Although amino acids (AA) synthesized by enteric microbiota in the upper gut of nonruminants can be absorbed, they do not necessarily make a net contribution to the host's AA supply. That depends on whether protein or nonprotein nitrogen sources are used for microbial protein production. We determined the contributions of urea, endogenous protein (EP), and dietary protein (DP) to microbial valine (M.VAL) at the distal ileum of growing pigs, based on isotope dilutions after a 4-d continuous infusion of l-[1-(13)C]valine to label EP and of [(15)N(15)N]urea. Eight barrows were assigned to either a cornstarch and soybean meal-based diet with or without 12% added fermentable fiber from pectin. Dietary pectin did not affect (P > 0.10) the contributions of the endogenous and DP to M.VAL. More than 92% of valine in microbial protein in the upper gut was derived from preformed AA from endogenous and DP, suggesting that de novo synthesis makes only a small contribution to microbial AA.

To determine the effect of dietary inclusion level of wheat shorts (WS; a high nonstarch poly-saccharide-containing feed ingredient) and casein (CS; a control) on the efficiency of utilizing ileal digestible Lys (kLys) and Thr (kThr) for whole body protein deposition (PD) in the growing pig, 2 separate N-balance studies were conducted with either Lys or Thr as first-limiting AA in cornstarch-based diets. For the Lys study, a basal diet (L-basal) was formulated to contain 0.24 g of standardized ileal digestible (SID) Lys per MJ of DE, to which 0.095 or 0.19 g of SID Lys per MJ of DE were added using either CS (L-CS2 or L-CS3, respectively) or WS (L-WS2 or L-WS3, respectively). A sixth diet was evaluated that was similar to L-CS3 but to which 6% pectin (L-pectin) was added as a source of soluble nonstarch polysaccharides. For the Thr study, the basal diet (T-basal) was formulated to contain 0.14 g of SID Thr per MJ of DE, to which 0.055 or 0.11 g of SID Thr per MJ of DE were added from CS (T-CS2 or T-CS3, respectively) or from WS (T-WS2 and T-WS3, respectively). A sixth diet was evaluated that was similar to T-CS3 but to which 6% pectin was added (T-pectin). Increasing SID Lys intake from CS did not influence kLys for PD (P > 0.10), whereas increasing SID Lys intake from WS reduced kLys for PD (P = 0.001; 89 vs. 79%). Inclusion of 6% pectin had no effect on kLys for PD (P > 0.10). Increasing SID Thr intake from CS also did not influence kThr for PD (P > 0.10), whereas kThr for PD was reduced at the greatest dietary inclusion level of WS (P < 0.001; 90 vs. 77%). Pectin inclusion had no effect on kThr for PD (P > 0.10). The inefficiency of utilizing ileal digestible Lys intake for PD may be attributed to nonreactive Lys in WS. The negative impact of including high levels of WS in the diet of pigs on kThr seems to be associated with fiber content of WS; it was not related to increased endogenous ileal AA losses at the distal ileum. The impact of dietary AA source on the use of ileal digestible Lys and Thr for PD, or other body functions, is substantial and should be considered in the formulation of pig diets. Further research is warranted to elucidate the mechanisms contributing to substantial dietary effects on Thr use for PD.

Use of dietary AA in growing pigs reflects digestion and use of digested AA for various body functions. Before evaluating dietary effects on use of digestible AA intake for body protein deposition, a digestibility study was conducted to investigate true ileal AA digestibility and endogenous ileal AA losses in growing pigs fed graded levels of wheat shorts (WS) or casein (CS; control). A casein-based basal diet (basal) was formulated to contain 0.27 g of standardized ileal digestible (SID) Lys per MJ of DE, to which extra Lys was added from WS (WS2, +0.10 g of SID Lys per MJ of DE; WS3, +0.20 g of SID Lys per MJ of DE) or casein (CS3, +0.20 g of SID Lys per MJ of DE). A fifth diet was formulated to be similar in CP level and source as CS3 but in which 6% pectin, a source of soluble non-starch polysaccharides (NSP), was included at the expense of cornstarch (CS3 + pectin). Five Yorkshire barrows (17.5 +/- 1.5 kg of BW) were fitted with a T-cannula at the distal ileum and randomly assigned to 1 of the 5 experimental diets in a 5 x 5 Latin Square design. Apparent ileal digestibility (AID), true ileal digestibility (TID), and endogenous ileal protein losses (EPL) were determined using the homoarginine method. Diet CS level did not influence (P > or = 0.10) TID of most essential AA or EPL (10.4 g/kg of DM intake). Including pectin in the diet did not influence TID of AA (P > or = 0.10) but increased EPL (15.6 g/kg of DM intake; P > or = 0.01). Inclusion of WS in the diet reduced TID of most essential AA (P < 0.01). The TID values for most essential AA, however, were the same (P > or = 0.10) for both dietary WS levels, except for Lys and Met, which were further reduced at the greatest dietary WS level. Increased EPL (P < 0.01) was only observed for WS3 (16 g/kg of DMI). We concluded that (1) the effects of dietary protein source on AID of AA can be attributed both to reduced TID of AA and increased EPL, (2) the impact of dietary WS level on TID of AA and EPL does not seem to be linear, (3) soluble NSP from pectin or WS exerts a greater effect on EPL than insoluble NSP, and (4) because of the metabolic cost associated with EPL and the impacts of feed composition on microbial fermentation in the gut lumen, the effects of feed ingredients on the use of ileal digestible AA for protein deposition should be investigated further.

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