there is intensive protein turnover at the
cellular level (Waterlow et al., 1978). The
gastrointestinal tract has a very strong
nitrogen (including endogenous protein
and non-protein nitrogen) recycling
mechanism. Considerable amounts of
endogenous protein are produced via
secretions (saliva, bile juice, pancreatic
juice and intestinal juice) and desquamated
mucosal cells. For example, the produc-
tion of endogenous protein was estimated
to be 113.1 g day^1 for pigs with a body
weight of 45 kg (Low, 1982b). Studies by
Krawielitzki et al. (1990) in pigs, with an
initial body weight of 30 kg, showed a
minimum production of endogenous
protein of 100.6 g day^1. However, the
direct determination of the production of
endogenous protein is difficult and, in
most cases, the results are possibly
underestimated as the entry and the
digestion/absorption of endogenous
protein occur simultaneously in most
regions of the gut. Many factors including
dietary conditions, physiological state and
differences in the method of deter-
mination may affect the estimation.
Endogenous protein enters the lumen of
the gastrointestinal tract and mixes with
exogenous protein. It has been reported
that about 70% of endogenous protein was
digested and reabsorbed at the distal end
of the ileum in pigs (Souffrant et al., 1986;
Krawielitzki et al., 1990). The reabsorbed
endogenous amino acids may rebalance
the dietary pattern of amino acids,
especially under conditions of amino acid
deficiency. However, the amount of non-
reabsorbable endogenous amino acids
recovered in ileal digesta has complica-
ted the determination of amino acid
digestibility values in feedstuffs for pigs.
In principle, true amino acid digestibility,
as defined previously in Equation 13.3,
should be the preferred criterion in
digestibility measurements. As apparent
ileal rather than faecal amino acid
digestibility values are determined, the
estimation of the recovery of non-
reabsorbable endogenous amino acids is
usually carried out in digesta collected
from the distal ileum. The estimation of
endogenous amino acid levels in ileal
digesta has been the focus in research for
many years. Several techniques have been
used including the feeding of protein-free
diets, the regression analysis technique
(Carlson and Bayley, 1970; Taverner et al.,
1981; Furuya and Kaji, 1989; Fan et al.,
1995; Fan and Sauer, 1997) and the^15 N
isotope dilution technique (Krawielitzki
and Smulikowaka, 1977; Souffrant et al.,
1981; De Lange et al., 1989). Furthermore,
the homoarginine labelling technique
(Hagemeister and Erbersdobler, 1985) and
[^15 N]amino acid isotope dilution
technique (Lien et al., 1993) have been
reported for estimation. Comparative
evaluation of these techniques was given
by Souffrant (1991) and Sauer and De
Lange (1992). Each of these techniques
is subject to certain limitations and
criticisms. Furthermore, evidence at
present suggests that the regulation of
endogenous protein secretions, digestion
and reabsorption is a highly complex
matter depending on many factors. It thus
seems prudent now to use apparent rather
than true ileal amino acid digestibility
values (Sauer and Ozimek, 1986; Low,
1990; Knabe, 1991).
With regard to the relationship
between apparent and true ileal amino acid
digestibility, true ileal digestibility values
are higher than their corresponding
apparent values as expressed in Equation
13.3. Recent studies (Fan et al., 1995)
showed that apparent ileal amino acid
digestibility values are quadratic functions
of the dietary amino acid content, whereas
true ileal amino acid digestibility values
are relatively constant and independent of
the dietary amino acid content (Fig. 13.2).
As is illustrated using methionine as
an example, apparent ileal amino acid
digestibility values initially increase
sharply then gradually reach their plateau
values. Upon reaching the plateau values,
apparent ileal amino acid digestibility
values become independent of the dietary
amino acid content; then, apparent ileal
amino acid digestibility values can be
related to their corresponding true digesti-
bility values.Methods for Measuring Ileal Amino Acid Digestibility 283