Plasma essential amino acid concentrations in response to casein infusion or ration change in dairy cows: A multilevel, mixed-effects meta-analysis

摘要

The objectives of the current meta-analysis were toreview the relationships between plasma individualessential AA concentrations and their respective AAdigestible flows (AADI) in 2 independent sets of studies.The first set of studies included 36 casein infusionstudies (CN; 83 treatment means) and was regarded asthe best comparison standard available, because differencesin AADI achieved by casein infusion (up to40% of total metabolizable protein supply) did not relyon any model assumptions and were directly estimatedfrom casein infusions. The second set of studies included42 feeding trials (FT; 94 treatment means) in whichAADI were predicted using the 2001 National ResearchCouncil model. The 2 sets of studies were not balancedfor dry matter intake and the supplies of metabolizableprotein and net energy for lactation; therefore, a subsetof 17 CN studies (35 treatment means) and 19 FT trials(49 treatment means) balanced for these variableswas assembled to allow the comparison of linear termsfrom CN and FT studies. In the subset of data set, thelinear terms of individual AA did not differ betweenCN and FT studies except for Met and Thr, with aslope lower by 23 and 62%, respectively, in CN versusFT studies. The agreement in linear slopes between CNand FT studies indicates, indirectly, that AADI werepredicted accurately by the National Research Councilmodel. In the large data set, the relationships betweenplasma concentrations of Ile, Leu, Val, and their sum(branched-chain AA; BCAA) and their respective AADIshared common characteristics that distinguished themfrom the other AA. For the plasma concentration ofBCAA, the linear terms were significant in CN and FTstudies, but the quadratic terms were significant onlyin FT studies. This decline in the response of plasmaconcentration of BCAA to increased BCAA digestibleflow in FT studies was associated with diets rich in energy,diets with a high concentrate level, or diets basedon corn silage. These dietary conditions can stimulateinsulin secretion and decrease plasma concentrationof BCAA. For the non-BCAA, a quadratic term wassignificant for plasma His, Lys, Met, and Thr in eachset of studies, indicating an increased removal of theseAA by the liver as AADI increased.