Estimating Bioavailable Amino Acid Content in Feedstuffs

Apparent ileal digestibility coefficients

In conventional digestibility studies, it is not possible to distinguish between nondigested dietary AAs and non-reabsorbed endogenous AAs that are present in the digesta captured at the distal ileum. Calculated digestibility coefficients are, therefore, referred to as apparent digestibility coefficients. However, apparent ileal digestibility coefficients of AAs in a mixed diet may underestimate the amount of AAs that are actually available to the pig. This is particularly true if low-protein feed ingredients such as cereal grains are included in the diet.[6] The reason for this underestimation is that apparent ileal digestibility coefficients may not be additive in a mixture of feed ingredients (Fig. 1), thus creating challenges in accurately formulating diets to supply the desired amount of AAs in the diet. The use of digestibility coefficients to estimate bioavailability is improved considerably when apparent ileal digestibility coefficients are corrected for endogenous gut AA losses. For a complete review of endogenous gut AA losses in pigs and the methods used for their estimation, please refer to Nyachoti et al.[5] and Boisen and Moughan.[7] Endogenous gut AA losses can be divided into two categories, namely, basal (diet-independent) losses and diet-specific (additional) losses. Basal endogenous AA losses are obligatory losses closely associated with the metabolic functions of the animal and are independent of the type of diet fed. Specific endogenous AA losses are dependent on the composition of the diet.

Standardized ileal amino acid digestibility coefficients

Standardized ileal AA digestibility coefficients, which are sometimes incorrectly referred to as true ileal digestibility coefficients, are derived by correcting apparent ileal digestibility coefficients for basal endogenous AA losses.[8] Compared with apparent digestibility coefficients, standardized ileal digestibility coefficients are believed to provide a better estimate of AA bioavailability in pig feeds. It is important, however, to recognize that estimates of basal endogenous AA losses vary widely among studies and that there is no general agreement on the best estimate of basal endogenous AA losses.[7]

True ileal amino acid digestibility coefficients

True ileal AA digestibility coefficients are estimated when the recovery of specific endogenous AAs in the ileal digesta is determined and used to correct apparent digestibility coefficients. This requires that the specific endogenous gut AA losses in ileal digesta be quantified. The required techniques are relatively tedious, thus making it more difficult to generate true ileal digestibility coefficients for routine feed formulation. However, when the goal is to understand how different feed ingredients or dietary components influence AA utilization in pigs, true ileal digestibility coefficients should be determined.

Apparent digestibility True digestibility

Apparent digestibility True digestibility

Fig. 1 Observed and calculated apparent and true ileal lysine digestibilities in a barley canola meal based diet fed to growing pigs. (Adapted from Ref. [9].) (View this art in color at www. dekker.com.)

Effects of gut microbes on estimation of amino acid digestibility

Although the majority of the gut microflora in monogas-tric animals reside in the hindgut, a large microbial population also inhabits the upper gut (i.e., small intestine). Gut microbes may deaminate dietary AAs or synthesize new AAs. This will influence the digestibility measurements. The significance of such an effect on determining ileal AA digestibility coefficients is not known yet and deserves further attention.

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