Introduction
The protein is an essential nutrient for every animal diet. It is a source for growth, maintenance, immunity and production for all animals. Actually, every animal species have they own requirement for amino acids. The amino acids are classified as essential amino acids or nonessential amino acids. The essential amino acids are the one not synthetized in vivo and need to be supplier for the diet. There is another amino acids that is called non-essential, because they could be metabolize in some metabolic route, by in some situation of stress or disease, the increase in their demand could be optimized by diet. There is also the concept of “functional amino acids”, for those that are fundamental key in specific metabolic route for cell surviving as arginine, cysteine, glutamine, glutamate, glycine, leucine, proline, and tryptophan which are known to improve the efficiency of utilization of dietary proteins in pigs (Kim and Wu, 2004; Li et al., 2007, Wang et al, 2008).
Normally in monogastric animals, the digestibility of proteins starts in the stomach and finish in the luminal enterocyte membrane. However, many factors can affect this digestibility of proteins. The amount of diet non-digested protein associated to endogenous proteins (from mucus and gut epithelial losses) could be fermented by the microbiota in large intestine and results in metabolites that could be an issue for the animal.
This presentation will review some important aspects of protein digestion, the factors that could affect it and the consequence of poor protein digestion for animal health and environmental.
Physiology of Protein Digestion
The digestion of proteins in monogastric animals starts in the stomach (acid phase), where the HCl and the pepsinogen are able to digest the big blocks of proteins in smaller particles. The second phase (neutral phase) is in small intestine where, at the basic pH, pancreas proteases (trypsin, chymotrypsin, elastases carboxypeptidases A and B) are able to reduce this big proteins parts in small peptides. However, the enterocytes, are just able to absorb di and tripeptides and the last part of digestion of proteins is finalized by proteases present at mucosal brushed border membrane of enterocyte.
Most of the free amino acids, di and tri-peptides are absorbed in the small intestine via active transport, simple diffusion, and facilitated diffusion.
However, this process is very efficient, there is some factors that could affect the protein digestibility. For example, in young animals, like in piglets, the low capacity of gastric secretion at birth may relate to immaturity of the parietal cells in piglets. The acidity of gastric contents in the post absorptive state is about pH 3 to 5 in milk-fed piglets during the early postnatal period due to low gastric secretory capacity and the high buffering capacity of sow’s milk (Reza et al, 2013). As a result, in 14-day-old pigs reared by sows and in 30-day-old pigs weaned at 21 days of age, only 70% and 55% of dietary amino acids are deposited in tissue proteins, respectively (Wu et al., 2010).
In chickens and turkeys also the activities of pancreatic enzymes are limited in young animals and this will results in the non-digested protein that could be fermented in the large intestine.
The health status of animal will also contribute for more efficient or poor diet digestion in any animal, because of increase in the flow rate in the intestine, effect on pancreas and other gastrointestinal organ efficiency on digestion.
Another factor that could affect the digestibility of proteins are related to quality of cereals. Proteases inhibitor in raw materials as anti-trypsin factor or gossypol, mycotoxins and other anti-nutritional factors could also affect the efficiency of protein digestion.
The remaining protein from the digestion associated to the endogenous lost by the epithelial mucosa must be degraded to CO2, NO, CO, H2S, methane, H2O, ammonia, urea, nitrate, and other nitrogenous metabolites (Wu et al., 2012, Li et al, 2017). Excretion of these products in urine and feces is a source of environmental pollution and can contribute to global climate changes.
Microbiota and Protein Fermentation in the intestine
However, the liver is the major site for metabolism of amino acids, the nutritional metabolism of amino acids in the intestine is important for normal gut function and intestinal mucosa growth;
Using genome it was possible identified 3499 metabolic reaction in the model of host/microbiota, these 1267 are unique to the microbiota and 1142 are shared with the host (Sridharan et al, 2014), showing the importante of microbiota in the metabolism of aminoacids.
Microbiota could use amino acids from feed to: 1- synthesis of protein; 2- conversion or fermentation for they metabolism; 3- De novo synthesis of other amino acids; 4- production of different metabolites. Most of these metabolites seems to be deleterious for the gut and induce inflammation.
During the fermentation process, similar to fiber fermentation, protein fermentation produces SCFA, but they are accompanied by branch-chained fatty acids (BCFA), derivate from branchchained amino acids (Leucine, Isoleucine and Valine), ammonia, amines, hydrogen sulfide, phenol and indoles (Macfarlane et al, 1992);
Shift of microbiota have been observed in small intestine after diet rich in protein (Qiu et al, 2018). Moderate reduction on protein level at diet reduce the Clostridiacea and biogenic amine in the ileum of pigs with increase of occluding and claudin (Fan et al, 2017);
In health humans high-protein diet revealed no change in the proportions of Clostridium and Bacteroides genera, or sulfate-reducing bacterial species, although a reduction in the pool of Bifidobacteria genus and others was observed (Brink- worth et al. 2009; Duncan et al. 2007).
Milk-fed piglets fed with higher protein presented good health and performance but higher gastrointestinal pH and a lower lactobacillus-to-coliform ratio (Partanen and Mroz 1999).
These results showing a dysbiosis as a consequence of the protein fermentation in animal intestine and this will affect the gut health and animal performance.
Alternatives to control Inflammation and dysbiosis as consequence of protein fermentation on the gut
To control or avoid the fermentation of proteins in the gut is associated to use of good quality of ingredients, formulation based in digestible amino acids and not crude protein, use of enzymes associated to gut modifiers additives and control of animal health and well-being.
Final Remarks
The concept that the digestibility of proteins can affect animal performance and health is presented.
This is associated to physiological limitation on the animals, quality of raw materials or diet formulation.
The products of the protein fermentation cause inflammation on TGI and also impact the environmental.
The knowledge of this could help us to find solution for this issue and make the animal production more sustainable.
