Farm Animal Metabolism and Nutrition

the topic of amino acid digestibility in pigs
during the last three decades. Evidence
indicates that ileal rather than faecal amino
acid digestibility values provide a more
reliable estimation of protein digestion,
peptide and amino acid absorption, and
usually predict amino acid availability
values in feedstuffs for pigs, as was dis-
cussed by Low (1982a), Austic (1983),
Tanksley and Knabe (1984), Sauer and
Ozimek (1986) and Sibbald (1987).
Nevertheless, special caution should be
exercised in interpreting amino acid
digestibility values in feedstuffs which
have been heat-treated. As some essential
amino acids including lysine, threonine,

methionine and tryptophan are susceptible

to the effect of heat, they may be partly

damaged and absorbed in a form which

renders these unavailable to the animal for

protein synthesis. In this case, ileal amino

acid digestibility values usually over-

estimate amino acid availability values

(e.g. Batterham et al., 1990a,b; Wiseman et

al., 1991).

Apparent ileal amino acid digestibility

values in a wide variety of feedstuffs have

been reported. It is easy to recognize that

there are large differences in ileal amino

acid digestibility values between feed-

stuffs. However, it comes as somewhat of a

surprise to note considerable variation in

280 W.C. Sauer et al.

True faecal digestible amino acids

Digestibility assay

Apparent faecal digestible amino acids

Dietary amino acids

Apparent ileal digestible amino acids

True ileal digestible amino acids

Blood plasma free amino acid pool

Body protein deposition Proteins for body maintenance

Protein synthesis Protein degradation

Oxidation of

amino acids

Metabolism of amino acids

in enterocytes

Distal ileal output of

endogenous amino acids

Distal ileal output of amino acids

Faecal output of amino acids,

disappearance of amino acids

in the form of ammonia, amines

and amides and synthesis of

bacterial amino acids

Faecal output of endogenous

amino acids

Protein synthesis Protein degradation

Excretion of heat-

damaged amino

acids in urine

Ileal analysis method

Faecal analysis method

The slope-ratio assay

Fig. 13.1.Schematic representation of amino acid utilization in growing pigs (Fan, 1994).