Agar (1953) confirmed for the first time that the intestine can completely absorb diglycidyl. However, due to the influence of traditional protein's digestion and absorption theory, scholars are not easy to accept other absorption methods. In addition, because diglycidyl is considered as a special 2- peptide, its molecular weight is very small, so the importance of this discovery is not recognized. It was not until the 1960s that Newey and others first proposed that small peptides could be completely absorbed. Hara et al. (1984) found a small peptide carrier on small intestinal mucosal cells, which further confirmed that small peptides can directly enter the circulation through small intestinal mucosal cells. In 1990s, small peptide carriers were cloned, and the absorption mechanism of small peptides was gradually recognized.
Known studies have found that the nutritional absorption mechanism of small molecular peptides has at least the following ten characteristics:
(1) small molecules can be absorbed directly without digestion.
Traditionally, only free amino acids can be directly absorbed and utilized by animals. Recent studies show that protein's digestive end products in the digestive tract are mostly small peptides, which can enter the human circulation completely through intestinal mucosal cells.
(2) The small molecular peptide has the advantages of fast absorption, low energy consumption and difficult saturation of the carrier.
It is found that mammals absorb amino acid residues in peptides faster than free amino acids. Hara et al. (1984) found that the absorption intensity of amino acids produced by protease degradation in rats was 70% ~ 80% higher than that of corresponding free amino acids. Daneil et al. (1994) think that the absorption capacity of peptide carriers may be higher than the sum of the absorption capacities of various amino acid carriers. Experiments show that small molecular peptides are more easily absorbed and utilized by the body than amino acids, and are not interfered by anti-nutritional factors.
(3) Small molecular peptides are fully absorbed by human body.
Compared with free amino acids, small molecular peptides not only absorb quickly, but also have high absorption efficiency, and are almost completely absorbed by the body.
(4) Small molecular peptides are absorbed in a complete form.
Small molecular peptides are not easy to hydrolyze further in the intestine, and can be completely absorbed into the blood circulation. Small peptides in blood circulation can directly participate in the synthesis of tissue proteins. In addition, small peptides can be fully utilized by tissues such as liver, kidney and skin.
(5) The transport mechanism of small molecular peptides is quite different from that of amino acids, and there is no problem of competing with or antagonizing the amino acid transport during absorption.
It is known that there are three transport systems for small molecular peptides:
The first is a pH-dependent H+/Na+ exchange and transport system, which does not consume ATP.
Secondly, the active transport process depends on the calcium ion concentration of H+ or Ca2+, which needs to consume ATP.
Thirdly, it is a transportation system combining glutathione (GSH).
(6) Because the competition of free amino acids in absorption is avoided, small molecular peptides can make the intake of amino acids more balanced, thus improving the synthesis efficiency of protein. For infants with immature digestive system, for the elderly whose digestive system is deteriorating, for athletes who urgently need to supplement nitrogen sources but cannot increase the burden of gastrointestinal function, and for people with poor digestive ability, nutritional deficiency, weak constitution and infirmity, supplementing amino acids in the form of small peptides can improve the absorption of amino acids and meet the body's demand for amino acids and nitrogen.
(7) Small peptides can promote the absorption of amino acids.
For example, when lysine and arginine exist in free form, they compete for absorption sites. Free arginine tends to decrease the level of lysine in hepatic portal vein, but when it exists in the form of peptide, it has no effect on the absorption of lysine. Absorption in the form of a mixture of small molecular peptides and amino acids is the best absorption mechanism for human body to absorb protein. Lenoard et al. (1976) showed that patients with hereditary amino acid metabolic diseases could not absorb free neutral amino acids, but could absorb peptide-bound neutral amino acids.
(8) Small molecular peptides can promote the absorption of minerals.
Small molecular peptides can form chelates with mineral ions such as calcium, zinc, copper, iron, etc., which increases their solubility and is beneficial to body absorption. Studies have proved that casein phosphopeptides (CPPS) formed in the process of organism digestion can promote the absorption of calcium, iron, zinc, manganese, copper, magnesium and selenium. This is because metal ions such as calcium and iron can only be effectively absorbed by the human body if they are dissolved in the intestinal mucosa. The environment of small intestine is alkaline, and calcium and iron are easy to form insoluble salts with phosphoric acid, which greatly reduces the absorption rate of calcium and iron. CPPS can form soluble complexes with metal ions such as calcium and iron, which can increase the concentration of soluble calcium and iron in the small intestine, thus enhancing the absorption of calcium and iron in the intestine.
(9) After being absorbed by human body, small molecular peptides can be directly used as neurotransmitters to indirectly stimulate the secretion of intestinal receptor hormones or enzymes.
(10) Small molecular peptides can promote the development of intestinal mucosal structure and function.
Small molecular peptides can be preferentially used as energy substrates for the structural and functional development of intestinal mucosal epithelial cells, which can effectively promote the development and repair of intestinal mucosal tissues, thus maintaining the normal structure and function of intestinal mucosa.
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