Biological assessment of protein quality. Optimal essential: non-essential amino acid ratios for maintenance of certain states of nitrogen balance in young rats.

Male weanling rats were used to assess the effect of replacing essential amino acid (EAA) mixtures with non-essential amino acid (NEAA) mixtures on nitrogen balance. Each of 4 essential amino acid mixtures, fed in quantities sufficient to maintain specific states of nitrogen balance, was replaced with a non-essential amino acid mixture on an isonitrogenous basis. The results indicate the following: 1. Within limits, replacement with NEAA mixture caused an increase in the efficiency of utilisation of the amino acid mixture when the nitrogen balance state investigated was + 100, +200 or +300 mg/100 g body mass/8 days, but not when nitrogen balance was zero. 2. In the maintenance of nitrogen balance = 0, the EAA mixture could be replaced with NEAA mixture to a greater extent (83%) than was possible in the maintenance of balance states of + 100, +200 or +300 mg (about 70%). The results support the view that (a) EAAs are converted to NEAAs at the cost of metabolically available nitrogen: and (b) with a decrease in the state of nitrogen balance maintained by the animal, the amino acid requirement is shifted partially from EAAs to NEAAs. The implications of the above findings are discussed with reference to dietary protein evaluation and related questions.