68 Introduction to Human Nutrition
● the extent to which the indispensable amino acids
are available to the host metabolism.
Digestibility and intestinal amino
acid metabolism
Traditionally, the assessment of the availability of
dietary proteins and amino acids under practical con-
ditions has been based on “apparent digestibility,”
i.e., the difference between nitrogen intake and fecal
nitrogen output. However, for two reasons, this
method is unsatisfactory for the precise estimation of
the digestibility of individual amino acids. First, fecal
nitrogen consists largely of bacterial protein, and
because the composition of bacterial protein differs
markedly from that of common dietary proteins, it
gives very little information on the digestibility of
different food-derived amino acids. Second, the bac-
terial nitrogen is not only derived from undigested
protein. This is because proteins secreted into the
intestinal lumen, as well as the urea nitrogen that has
diffused from the blood, are important contributors
to colonic nitrogen fl ow. Studies in both animals and
humans using^15 N-labeled amino acids suggest that at
least 50% of the fecal nitrogen is derived from the
body rather than directly from undigested dietary
protein.
Recently,^15 N-labeled dietary proteins have been
given to adults and by measuring the fl ow of^15 N from
the terminal ileum it is possible to calculate the “true”
digestibility of the dietary source. There have also
been a number of studies in pigs in which^15 N-labeled
amino acids have been infused intravenously for pro-
longed periods. This labels the host proteins so that
the^15 N labeling of ileal proteins allows the calculation
of the endogenous contribution of the luminal protein
pool. By and large, the results of all these studies lead
to the same conclusion; namely, that the true digest-
ibility of most dietary proteins is very high and that
at least 50% of the fecal nitrogen is derived from host
metabolism rather than from the diet.
Most of the evidence favors the conclusion that
there is an almost complete digestion of most dietary
proteins in the small bowel. It is also quite clear that
a considerable amount of amino acid metabolism
occurs in the tissue of the splanchnic bed, in general,
and in the intestinal mucosa, in particular, before the
amino acids, liberated from food proteins during the
digestive process, reach organs such as the liver,
kidneys, and skeletal muscles. Calculations based on
recent isotopic studies suggest that intestinal amino
acid utilization (both from the diet and via the blood
supply to the intestine; the mesenteric arterial circula-
tion) can account for as much as 50% of the body’s
utilization of amino acids. It is also important to note
that the degree to which individual amino acids are
utilized by the gut varies markedly (Table 4.11).
Among the indispensable amino acids, threonine uti-
lization is particularly high and virtually all of the
dietary glutamate and aspartate are utilized within the
mucosa. In addition, the magnitude of splanchnic
amino acid metabolism varies with age, being appar-
ently greater in infants and also perhaps in the elderly.Table 4.10 The earlier and three contemporary suggested patterns of amino acid requirements in healthy adults
Amino Acid United Nationsa 1985 University of Surreyb 1999 MITc 2000 IOMd 2002
Isoleucine 10 e (13)f 18 (30) 23 (35) (25)
Leucine 14 (19) 26 (44) 23 (65) (55)
Lysine 12 (16) 19 (31) 30 (50) (51)
Methionine and cystine 13 (17) 16 (27) 13 (25) (25)
Phenylalanine and tyrosine 14 (19) 20 (33) 39 (65) (47)
Threonine 7 (9) 16 (26) 15 (25) (27)
Tryptophan 3.5 (5) 4 (6) 6 (10) (7)
Valine 10 (13) 14 (23) 20 (35) (32)
a FAO/WHO/UNU. Technical Report Series No. 724. Geneva: World Health Organization, 1985.
b Millward DJ. The nutritional value of plant-based diets in relation to human amino acid and protein requirements. Proc Nutr Soc 1999; 58 :
249–260.
c Young VR, Borgonha S. Nitrogen and amino acid requirements: the Massachusetts Institute of Technology Amino Acid Requirement Pattern.
J Nutr 2000; 130 : 1841S–1849S, reproduced with permission of the American Society of Nutrition.
d US National Academies of Science Institute of Medicine.
e Values expressed as mg/kg/d.
f Values expressed as mg amino acid/protein required for effectively meeting total protein and amino acid needs.