Efficiency of utilization of essential amino acids in growing rats at different levels of intake.

Nitrogen balance was studied in growing male SPF-rats fed on diets in which each essential amino acid was varied from zero to about 120% of optimum requirement. From the balance results, optimum and maintenance requirements were estimated as well as the efficiency of utilization of amino acids for growth and growth + maintenance. N balance increased with increasing dietary level of the deficient amino acid; the response gradually diminished as the content of the amino acid approached optimum. At zero level of intake, negative N balance was found for all amino acids except histidine. The highest loss of body N was found in the sulphur-amino-acid-free diet and the lowest one in the lysine-free diet. Maximal utilization of essential amino acids for growth was found at dietary levels corresponding to 30-60% of optimum requirement and ranged from about 0.65 to 0.85 except for S amino acids and histidine. The utilization of S amino acids was about 0.55 while that of histidine exceeded 1.0. The utilization of amino acids for growth + maintenance was maximal at the lowest levels of intake and gradually decreased as the dietary concentration of the limiting amino acid increased. At dietary levels near optimum the utilization was about 0.6-0.7, except for S amino acids where the utilization was less than 0.5.