Protein digestion kinetics in pigs and poultry

Increasing the protein efficiency is considered a main strategy for sustainable feeding of pigs and poultry. In practice, protein in pig and poultry diets originates from different ingredients, selected in diet formulation based on their nutritional value and cost. Currently, the nutritional value of protein sources in pig and poultry diets is based on the concentration of essential amino acids (AAs), and their digestibility up to the end of the ileum or the gastrointestinal tract (GIT) (NRC, 2012; CVB, 2016). The ileal and faecal digestibility of protein and AAs, however, only provide information on the quantity of protein and AAs apparently absorbed up to the end of the ileum or over the entire GIT, respectively. They, however, do not provide information on the kinetics of protein digestion, which might affect the post-absorption metabolism of dietary AAs. The aim of this thesis, therefore, was to provide further insights into digestion kinetics of dietary protein sources in the GIT of pigs and poultry, and the consequences of differences in digestion kinetics of dietary protein for the growth performance of broilers. Protein digestion kinetics in pigs and poultry In Chapter 2, in vitro protein digestion kinetics of various protein sources (soybean meal (SBM), wheat gluten (WG), rapeseed meal (RSM), whey powder (WP), dried porcine plasma protein (DPP), yellow meal worm larvae (MW), and black soldier fly larvae (BSF)) were determined using a two-step method. Protein sources were incubated with pepsin at pH 3.5 for 0-90 min and subsequently with pancreatin at pH 6.8 for 0-210 min at 39 °C. Protein sources showed substantial differences in in vitro protein digestion kinetics as measured by the kinetics of N solubilisation and the release of low molecular weight peptides ( Based on the in vitro results, SBM, RSM, WG, DPP and BSF were selected for further investigations into in vivo protein digestion kinetics in both pigs (Chapter 3) and broiler chickens (Chapter 4). Forty pigs were randomly allocated to one of the five experimental diets containing the respective protein sources as the only source of protein. Four pigs per experimental diet were fitted with an ear-vein catheter and blood samples were collected before and after a morning meal. At dissection, digesta samples from the stomach and the small intestine, divided into four segments of equal length, were quantitatively collected. Apparent digestibility of crude protein (CP), and retention time (RT) of the solid fraction of digesta along the stomach and the SI were determined to calculate protein digestion kinetics. The initial protein digestion rate ranged from 0.68 % · min-1 for the RSM based diet to 3.04 % · min-1 for the DPP diet. A higher digestion kinetics of dietary protein resulted in a more rapid and pronounced postprandial appearance of AAs and peptides in systemic blood of pigs. In the broiler trial, a total of 378 26-day-old male broilers with average body weight of 1430 ± 48 g were randomly allocated to 42 pens. Pens were randomly allocated to one of the seven diets (i.e. a basal diet and six experimental diets with SBM, soy protein isolate (SPI), WG, RSM, DPP or BSF as the main protein source). At dissection, digesta samples from the crop, gizzard, duodenum, proximal jejunum, distal jejunum, and ileum were quantitatively collected. The CP digestion kinetics of the experimental diets were calculated by relating the apparent CP digestibility coefficient at each segment of the small intestine to the sum of digesta retention up to that segment. The initial protein digestion rate ranged from 1.76 % · min-1 for the RSM based diet to 30.7 % · min-1 for the WG based diet. Mechanism of protein hydrolysis in the GIT of pigs and poultry It was hypothesised that proteins present in highly digestible protein sources (i.e. WG and DPP) are more susceptible to hydrolysis by digestive enzymes than slow digestible protein sources (i.e. SBM, RSM and BSF) and that enzymatic hydrolysis of protein progress stepwise in the small intestinal intestine, resulting in hydrolysis products (peptides) becoming smaller in size towards the end of the small intestine. As a consequence, relatively more low and intermediate molecular weight peptides were expected to be present in ileal digesta of pigs and broilers fed highly digestible protein sources, compared to sources with a lower digestibility. The molecular weight distribution of soluble proteins and peptides in digesta from the different segments of the GIT of pigs and broilers was analysed using size exclusion chromatography (Chapter 3 and 4). The molecular weight distribution of proteins and peptides in ileal digesta of pigs and broilers fed highly digestible protein sources was comparable to those of pigs and broilers fed low digestible protein sources. In addition, the molecular weight distributions were rather similar throughout segments of the GIT. These results indicate that proteins from both highly and low digestible sources follow a “one-by-one” type of hydrolysis mechanism, meaning intact proteins are hydrolysed to low molecular weight peptides and free AAs and absorbed by the intestinal mucosa in one sequence. As a result, proteins and peptides with a wide range of molecular weights were not observed in digesta of different segments of the GIT. Approximately 30 % of peptides present in ileal digesta of pigs are Synchronisation the supply of dietary starch and protein The effects of synchronising the supply of dietary protein and starch using information on their kinetics of digestion on the growth performance and carcass characteristics in broilers was investigated (Chapter 5). Two starch and two protein sources were used: pea starch (PS) and SBM as slowly digestible sources while rice starch (RS) and SPI as fast digestible sources. Broilers fed diets synchronised for digestion rate of starch and protein (i.e. PS-SBM (slow-slow) and RS-SPI (fast-fast)) did not show a higher growth performance and breast meat yield compared to broilers fed the asynchronised diets (i.e. RS-SBM (fast-slow) and PS-SPI (slow-fast)). The evaluation of the effect of synchronising the supply of dietary starch and protein, however, was hindered by feed intake being affected by dietary protein and starch source. Feed intake of birds was higher when fed diets with SBM compared to SPI and when PS was fed instead of RS. Conclusions The results of the present thesis indicate that the kinetics of protein digestion in the GIT of pigs and poultry differs substantially among protein sources. Wheat gluten and DPP can be regarded as fast digestible protein sources while SBM, RSM and BSF are more slowly digestible protein sources in both pigs and broilers. Broilers showed on average a 2.7-fold higher small intestinal protein digestion rate than pigs, excluding and with the exception of WG, for which the protein digestion rate was very high in broilers compared to pigs. However, despite differences in intrinsic characteristics (e.g. AA composition, protein conformation, physicochemical properties) of protein sources and in digestive physiology of pigs and poultry, the mechanism of hydrolysis of dietary proteins in the gut seems rather similar. Synchronising the digestion kinetics of dietary starch and protein using both fast digestible sources or both slowly digestible sources did not improve the performance nor the breast muscle yield of ad libitum fed broilers kept under an intermittent light regime.