Determination of amino acid requirements of young pigs using an indicator amino acid.

Mixtures of skim milk and free amino acids were compared as diets for pigs which would allow manipulation of dietary amino acid levels. Piglets gained 208 g/d between 3 and 14 d of age on the skim-milk diet, but replacement of 600 g/kg of the dietary nitrogen with free amino acids reduced growth rate to 148 g/d. Supplementation of a lysine-deficient diet with lysine reduced the catabolism of [14C]phenylalanine showing that phenylalanine catabolism could be used as an indicator of the adequacy of diet with respect to another essential amino acid. The dietary level of phenylalanine which would provide an excess for catabolism by the piglet was estimated directly by measuring the influence of dietary phenylalanine level on [14C]phenylalanine oxidation. Reduction of the dietary phenylalanine level below 7 g/kg had no effect on phenylalanine oxidation, whereas increasing the dietary phenylalanine level above 7 g/kg resulted in a linear increase in phenylalanine oxidation. An indirect estimate of histidine requirement was made by examining the influence of dietary histidine level on [14C]phenylalanine oxidation. In diets containing more than 4 g histidine/kg, phenylalanine oxidation was minimal. In diets containing less than 4 g histidine/kg, [14C]phenylalanine oxidation increased as the level of dietary histidine was reduced. This showed that the utilization of the essential amino acid phenylalanine, for protein synthesis, was not limited by histidine supply in diets containing more than 4 g histidine/kg. A direct estimate of histidine requirement was made by examining the influence of dietary histidine level on [14C]histidine oxidation. Diets with more than 4 g histidine/kg contained an excess which was catabolized: there was a linear increase in histidine oxidation in response to dietary histidine levels greater than 4 g/kg. This confirmed the previous indirect estimate of histidine requirement.