Efficiency of Utilization of Indispensable Amino Acids for Growth by the Rat

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Abstract
Rats fed an amino acid diet containing 14C-labeled algal hydrolysate or individual amino acids, were used to estimate the efficiency of utilization of amino acids for growth. When algal protein hydrolysate was used 35.8% of the absorbed dose was recovered in CO2 in 24 hours while 2.9 and 61.2% were recovered, respectively, in the urine and carcass. Approximately 75% of the 14C in the carcass at 24 hours was recovered there on day 14. When the same diet was labeled so that the 10 indispensable amino acids had the same specific activity, 22.0% of the absorbed dose was recovered in CO2 in 24 hours, while 3.7% and 75.0% were recovered in the urine and carcass, respectively. At 7 and 14 days, 84.5 and 76.6% of the activity that was in the carcass at 24 hours was recovered. Calculation of t0.5 for 14C retention in the carcass gave 28 days for these two experiments. Widely different proportions of the absorbed dose were recovered in CO2 and urine, respectively, over a 24-hour period when the diet was labeled with only one of the essential amino acids (His 12.0, 11.7; Trp 14.0, 4.1; Leu 16.5, 2.7; Thr 18.2, 1.3; Lys 20.3, 9.9; Val 23.5, 1.9; Phe 24.2, 2.8; Ile 24.5, 2.6; Arg 27.8, 10.5; Met 41.6, 1.5). Varying the methionine level above and below its requirement revealed that it is oxidized more extensively than the other indispensable amino acids, suggesting that the requirement of methionine as a methyl and sulfur source may be responsible for its high rate of oxidation.