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A study was conducted to investigate the biochemistry of digestion of field pea (Pisum sativum L.) albumins and globulins in the stomach and along the small intestine of weaned piglets with a particular emphasis on the respective roles of these compartments in pea protein digestion. Twenty-four piglets were weaned at 28 d of age. They were allocated to 2 diets (control and pea) and 3 slaughter times (3, 6, or 9 h after the last meal) in a 2 x3 factorial arrangement of treatments in a randomized complete block design. Pea flour provided 30% of total dietary protein in the pea diet. The diets were fed for 2 wk after weaning. After slaughter, gastrointestinal tract (GIT) compartments were weighed, digesta were collected, and pH was measured. Digesta from the stomach and cranial, middle, and caudal small intestine (SI) were extracted for soluble proteins and analyzed for specific pea proteins using SDS-PAGE, immunoblotting, and mass spectrometry. Tissue weight of the whole GIT (P = 0.015), cecum (P <0.001), and colon (P <0.001) was greater in the pea diet. Digesta pH in the stomach and caudal SI was lower (P = 0.02) in the pea diet than the control diet. In the stomach, vicilin, lectin, and pea albumin 2 were not digested, whereas legumin was only partly digested. Legumin and vicilin were totally digested in the SI in less than 3 h. A resistant peptide of 15 kDa located at the N-terminus of pea albumin 2 was transiently detected at 3 h. A protein band at 20 kDa was consistently identified as lectin. It was present in high intensity in intestinal digesta of pea-fed piglets at all times after the meal compared with those fed the control diet (P <0.001). Various proteins of, presumably, endogenous origin displayed differential digestion patterns between the control and the pea-fed piglets (P<0.05). In conclusion, differences in digestion between specific pea proteins were observed along the GIT of piglets. They could be partly explained by differences in protein digestion in the stomach.

In this review, the terminology that is used to describe the bioavailability and ileal digestibility of AA in pig feed ingredients is defined. Aspects of the methodology to establish bioavailability and ileal digestibility values also are discussed, and recommendations about the use of these values are provided. Two main factors can contribute to differences between bioavailability and ileal digestibility of AA. First, some AA, such as Lys, may be absorbed in chemical complexes that preclude their use for metabolism. Second, fermentation in the upper gut may result in a net loss or gain of AA to the animal. In addition, dietary effects on the efficiency of using bioavailable AA intake for tissue growth or milk production should be considered and may be attributed to endogenous AA losses in the hindgut and the metabolic costs associated with endogenous gut protein synthesis and losses. Ileal digestibility values may be expressed as apparent ileal digestibility (AID), standardized ileal digestibility (SID), or true ileal digestibility (TID). These terms are used to specify how ileal endogenous AA losses are reflected in digestibility values. Ileal endogenous AA losses may be separated into basal losses, which are not influenced by feed ingredient composition, and specific losses, which are induced by feed ingredient characteristics such as levels and types of fiber and antinutritional factors. Values for AID are established when total ileal outflow of AA (i.e., the sum of endogenous losses and nondigested dietary AA) is related to dietary AA intake. A concern with the use of AID values is that these are not additive in mixtures of feed ingredients. This concern may be overcome by correcting AID values for defined basal endogenous losses of AA, which yields SID values. Furthermore, if the AID values are corrected for basal and specific endogenous losses, then values for TID are calculated. However, reliable procedures to routinely measure specific endogenous losses are not yet available. It is recommended that basal ileal endogenous losses of AA should be measured in digestibility experiments using a defined protein-free diet and that these losses are reported with observed AID and SID values. It is suggested that SID values should be used for feed formulation, at least until more information on TID values becomes available.

Intestinal mucin, a family of glycoproteins secreted by goblet cells, is the main constituent of the mucus protecting the gastrointestinal tract. For optimal mucosal protection, both the quantitative and qualitative characteristics of mucin are essential. To evaluate how viscosity influences ileal apparent digestibility and mucin biology, a highly viscous nonfermentable soluble polysaccharide, carboxymethylcellulose (CMC), was fed to weaned piglets for 15 d. The ileal crude mucin concentration was determined by ethanol precipitation, and changes in goblet cell subtypes were analyzed by the histochemistry of ileal and colonic tissues. As expected, CMC increased the viscosity of ileal digesta and the moisture of feces (P < 0.001). The crude mucin concentration and output at the ileum were higher (P < 0.05) in pigs fed CMC than those fed the control diet. Increasing intestinal content viscosity in pigs fed CMC had no significant effects on the ileal apparent digestibility of dry matter, organic matter, nitrogen, and minerals. The number of total ileal goblet cells per villus also was higher (+30%, P < 0.05) in pigs fed the CMC diet compared with controls. This increase was essentially accounted for by increased numbers of acidic and acidic sulfated mucin-containing cells (+30%, P < 0.05). Trends (P = 0.06) toward decreased numbers of neutral and acidic mucin-containing cells in ileal crypts were also noted. In conclusion, increasing intestinal content viscosity in weaned piglets fed CMC increased the ileal mucin output and numbers and maturation of goblet cells in ileal villi without effects on the apparent digestibility of the diet.

