Maintenance studies

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Induction of deficiency states is an integral component of the process to determine maintenance requirements of individual amino acids (Table 1.7). The approach involves the formulation of diets with varying degrees of deficiency. For example in their studies on threonine requirements for maintenance in the chick, Edwards et al. (1997) used diets varying in threonine concentrations from 5 to 95% of assumed requirements for maximal growth. Chicks fed diets containing threonine at 5, 10 and 15% of ideal requirements lost weight commensurate with the degree of deficiency. Threonine retention was also negative for these groups, in proportion to the extent of threonine deficiency. Over the entire range tested, whole-body accretion of threonine occurred with an efficiency of 82%, considerably higher than that reported for pigs (60%; Adeola, 1995). Chicks fed lysine-deficient diets retained lysine in carcass with an efficiency of 79% (Edwards et al., 1999). In contrast, chicks fed graded levels of total SAA (methionine + cystine) from 5 to 95% of ideal requirements gained weight, and retained SAA in carcass

Table 1.7. Whole-body accretion of amino acids in chicks.

Dietary amino Whole-body accretion (mg day 1)

Table 1.7. Whole-body accretion of amino acids in chicks.

Dietary amino Whole-body accretion (mg day 1)

acid level (% of ideal)3

Thr

Val

Lys

SAAb

5

-11.9

-8.7

-20.9

2.8

10

-6.4

-2.4

-

6,3

15

-2.3

-

-

-

40

40.0

53.1

15.1

50.4

55

83.2

100.0

50.5

67.9

70

116.9

139.6

100.9

93.4

95

158.4

190.1

167.2

109.8

Reference

Edwards et al.

Baker et al.

Edwards et al.

Edwards and

(1997)

(1996)

(1999)

Baker (1999)

aDiets deficient in the single amino acids indicated in the next four columns. bSAA, sulphur amino acids.

aDiets deficient in the single amino acids indicated in the next four columns. bSAA, sulphur amino acids.

in a positive and dose-related manner over the entire range of dietary concentrations (Edwards and Baker, 1999). Furthermore, efficiency of whole-body SAA accretion was only 52%, presumably reflecting the diverse functions of methionine in addition to its role as a component of tissue proteins. Although chicks fed the lysine-deficient diet maintained a stable body-weight (Edwards et al., 1999) those fed the valine-deficient diet lost weight (Baker et al., 1996) when each amino acid was set at 5% of ideal. However, carcass retention of lysine was -20.9 mg day-1 and of valine was -8.7 mg day-1 (Table 1.7). Thus equivalent degrees of deficiency induced different responses, with lysine again emerging as an amino acid with unique effects (Velu et al., 1972).

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