Agriculture Reference
In-Depth Information
in particular across those of lesser protein quality, an amino acid requirement based
on a total basis will differ depending on the feed ingredients used for a particular diet.
Therefore, it is now common practice to express the amino acid requirements of sows on
an 'ileal digestible' basis. Nutritionists implicitly assume that ileal digestibility provides
reasonable estimates of amino acid bioavailability (Stein
et al.
, 2007).
Ileal digestible
Why ileal? In this context, the term 'ileal' refers to the most distal end of the small intestine
where small intestinal digesta is collected before the small intestinal junction with the
proximal colon. Amino acid concentrations are determined in the small intestinal digesta
to assess their disappearance relative to their dietary intake, hence the term 'digestibility'
(Sales and Janssens, 2003). The biological rationale for this approach is that sampling
at this site instead of the less invasive traditional fecal collection approach avoids
microbial metabolism of dietary amino acids in the large intestine (cecum and colon) that
confounds interpretation of traditional fecal collections. The massive microbial activity
in the large intestine alters the amino acid composition of the ingested protein (Metges,
2000). The much greater microbial count and longer transit time in the large intestine
relative to the small intestine (Hovgaard and Brondsted, 1996) favor fermentative protein
and amino acid degradation as well as production of amino acids by microbes from both
nonspecific and dietary nitrogen sources (Sauer
et al.
, 1975). Consequently, fecal amino
acid digestibility may either overestimate (for most of the amino acids) or underestimate
ileal digestibility depending on diet composition and the amino acid in question. Because
there is no absorption of dietary amino acids from the large intestine (Darragh
et al.
,
1994; Moughan and Stevens, 2012; Torrallardona
et al.
, 2003), the ileal sampling approach
provides a better measure of amino acids digested and absorbed by the animal, and thus
amino acid bioavailability, than fecal digestibility.
While ileal amino acid digestibility does not necessarily equate to bioavailability, it provides
estimations that can be measured routinely. The ileal digestible method of expressing
amino acid requirements, however, does have limitations. A key concern remains the
accurate differentiation between non-digested dietary amino acids and endogenous
amino acids that are secreted or lost into the digestive tract during the passage of feeds
through the digestive tract. Another concern is that in heat-treated ingredients, ileal
digestibility may overestimate bioavailability of amino acids and especially that of lysine
(Stein
et al.
, 2007).
Apparent, standardized and true ileal digestible
In routine ileal digestibility assays and when sampling ileal digesta, no differentiation
is made between non-digested dietary amino acids and endogenous amino acid losses,
i.e. the losses associated with digestive enzymes, mucus and other proteins secreted by
pigs into the digestive tract. Therefore, routine digestibility values should be referred to
as apparent digestibility. There are two main fractions of endogenous protein losses in
pigs: (1) basal losses which are independent of diet and can be estimated routinely by
feeding pigs protein-free diets; and (2) diet- or ingredient-specific endogenous losses
