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.
