Assessment of nitrogen requirements

Abstract

The estimated protein needs obtained over approximately 100 years and always by nitrogen balance studies indicate an average need of somewhat more than 0.5 g/kg per day. The range of values obtained within the same laboratory or between different laboratories is large. If the average need of a 70 kg man is of the order of 40 g/day, the lower limit would be near 25 g and the upper limit near 60 g. We simply do not know whether this large range is representative of inherent differences in subjects, prior treatment of subjects, or errors in the balance technique itself. It is important, nevertheless, to emphasize that the requirements of young men, for which the most data are available, are of the order of 5 to 7% of the total energy need, a value considerably less than even high cereal diets. In general, the data indicate that in the adult subject, protein quality is not of great significance unless exceptionally poor quality proteins are fed. It has become obvious in recent years that interpretations of balance studies are not simple. The method appears to yield uninterpretable findings when high levels of protein are fed. Enzymatic adaptation occurs when low levels of certain essential amino acids are fed which allow conservation of these amino acids. The energy intake of the subject studied is of crucial importance. We suggest that much of the nitrogen balance data probably cannot be translated to chronically restricted populations or patients because the adaptive processes to energy restriction and amino acid restriction and the interaction between the two is unknown.