Poor utilization of dietary free amino acids by white sturgeon

Three separate experiments were conducted to investigate the utilization of dietary free amino acids by white sturgeon, Acipenser transmontanus. In the first experiment, sturgeon were fed for 8 weeks with isoenergetic and isonitrogenous diets containing either intact protein, an amino acid mixture, or a neutralized amino acid mixture of similar composition. Sturgeon fed the two amino acid diets grew significantly (p < 0.05) less, mainly because of reduced feed intake. In the second experiment, postprandial changes in plasma free amino acids of sturgeon force-fed the above diets were monitored at 2, 4, 6, 8, 12, and 24 h. Total plasma amino acids in sturgeon force-fed the intact protein diet showed a gradual increase and reached peak levels at 8 h. In contrast, plasma amino acids of fish force-fed the amino acid diets showed a rapid increase and reached maximal levels at 2 h. In the third experiment, free amino acid excretion in the urine of sturgeon force-fed the three diets was monitored at 4, 8, 12, 16, 20, and 24 h. In all dietary groups, peak excretion of indispensable amino acids was detected at 8 h after feeding but with significantly higher levels from fish force-fed the amino acid diets. Sturgeon force-fed the neutralized amino acid diet excreted 35% less amino acids compared to fish force-fed the non-neutralized diet. Sturgeon force-fed the intact protein diet excreted significantly higher amounts of ammonia. Under amino acid feeding conditions, sturgeon excreted more amino acids in its urine, but the urinary excretion of amino acids was not a major route in dealing with the rapid absorption of dietary free amino acids.