Nitrogen balance and definition of requirement

Nitrogen balance and definition of requirement

The starting point for estimating total protein needs has been, in most studies, the measurement of the amount of dietary nitrogen needed for zero nitrogen balance, or equilibrium, in adults. In the growing infant and child and in women during pregnancy and lactation, or when repletion is necessary following trauma and infection, for example, there will be an additional requirement associated with the net depo-sition of protein in new tissue and that due to secre-tion of milk. Thus, a United Nations (UN) Expert Consultation in 1985 defined the dietary need for protein as follows.

The protein requirement of an individual is defined as the lowest level of dietary protein intake that will balance the losses from the body in persons maintaining energy balance at modest levels of physical activity. In children and pregnant or lactating women, the protein requirement is taken to also include the needs associated with the deposition of tissues or the secretion of milk at rates consistent with good health.

Most estimates of human protein requirements have been obtained directly, or indirectly, from measurements of nitrogen excretion and balance (Nitrogen balance = Nitrogen intake – Nitrogen excretion via urine, feces, skin, and other minor routes of nitrogen loss). It must be recognized that the nitro-gen balance technique has serious technical and inter-pretative limitations and so it cannot serve as an entirely secure or sufficient basis for establishing the protein and amino acid needs for human subjects. Thus, there are:

●      a number of inherent sources of error in nitrogen balance measurements that should be considered

●      a number of experimental requirements that must be met if reliable nitrogen balance data are to be obtained.

These include

●      the need to match closely energy intake with energy need, for the various reasons discussed earlier

●an appropriate stabilization period to the experi-mental diet and periods long enough to establish reliably the full response to a dietary change

●timing and completeness of urine collections

●absence of mild infections and of other sources of stress.

Reference to detailed reviews of the concepts behind and techniques involved in the nitrogen balance approach.

 When direct nitrogen balance determinations of the protein requirement data are lacking, as is the case for a number of age groups, an interpolation of requirements between two age groups is usually made simply on the basis of body weight considerations. A factorial approach may also be applied; here, the so-called obligatory urine and fecal nitrogen losses are determined (after about 4–6 days of adaptation to a protein-free diet in adults), summated together with other obligatory losses, including those via sweat and the integument. For children, estimates of nitrogen deposition or retention are also included. In the case of very young infants the recommendations for meeting protein requirements are usually based on estimated protein intakes by fully breast-fed infants.