___ The effect of frequency of feeding (once daily or six equal portions at intervals of 3 h) of diets containing lysine (6.4 g/kg) supplied as either protein-bound or free lysine on the metabolism of essential amino acids was investigated in pigs over the 23-34 kg weight range. The amounts of radioactivity from 14C02 produced by the pigs given oral doses of 114Cllysine or [14Cjphenylalanine were determined. [14C1Lysine was confirmed to be unsuitable presumably because the oral dose was diluted

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... by the dietary free lysine or was absorbed ahead of the protein-bound lysine, or both. With ['4C]phenylalanine addition, there was a significantly (P < 0.01) greater production of 14C02 by pigs fed once daily on the diet containing free lysine. However, with frequent feeding, there was no significant difference (P > 0.05) in the amount of 14C0, produced by pigs from [14Cjphenylalanine for either diet. Growth responses of the pigs were lower when the diet containing free lysine was given once daily (P < 0.01) but were similar when the diets were given frequently. It is concluded that the more rapid absorption of free lysine by growing pigs fed once daily results in an imbalance of amino acids a t the sites of metabolism. This leads to greater oxidation of essential amino acids than occurs when all the amino acids are supplied in the protein-bound form. With frequent feeding, a better balanced supply of amino acids is absorbed, leading to similar rates of oxidation of excess essential amino acids from diets containing either free or protein-bound lysine.