Nitrogen Balance of Adult Rats Fed Diets Low in L- and DL-Threonine

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Abstract
Nitrogen balance was determined on two groups of adult male albino rats which derived all of their dietary nitrogen from mixtures of amino acids. Each experiment included the following dietary regimes in the order given: 14 days on maintenance diet (9.6% whole egg protein); 7 days on N-free diet; 7 days on amino acid diet supplying approximately half of the maintenance requirement of total nitrogen; 7 days on double the quantity of the amino acid mixture fed in the previous period. These diets, except the maintenance diet, were fed by stomach tube in two equal portions daily, and each rat received the same quantity of diet each day. At and below the limiting concentration, threonine intake is linearly related to nitrogen balance. The natural isomer of threonine is twice as effective as the racemic mixture and hence it is concluded that D-threonine cannot be utilized by the adult rat. The requirement of the rat, under these conditions, is computed to be 2.36 mg of L-threonine nitrogen/day/kg¾ which is equivalent to 20.1 mg of the amino acid. The requirement for man, recomputed on the same basis from the data of Rose ('49), is 20.7 mg of L-threonine/day/kg¾. The rats on diets low in threonine deveoped edema of the esophagus to such an extent that it was difficult or impossible to pass the stomach tube. This condition developed rapidly and was the cause of death of several animals.