There is the possibility that the metabolic requirement of indispensable amino acids in monogastric mammals is met not only by the diet but also by amino acids synthesized de novo by the gastrointestinal microflora, which are then absorbed. It is therefore crucial to better understand and quantitate the microbial biosynthesis of amino acids in the gastrointestinal tract and its potential role in providing amino acids to meet amino acid requirement. This paper summarizes the available evidence on a contribution of microbial lysine to the host's lysine homeostasis, applying isotope tracers in humans, pigs, and rats. Between 2 and 20 % of circulating plasma lysine, urinary lysine and body protein lysine of the host, respectively, is derived from intestinal microbial sources. Factors affecting estimates of net microbial IAA contribution are discussed. It was estimated that the porcine small intestine is responsible for more than 90% of microbial lysine uptake. Microbial amino acid synthesis in the gastrointestinal tract utilizes a mixture of various nitrogen sources, i.e. endogenous amino acids, urea and ammonia. Acetate and CO2 and to a lesser degree propionate derived from microbial carbohydrate fermentation form an active precursor pool of carbon for amino acid synthesis. Certain dietary non-starch polysaccharides and oligosaccharides, poorly digestible by mammalian enzymes can affect the composition and metabolic activity of the intestinal microflora, and are demonstrated to serve as carbon precursors for de novo amino acid synthesis in the intestinal microflora. This opens the possibility of manipulation of the microbial composition, and thus its fermentation products potentially available to the host. The intestine is a highly dynamic tissue of continuous replacement. Due to its direct vicinity to the intestinal flora it controls the effect of intestinal microbes on whole-body physiology. There is evidence that at low dietary protein intakes, or lysine concentrations splanchnic tissues benefit more from microbial amino acid sources than peripheral tissues. In conclusion, using the N-15 labeling paradigm a significant contribution of microbial lysine to the host lysine homeostasis is found. However, to assess net contribution of microbial amino is complicated by the nitrogen and amino acid recycling in the gut and the uncertainty of the precursor pool of absorption. Evidence based on C-14 data and digesta exchange experiments supports the view that the de novo indispensable amino acid (i.e. isoleucine, leucine, valine, phenylalanine, lysine) synthesis by the small intestinal microflora represents a net addition to dietary amino acids absorbed from the gut.

Among the reasons suggested for the discrepancy between N balance and tracer-derived indispensable amino acid (IAA) requirement estimates is the possibility that the metabolic requirement is met not only by the diet but also by IAA synthesized de novo by the gastrointestinal microflora, which are then absorbed. It is therefore crucial to better understand and quantify the microbial biosynthesis of amino acids in the human gastrointestinal tract and its potential role in providing IAA to meet human amino acid requirement. Here, the available evidence on the contribution of microbial amino acids to the host's amino acid homeostasis, applying the N-15 labeling paradigm, is summarized. Between 1 and 20% of circulating plasma lysine, urinary lysine and body protein lysine of the host, respectively, is derived from intestinal microbial sources and corresponds to a gross microbial lysine contribution of 11-68 mg . kg(-1) . d(-1) in adult humans with an adequate protein intake when fecal or ileal microbial lysine enrichment is used as precursor. Factors affecting estimates of net microbial IAA contribution are discussed. It appears that the small intestine is responsible for a large part of microbial lysine uptake, although some absorption from the large intestine cannot be excluded. Nonoxidative lysine losses from the human gastrointestinal tract, which were found to be between 3.9 to 8.5 mg . kg(-1) . d(-1), are necessary to estimate the net contribution of microbial IAA. It is reasonable to assume that microbial amino acid synthesis in the human gastrointestinal tract utilizes a mixture of various nitrogen sources, i.e., endogenous amino acids, urea and ammonia. Microbes in the small intestine may rely more on endogenous amino acids. Deprivation of nutrients, the intake of certain dietary nonstarch oligosaccharides, lipids, as well as protein intake level and source and level of consumption of certain amino acids can affect the composition and metabolic activity of the intestinal microflora and thus its fermentation products potentially available to the host. In conclusion, with the use of the N-15 labeling paradigm, a significant contribution of microbial lysine to the host lysine homeostasis is found. However, to assess the net contribution of microbial IAA and its importance in defining the adult IAA requirement, this is not the ultimately successful experimental strategy because the interpretation of results is complicated by the nitrogen recycling in the gut, the uncertainty of the precursor pool of absorption and the limited data on nonoxidative IAA losses from the human gastrointestinal tract.

The GC-C-IRMS technique represents a new and useful tool to study the (SN)-S-15 abundance in amino acids for metabolic studies. Here we present N-15 analysis data for 13 human plasma protein amino acids at natural abundance and in slightly enriched samples using GC-C-IRMS analysis of N-pivaloyl-i-propyl esters of amino acids. A mean precision of +/- 0.71 parts per thousand, delta(15)N or +/- 0.77 parts per thousand, delta(15)N was determined for all amino acids analysed at natural abundance values and for the slightly enriched samples (< 100 parts per thousand, delta(15)N), respectively. However, for individual amino acids at natural abundance derivatized and injected in duplicate the standard deviation ranged from 0.24 to 1.43 parts per thousand. The lowest natural N-15 abundance was found for threonine(about -5 parts per thousand delta(15)N) and the highest for proline and glutamic acid (about 15 parts per thousand delta(15)N). A standard deviation JSD) range of +/- 0.71 (glutamic acid) to +/- 4.35 parts per thousand delta(15)N (phenylalanine) (mean SD = +/- 1.52 parts per thousand delta(15)N) was obtained in protein amino acids in a group of 7 volunteers at natural abundance level, A single oral [N-15(2)]urea bolus was administreted to one healthy volunteer to demonstrate the feasibility of the GC-C-IRMS technique to trace human nitrogen kinetics in plasma protein amino acids. Small but significant differences in N-15 abundance were found between amine acids, presumably due to their differential participation in transamination reactions.