Protein secretion into the intestine continues even under conditions of protein-free feeding, and fecal nitrogen losses (i.e., nitrogen lost as bacteria in the feces) may account for 25 percent of the obligatory loss of nitrogen (Fuller and Reeds, 1998). Under this dietary circumstance, the amino acids secreted into the intestine as components of proteolytic enzymes and from sloughed mucosal cells are the only sources of amino acids for the maintenance of the intestinal bacterial biomass. In those studies in which highly digestible protein-containing diets have been given to individuals previously ingesting protein-free diets, fecal nitrogen excretion increased by only a small amount. For highly digestible proteins, it also is likely that when humans consume diets that do not provide an excessive quantity of protein, a high proportion of the fecal nitrogen losses
originate from a combination of gastrointestinal secretions and the partial capture of the significant quantities of secreted urea that are hydrolyzed and subsequently used by the microflora in the large intestine (Jackson, 1989).
The following points support the view that the intestinal route of protein (amino acid) loss is of quantitative significance to maintenance protein needs. First, continued mucosal cell turnover and enzyme and mucin secretion are necessary for maintaining the integrity of the gastrointestinal tract and its normal digestive physiology. Second, animal studies show that the amino acid composition of the proteins leaving the ileum for bacterial fermentation in the colon is quite different from that of body protein (Taverner et al., 1981). In particular, the secretions are relatively rich in dispensable amino acids as well as threonine and cysteine (Dekker et al., 1991; Khatri et al., 1998; Taverner et al., 1981), probably because mucin secretions make a substantial contribution to the endogenous outflow. These two amino acids are of significance in meeting amino acid needs when intake is close to the requirement (Laidlaw and Kopple, 1987).
Other routes of loss of intact amino acids are via the urine and through skin and hair loss. These losses are small by comparison with those described above, but nonetheless may have a significant impact on estimates of requirements, especially in disease states (Matthews, 1999).
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