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obtained to the supplementation of the low-
est level of each of the amino acids with the
synthetic form of the amino acid.
The results of protein deposition and
body lipid content presented in this chapter
confirm the reports of Leclercq (1998) and
Gous (1998, 2007), that increasing the con-
tent of a limiting amino acid in the diet en-
hances deposition of protein and reduces
the body lipid content in broilers, thereby
changing the composition of the body
weight gain. But this statement is only valid
for birds fed diets formulated with the dilu-
tion technique. According to D'Mello (2003a),
when comparing the dilution and graded
supplementation techniques, it is possible
to observe contrasting effects on carcass fat
content of chicks fed levels of individual
amino acids, and it is important to consider
methodological aspects when interpreting
carcass fat responses to amino acid intake.
This is due to the fact that the birds have a
genetic potential for growth and protein de-
position, and offering them a diet with the
same energy content, but deficient in pro-
tein or amino acid, will force them to con-
sume more feed in an attempt to reach their
potential growth, and in so doing consume
more energy than necessary and deposit
more fat (Emmans, 1981, 1989).
Results of the Thr response trial indi-
cate that even the highest levels of this
amino acid were not sufficient to decrease
body lipid content to the same level as was
measured in the Met+Cys trial. This lack of
effect of Thr on carcass fat has been reported
previously (Leclercq, 1998; Kidd et al .,
1999; Dozier et al ., 2000a,b, 2001). The ab-
dominal fat depot is suggested to be the
least sensitive response criterion for dietary
Thr, but not for Met+Cys (Dozier, 2000b).
This fact is related to the utilization of Thr
by broilers, since this amino acid is heavily
involved in feather formation, although the
percentage of Thr in the whole carcass is
similar to that of feathers (Fisher et al .,
1981; Stilborn et al ., 1997). Once assimi-
lated in feathers, there is no turnover of Thr
for reutilization in protein synthesis (Dozier,
2000a). On the other hand, Met+Cys acts as
a lipotropic agent (Andi, 2012), playing a
role in carnitine synthesis, which stimulates
the oxidative metabolism of lipids (Schutte
et al ., 1997) thus reducing abdominal fat
(Andi, 2012) and consequently, body lipid
content.
For the reason cited above, protein or
amino acid deposition was used to calculate
the efficiency of utilization of each amino
acid, rather than body weight gain. In the
present study, the contents of Met+Cys and
Thr measured in the feather-free body and
feathers are consistent with the literature
(Sklan and Noy, 2004; Stilborn et al ., 2010;
Silva, 2012).
Some authors suggest that the efficien-
cies of utilization are different for each
amino acid and that differences must be due
to species and genotype (Edwards et al .
1999; Fatufe et al ., 2004). Other authors
report that these factors and some others
like age, sex, environmental temperature,
immunological stress and diet nutritional
composition exert their effects by affecting
food intake, and not specifically the effi-
ciency of utilization of the amino acids
(D'Mello, 2003a).
In the case of poultry, feathers grow at
a different rate to that of the rest of the body
(Emmans, 1989; Gous et al ., 1999) and also
the rates differ between sexes, since birds
are feather-sexable, thus females have fea-
ther growth rates greater than males (Marcato
et al ., 2009). Therefore, we expected that
there would be differences in the efficiency
of utilization of amino acids, especially
Met+Cys, between phases and between the
sexes. However, no differences were ob-
served for any of the amino acids studied,
since the slopes of the lines were similar for
both sexes, as well as in the different growth
phases. Therefore we determined a single
efficiency of utilization to describe the
average daily requirement of these amino
acids.
Heger and Frydrych (1989) reported
that the relative concentration of the amino
acid in the diet is the main factor that affects
the efficiency of utilization, the efficiency being
higher when the amino acid is deficient, and
the increased consumption of the limiting
amino acid reduces its efficiency of utiliza-
tion, because a greater proportion of this amino
acid is allocated to alternative metabolic
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