract development and lower digestive enzyme activity in piglets. It induces the development of digestive function of the small intestine by inducing the increase of some enzyme activities in the small intestine, promoting the health of piglets and improving their production performance. And dipeptides, tripeptides have a certain role in the prevention and treatment of diarrhea in piglets. Feeding diets supplemented with small peptides in early weaned piglets can significantly increase the activity of fatty acids and pancreatic amylase in piglets, increase the villus height of the small intestine, and make the crypts shallow, thereby increasing the digestion and absorption of nutrients and reducing diarrhea in piglets. And promote growth. Production trials have shown that small peptides can improve the performance of lactating sows, increase their milk production, and allow quicker and easier reassortment. Piglets have higher weaning weight and can reduce sow weight loss. The results of Gao Xin et al. (2000) showed that using small peptides can improve digestion and absorption of nutrient substances in piglets, increase production performance, and reduce diarrhea index. Wang Bilian et al. (2000) used a certain amount of small peptides to feed the piglets. The piglets in the experimental group gained 12.93% more weight than the control group, the diarrhea rate decreased by 60%, and the economic benefit was 15.62% higher than that of the control group. 4. Application prospects of small peptides in pig feed Because small peptides have the following characteristics when compared with free amino acids: 1 The absorption of both peptides is a completely different independent mechanism; 2 The absorption rate of small peptide amino acid residues is greater than that of the corresponding free radicals. The rate of acid absorption; the absorption of 3 small peptides can avoid the competition among amino acids, so small peptides have more advantages than free amino acids. In recent years, great progress has been made in the study of small peptides. The study found that the amount of small peptides released during digestion and hydrolysis of proteins is related to the quality of the protein. Proteins are high, and small peptides are released more; otherwise, they are less. The order of peptide peptides released from protein feeds was as follows: casein, fish meal, silkworm cocoon, soybean meal, soybean cake, rapeseed meal, and corn gluten meal. In the past, ideal protein models for pigs were based on available amino acids. Some trials have shown that using either homozygous diets or low-protein balanced amino acid diets, optimal production performance cannot be achieved. Therefore, it should be further combined with feeding experiments to explore the proportion of small peptides and free amino acids when pigs use the best amino acids, and then design a more reasonable feed formula, and make full use of protein resources to improve production efficiency. At the same time, the composition of the small peptides in the feed and the ratio of amino acids and the mechanism of action on the growth and immune regulation of pigs need to be further studied to further explore the absorption and metabolism of small peptides and their forms of action, in order to open up a new way for the development of protein nutrition theory.