Pigs' gastrointestinal microflora provide them with essential amino acids.

The synthesis of essential amino acids by the gut microflora of pigs, and their absorption, were assessed from the incorporation of (15)N from dietary (15)NH(4)Cl and of (14)C from dietary (14)C-polyglucose into amino acids in the body tissues of four pigs. Both (15)N and (14)C were incorporated into essential amino acids in body protein. Because pig tissues cannot incorporate (15)N into lysine or (14)C into essential amino acids, the labeling of these amino acids in body protein indicated their microbial origin. The absorption of microbial amino acids was estimated by multiplying the total content of each amino acid in the body by the ratio of the isotopic enrichment of the amino acid in the body to that in microbial protein. Because the ratio of (14)C:(15)N in body lysine was closer to that in the microflora of the ileum than to that of the cecum, absorption was assumed to take place exclusively in the ileum. The estimates of microbial amino acid absorption from (14)C-labeling were as follows (g/d): valine 1.8, isoleucine 0.8, leucine 2.0, phenylalanine 0.3 and lysine 0.9, whereas for lysine, the estimate from (15)N-labeling was 1.3 g/d. We conclude that the gastrointestinal microflora contribute significantly to the amino acid requirements of pigs.

Lysine synthesized by the gastrointestinal microflora of pigs is absorbed, mostly in the small intestine.

This study used a digesta transfer protocol to determine the site of absorption of lysine synthesized by the gastrointestinal microflora of pigs. Eight pigs were used, four with reentrant cannulas in the terminal ileum, two with simple T cannulas in the terminal ileum, and two intact. All pigs were given, for 5 days, the same low-protein diet that included fermentable carbohydrates. The diet of two pigs with reentrant cannulas (donor) and of the two intact (control) pigs was supplemented with (15)NH(4)Cl. The two other pigs with reentrant cannulas (acceptor pigs) and those with simple cannulas (used to supply unlabeled digesta) were given the same diet but unlabeled NH(4)Cl. Ileal digesta were collected continuously from all of the reentrant cannulas and kept on ice. All digesta from each donor pig were reheated and returned to the distal cannula of its companion acceptor, whose ileal digesta were discarded. Unlabeled ileal digesta from the pigs with simple cannulas were instilled into the distal cannulas of the donor pigs. At the end of the experiment, the average (15)N enrichment in the plasma free lysine of control pigs was 0.0407 atom % excess (APE); that of donor pigs was 0.0322 APE (79% of controls), whereas that of acceptor pigs was only 0.0096 APE (24% of controls). Due to nitrogen recycling, acceptor pigs had labeled lysine in the digesta of the stomach and small intestine, and donor pigs had labeled lysine in the digesta of the large intestine. If account is taken of the higher (15)N enrichment of microbial lysine in the large compared with the small intestine, it can be estimated that >90% of the absorption of microbial lysine took place in the small intestine.