We assessed the influence of sampling site when using the isotope dilution method to determine ileal endogenous N losses. Three growing pigs were prepared with ileorectal anastomoses and fitted with three catheters (portal, jugular and carotid). A 15N-leucine solution was infused for 24 d, alternating between the carotid artery and the jugular vein. Blood was sampled from the portal catheter and from the systemic catheter not used for the infusion. The pigs were fed successively a protein-free diet, an isolated pea protein diet and a hydrolyzed pea protein diet according to a Latin-square design. The 15N was transferred from leucine to isoleucine, valine, alanine, glycine and proline. Free 15N alanine, glycine and valine enrichments were closer to the respective amino acid enrichments in secretory tissues in the portal vein than in the systemic blood. The enrichment of total nitrogen was higher in the trichloroacetic acid-soluble fraction of the plasma than in the ileal digesta of pigs fed the protein-free diet. Lysine enrichment was significantly different from zero in all tissues except muscle, suggesting that essential amino acids can be synthesized by microflora and used for protein synthesis in the host. We conclude that the total nitrogen isotope dilution method is inappropriate to determine the endogenous loss of amino acids. Moreover, the amino acid dilution method should be performed with portal blood sampling. The main limitation of this method is that a number of essential amino acid losses cannot be determined.

Endogenous protein loss at the end of the small intestine was determined in two experiments using 10 pigs surgically prepared with end-to-end ileo-rectal anastomosis to allow total collection of ileal digesta. In the first experiment pigs were fed graded protein levels of 0 (protein-free), 55, 110 or 165 g/kg diet. Optimal durations for the adaptation and collection periods were found to be 4 and 3 d, respectively (combination 4:3), as shown by the higher correlation coefficient (r2 = 0.95) between excreted and ingested nitrogen compared with the other combinations tested (5:2, 5:3, 9:3, 9:5). The estimated amounts of endogenous N and amino acids were less accurate and tended to be smaller (P < 0.20) when obtained by extrapolation to zero nitrogen intake than when measured in pigs fed the protein-free diet. The endogenous protein was rich in proline, glutamic acid, glycine, aspartic acid, serine and threonine. In comparison to other amino acid patterns, this composition suggested a low bacterial contamination of the digesta. In the second experiment three levels of dietary fiber from wheat straw, corn cobs and wood cellulose were studied in pigs fed protein-free diets. Between 17 and 34 g crude fiber/kg diet, fiber increased the endogenous losses of nitrogen and amino acids per kilogram of dry matter intake (P < 0.05), but the excretion reached a plateau at higher dietary fiber concentration (102 g/kg). In contrast, glucosamine and galactosamine excretion increased continuously and linearly (P < 0.05) with fiber intake. We conclude that endogenous amino acid loss may be considered constant at usual and high levels of the fibrous mixture under study.

The current 15N-leucine infusion technique may overestimate the ileal endogenous nitrogen losses in pigs. To determine the reason, we infused four cannulated pigs intravenously, fed them a pea-based diet with 15N-leucine, and examined some methodological variables. Neither the blood sampling time nor the choice of precursor pool (total N or amino acid N of deproteinized plasma) or the method of estimation of the isotopic equilibrium level significantly affected the results. On the other hand, the 15N-enrichment of purified mucin, isolated from ileal digesta, was higher than that of the plasma amino acid pool (0.114 vs. 0.077 atom % excess). The endogenous proportion of the labeled amino acids (Ala, Gly, Ile, Leu and Val) in the ileal digesta ranged from 23 (Leu) to 74% (Ala), compared with 70% for total N. The low value of leucine was ascribed to the constant marker infusion condition. In pigs infused with 13C-leucine, a similar endogenous proportion was obtained for lumenal leucine with 13C-leucine and 15N-leucine infusion. However, the 13C-enrichment of the leucine bound to mucin was markedly lower than that of plasma leucine (38%). The endogenous amino acid flows were also estimated by combining the ileal N flow measured with 15N and the endogenous amino acid profile obtained by means of an N-free diet. They were different from those obtained with the 15N-amino acid dilution technique. We conclude that the precursor pool currently used (plasma total N or total alpha-amino acid N pools) is a poor indicator of the enrichment of the secretions and that the infusion of one labeled amino acid is not sufficient to extend the method at the amino acid level. [ABSTRACT FROM AUTHOR]