The effect of dietary lysine level, restriction of food intake and sampling time on the levels of amino acids in the blood plasma of chicks

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Abstract
A suitable procedure for obtaining blood samples was established from an investigation of (a) diurnal variations in plasma amino acid concentrations and (b) the effect of controlling food intake on plasma amino acid levels. Restriction of feeding to a 40‐min period, 105 min before sampling, significantly increased total plasma amino acid levels (P< 0.05) and individual concentrations of serine and proline (PP<0.01). The cause of these changes is not clear. Amino acid concentrations in samples taken at different times during the day were fairly constant. In the sampling procedure adopted routinely, birds were allowed free access to their diets for two weeks and sampled at 14.00 h on day 14. The effect of dietary lysine level, at two protein concentrations, on plasma amino acid levels was evaluated. With inadequate and excessive dietary lysine concentrations total plasma amino acid levels were generally elevated. Plasma levels were least with diets containing 0.9 to 1.15% lysine. The lysine requirements for optimum performance measured by growth rate, food conversion efficiency and nitrogen retention were identified as 1.2% with a 21% protein diet and approximately o‐8% with a 16% protein diet. The data indicate that efficient utilisation of dietary protein and relatively low levels of amino acids in the plasma are fairly closely related.