一 . The largest organ of the body
The gastrointestinal tract is the largest organ of the body and the main user of metabolic energy, accounting for about 20% of the total energy of eating. For monogastric animals, the rate of protein synthesis in the gastrointestinal tract is the highest, accounting for more than 20% of the total body protein. The gastrointestinal tract is the largest endocrine gland in the body, which can secrete at least 20 kinds of hormones, regulatory peptides and their receptors. The gastrointestinal tract is the largest part of the body's immune system, and it is also the main site for most lymphocytes and other immune cells in the intestinal lymphoid tissue (GALT).
二 . Two chemical processes: enzymatic digestion and microbial fermentation
The gastrointestinal tract in ruminants and monogastric animals has evolved into different structures with different modes of action, and is therefore a peculiar organ. For all animals, there are two main biochemical processes in the gastrointestinal tract, enzyme digestion and microbial fermentation. Nutrients ingested by ruminants are first fermented by microorganisms in the rumen, and then undigested nutrients are digested by enzymes in the small and large intestine. The nutrients ingested by monogastric animals are first digested by enzymes in the stomach and small intestine, and then the undigested nutrients enter the large intestine and are fermented by microorganisms. Microbial fermentation plays an important role in supplying nutrients to host animals and only a minor role in ruminants and monogastric animals.
The gastrointestinal tract is home to a large community of microbes, whose number of microbial cells far exceeds the total number of cells in the host animal body. Bacteria in the human colon make up more than 90 percent of the body's cells. Kleessen et al. (1997) reported that 627 microbial colonies had been identified in rat cecum and feces.
三 . Sterile intestinal tract and flora intestinal tract
Some studies suggest that microbial communities or microecosystems may not be necessary for animals to survive and grow, such as sterile chickens that are better than normal chickens or even animals treated with antimicrobial growth promoters. The gastrointestinal tract of sterile animals is different from that of normal animals (Ewing and Cole,1994). Germ-free animals had lower intestinal weight and lower metabolic energy consumption and endogenous loss. Generally speaking, the adverse effects of metabolic consumption and intestinal diseases on the body of monogastric animals far outweigh the beneficial effects of the microbial colonies in the gastrointestinal tract. This is not the case, however, for ruminants, whose ability to feed on low-quality, fiber-rich roux is largely due to the rumen microflora.
The gastrointestinal microbial community of monogastric animals often functions in autonomic ways that may be harmful to the animal. Some of the microbial species in the gastrointestinal tract can be considered as microbes that often compete with host animals. It is, therefore, an interesting question whether the gastrointestinal tract is an indispensable part of an animal, or simply a structure that protects and provides nutrients to the microbial community that inundate it.
四 . Omni-trophic regulation of "competitive organisms" benefits the host
One of the main challenges of omni-nutrition is to regulate this "competitive organism" for the benefit of the host. The gastrointestinal tract of any animal is a large organ that requires high levels of energy and protein for maintenance and requires a large amount of nutrients for maintenance, tissue renewal, and nutrient utilization. Promoting gastrointestinal health and improving its utilization efficiency will enhance the health of the whole animal body and improve animal production performance. In fact, a variety of gastrointestinal diseases are the cause of disease in all animals, including humans.
Stable microbial fermentation in the gastrointestinal tract of monogastric animals can affect the establishment of pathogenic bacteria, because stable fermentation in the intestinal tract has a resistance to the establishment of pathogenic bacteria (Hil 'man et al., 1994). However, it is difficult to detect the stability of animal microbial fermentation, so it is necessary to study the changes of intestinal contents. Some microorganisms in the intestinal tract can effectively inhibit the proliferation of pathogenic bacteria, especially lactobacillus. (JianMing industrial) so the determination of the intestinal tract can inhibit proliferation of pathogenic bacteria colonies of effective potential can be based on lactic acid bacteria in the intestinal contents and the ratio between the escherichia coli, in general, if the number of beneficial bacteria lactobacillus is relatively higher than harmful bacterium escherichia coli, suggests that the pathogen has inhibitory effect on the number of bacteria increased, and this is likely to be whether animals a useful indicator of the health.
五 . The effects of gastrointestinal viruses
Gastrointestinal virus infection is also a major risk factor for animal health. Pigs and poultry are susceptible to a variety of intestinal viral diseases. The consequences of infection are mainly determined by a variety of factors, such as the animal's age, immune status, nutritional level and environment. In addition, gastrointestinal viral infections may trigger other disease syndromes. Damage to the gastrointestinal mucosa caused by viruses provides a pathway for other pathogens, such as E. coli or Salmonella, to enter the body. Viral damage to the gastrointestinal tract also leads to low feed digestibility and poor nutrient absorption, which leads to nutrient deficiency, which leads to a cascade of adverse animal health and growth, reduced resistance to other infections and other effects.
The effect of dietary changes on intestinal epithelial differentiation and growth is particularly evident at birth and weaning. Many changes occur in the first few weeks of life, such as changes in intestinal morphology, increased DE novo protein synthesis, and changes in digestive tract function caused by age and diet. During this period, intestinal bacterial communities alternate over rapidly changing mucosal surfaces. For most animals, a dynamic balance between the host itself, the dietary intake, and the gastrointestinal microflora contributes to the establishment of a stable microbial ecosystem. This microbial ecosystem in turn is characterized by the presence of symbiotic microbes that maintain and build an active immune system for gut health. However, intestinal microecosystem disorders often occur during the neonatal and early weaning period of animals, which is the reason for the highest morbidity and mortality during this period. Poor nutrition can cause intestinal infections, which can lead to mucosal damage and diarrhea in piglets and other newborn animals. The symptoms of these diseases can be exacerbated in two ways: first, the relative imperfection of the mucosal immune system; The second is the toxicity of opportunistic pathogens, which colonize the intestinal tract and rapidly change the mucosal surfaces of newborn animals.
六 . Utilization of sugar
The gastrointestinal tract consumes a lot of energy for its own metabolism, so changes in intestinal function can affect an animal's productive performance. Some microbial fermentation occurs in the small intestine, and these microorganisms also compete with host animals for digestible nutrients. A net 6% of pig feed is lost due to microbial fermentation in the small intestine (Mikkelsen and Jensen, 2000). Regulating the efficiency of intestinal chyme circulation may improve livestock production performance. Animals eating energy-dense diets increase the ability to transport glucose and other nutrients, which reduces the amount of glucose entering the large intestine. Glucose is not efficiently used in the large intestine.
