Nitrogen Balance Index in the Adult Rat as Affected by Diets Low in L- and DL-Methionine

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Abstract
Nitrogen balance was determined on adult rats receiving amino acid mixtures as the sole source of nitrogen. Each experiment included: a 14-day period on a maintenance diet; a 7-day period on an N-free diet; a 7-day period on an amino acid mixture that supplied approximately half of the maintenance requirement of total nitrogen; and a 7-day period on the same amino acid mixture but with the quantity of nitrogen doubled. These diets, except the maintenance one, were fed by stomach tube in two equal portions daily and each rat received the same quantity of diet each day. The weighted mean nitrogen balance index, K′, for three experiments (I, IV, V, in fig. 1) with the “complete” amino acid mixture was 0.98. When L-methionine was reduced to one-half of the amount in the complete amino acid mixture, K′ = 0.79. Data on nitrogen balance suggest that the D-component of DL-methionine is fully utilized at or near nitrogen equilibrium but may not be utilized at all when nitrogen balance is decidedly negative. The requirement of methionine for maintenance of nitrogen equilibrium under these conditions is computed to be 3.76 mg of the methionine nitrogen/day/kg¾, or 40.0 mg as the amino acid. On the same basis this is very close to the requirement for man. A change in the calories derived from fat from 36% to 10% failed to influence the utilization of the complete amino acid mixture.