Effect of Dietary Protein Level on Plasma Free Amino Acids in the Chicken

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Abstract
Plasma free amino acids in 5-month-old chickens were measured 2 hours after the last meal. Response curves of the plasma amino acid concentration to changes in the dietary casein level could be grouped into four types. Threonine, lysine, valine, leucine, tyrosine, isoleucine, methionine and proline (type A) increased linearly with increases in the dietary casein level from 3 to 21%. Alanine (type B) increased with increases in the dietary casein level from 3 to 15% and thereafter decreased with increased dietary casein level. Serine and glycine (type C) showed no significant trend with increases in dietary casein level from 3 to 12%, but thereafter decreased with increased dietary casein level. Histidine, tryptophan, arginine, phenylalanine, cystine, glutamic acid and aspartic acid (type D) were not consistently influenced by the dietary casein level from 3 to 21%. As a whole, the total indispensable amino acids were increased linearly with increases in dietary casein level. Such accumulated amino acids were considered to be a part of protein reserve. There was a highly significant correlation between the body weight gain and the total plasma indispensable amino acids.