Abstract: in this paper, the current research work of peptides in the digestive tract of monogastric animals is briefly summarized from the aspects of peptide release, peptide absorption, peptide types and sources in blood circulation, the influence of peptides on protein turnover and metabolism of animals, and the physiological activity of peptides.

Keywords peptide; oligopeptide; small peptide; amino acid; nutrition

Small peptide (SP) is composed of 2-3 amino acids

Acid, AA) constitute oligopeptide (OP). In the past half century, with the in-depth study of AA and ideal protein available to animals, peptide nutrition has attracted more and more attention and made positive progress. It is generally believed that peptide nutrition research is of great significance to enrich traditional animal protein nutrition theory, improve protein utilization efficiency, save limited protein feed resources and reduce nitrogen pollution.

In this paper, the absorption mechanism, absorption characteristics, factors affecting SP absorption, peptide release in the process of protein digestion, the types and sources of peptides in blood circulation, the effects of peptides on protein turnover and metabolism in animals, and the physiological activities of peptides are briefly reviewed.

1. Theoretical basis of SP nutrition

The presentation of animal SP nutrition depends on the discovery of several experimental results. In addition to free amino acid (FAA), the final hydrolysate of protein in small intestine also contains a lot of sp. This result prompted the researchers to question the classical theory of protein digestion and absorption (dogma theory); a series of studies by Newey and Smyth from 1957 to 1962 found that a large number of SP produced by protein in the small intestine can directly enter the intestinal mucosal cells, and it is the Research Institute of agar et al. (1953), Adibi et al. (1968), Adibi (1971), Cheng et al. (1971) and buston et al. (1972) It was confirmed that there was a formal challenge to dogma theory; craft et al. (1968), Addison et al. (1974a, b) and Adibi et al. (1975) found SP transport system in animal intestinal cells, Hara et al. (1984) found SP carrier in intestinal mucosal cells, and then SP type I carrier (Fei et al., 1994) and type II carrier (Adibi, 1996) were cloned respectively. This provides direct evidence for the objective existence of SP nutrition in animals from the perspective of histology. The comparison of the growth test between the whole AA diet and the whole protein diet shows that when animals eat the pure diet or the low protein diet with AA balance, they can not achieve the best production performance (pinchasou et al., 1990; Kephart et al., 1990; KESHAVARZ, 1991; Jensen, 1991; COLNAGO et al., 1991). This suggests that The results showed that the protein requirement of animals could not be completely met by FAA, and SP might be the essential nutrient of animals. Table 1 shows the results of a test conducted by the University of Georgia in the United States.

Note: the metabolizable energy of the diet is 13.38mj/kg; the diets with crude protein lower than 23% contain AA, lysine, arginine, tryptophan, threonine, isoleucine and valineThe equivalence was the same as that in the diet with 23% crude protein

2 absorption mechanism of SP

The exploration of SP absorption mechanism began in 1970s. Up to now, the mechanism of SP absorption has been speculated on the basis of the absorption of a few kinds of SP, such as Gly Gly dipeptide, l-glu-l-glu dipeptide, l-lys-l-lys dipeptide, gly-l-lys dipeptide, carnosine dipeptide and Gly Gly Gly tripeptide, and the study has been carried out with frogs, rats, mice, guinea pigs, rabbits and humans. The results show that SP has the ability to enter intestinal cells in a complete form and has a different absorption mechanism from FAA.

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