Nitrogen Balance Index and Specific Dynamic Action in Rats Receiving Amino Acid Mixtures Low in Isoleucine, Methionine or Valine

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Abstract
Simultaneous nitrogen balance and energy metabolism studies were conducted on adult rats receiving amino acid mixtures in a diet otherwise very low in nitrogen. Each amino acid diet period was preceded by a 7-day depletion period in which an isocaloric N-free diet was fed. The diets were administered by stomach tube twice daily in order to insure equal intake by every animal every day. Resting energy metabolism determinations were made during a 10-hour period on the last day of the amino acid diet and again two days later after a fast of 36 hours. The difference between these rates was taken as representing the specific dynamic action of the diet. When a “complete” amino acid mixture simulating whole egg protein was fed the N balance index was invariably greater than unity. The N requirement for maintenance on this amino acid mixture averaged 162 mg N/day/kg3/4. In 6 experiments with two groups of rats the maintenance N requirement proved a more consistent characteristic of the amino acid mixture than the N balance index. Reducing the DL-isoleucine to one-third the quantity in the “complete” amino acid mixture caused a decrease in N balance index, an increase in maintenance N requirement to 212 mg N/day/kg3/4, and an increase in specific dynamic action of 3 Cal./day/rat. A similar reduction in DL-methionine caused a decrease of N balance index to about half and an increase of maintenance N requirement to 323 mg N/day/kg3/4 but no significant change in specific dynamic action. Reduction of DL-valine to one-third the quantity in the “complete” amino acid mixture caused no significant change in either N utilization or energy metabolism. Substituting glycine for glutamic acid caused no significant change in N balance index, maintenance N requirement or specific dynamic action. Reducing the quantity of an essential amino acid will, if carried far enough, lead to decreased efficiency of utilization of N (decreased N balance index and increased maintenance N requirement), and probably to a parallel decrease in efficiency of energy utilization (increased specific dynamic action). Such a decrease in efficiency of N utilization was found when either isoleucine or methionine was reduced to one-third. The corresponding decrease in efficiency of energy utilization was found only in the case of the low isoleucine diet.