Abstract
Plasma free amino acids in adult cockerels were measured 2 hours after a meal to study the effect of the dietary lysine level on the plasma free amino acid concentration. L-Lysine was added to a 3% casein diet (lysine level, 0.21%) to provide lysine concentrations of 0.42 to 1.47% in the diet. The response curves of the plasma free amino acid concentration to the dietary lysine level could be grouped into four types. Lysine was constant when dietary lysine increased from 0.21 to 0.63%, but increased with a further increase of dietary lysine. Valine, tyrosine, leucine, histidine, phenylalanine, isoleucine, cystine, methionine, proline, glutamic acid, glycine and aspartic acid remained constant when the dietary lysine increased from 0.21 to 1.47%. Threonine, tryptophan, serine and alanine increased with an increase of dietary lysine from 0.21 to 1.47%. Arginine was constant when dietary lysine increased from 0.21 to 0.84%, but decreased with a further increase of dietary lysine. Body weight was unchanged when the dietary lysine level was between 0.21 and 0.63%, whereas it was reduced by further increases in dietary lysine. Results suggest that this loss in body weight was a major factor affecting the observed plasma levels of the amino acids, along with differences among the amino acids in their susceptibility to catabolism and in the amounts ingested.