The bioavailability of an AA refers to the proportion of that AA that is in a form that can be digested, absorbed, and used for metabolic functions.1-1-1 Bioavailability coefficients of AAs in feedstuffs are determined in slope ratio growth assays. These assays are laborious and time consuming and only allow determination of the bioavailability of a single AA at a time, which is costly. Consequently, there are only a limited number of ingredients and AAs for which bioavailability coefficients have been determined and such coefficients of AA are not routinely used in feed formulation.
The use of bioavailability coefficients is important for heat-processed or long-stored ingredients because of the negative impact of these treatments on the bioavailability of the AA lysine. Lysine is rendered biologically unavailable by complexing with reducing compounds in the feed.
There are considerable efforts to develop simpler methods for estimating AA bioavailability in feedstuffs. A method known as the reactive lysine technique for assessing the bioavailability of lysine in feedstuffs that have undergone heat processing or that have been stored for a long time under conditions that might compromise lysine availability has been proposed. A method for determining the so-called metabolic availability of AAs in feed ingredients has also been developed.1-3-1 Thus far, this method has been used to estimate the metabolic availability (i.e., the proportion that is used for body protein synthesis) of lysine in a small number of ingredients and there are still several questions that must be addressed before it can find use in routine assessment of AA bioavailability in feedstuffs.
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