Factors Influencing Proteolysis and Protein Utilization in the Intestine of Pigs: A Review

SIMPLE SUMMARY: In order to minimize protein requirements and nitrogen emissions in pig production, it is important to understand the physiology of protein breakdown in the pig’s gastrointestinal tract and to find ways to improve the protein utilization efficiency of the animals. In this review, we summarize and discuss factors influencing protein degradation and thereby affecting the efficiency of the animals. We shed light on the individual pillars of protein breakdown, starting with the enzymatic breakdown of the fed protein, via the amino acid transporters absorbing in the intestine, to the proteolytic capacity of the microbial and animal-specific proteome. The available literature shows the specific activities and expression levels of proteolytic enzymes and AA transporters depending on the availability of free or bound AA in the feed. Improvements in nutrient digestibility result from changes in nutrient transporter and enzyme gene expression, as well as a change in microbial activity. We are of the opinion that in the future, also with the help of transcriptomics, more time should be invested in researching the physiology of protein degradation and the associated protein utilization efficiency using highly standardized animal trials. ABSTRACT: Pigs are among the most important farm animals for meat production worldwide. In order to meet the amino acid requirements of the animals, pigs rely on the regular intake of proteins and amino acids with their feed. Unfortunately, pigs excrete about two thirds of the used protein, and production of pork is currently associated with a high emission of nitrogen compounds resulting in negative impacts on the environment. Thus, improving protein efficiency in pigs is a central aim to decrease the usage of protein carriers in feed and to lower nitrogen emissions. This is necessary as the supply of plant protein sources is limited by the yield and the cultivable acreage for protein plants. Strategies to increase protein efficiency that go beyond the known feeding options have to be investigated considering the characteristics of the individual animals. This requires a deep understanding of the intestinal processes including enzymatic activities, capacities of amino acid transporters and the microbiome. This review provides an overview of these physiological factors and the respective analyses methods.