Relationship between fasting urinary nitrogen loss and obligatory nitrogen loss in rats

The relationship between obligatory nitrogen loss (ONL) and fasting urinary nitrogen loss (FUNL) was investigated in a longitudinal study in rats aged 30–430 days. Although both measurements increased with body size, when scaled allometrically they showed a decline. The rates of decline were parallel, such that at all ages the relationship between the two was FUNL=1.5 ONL. At any age inter-individual variability in this ratio was relatively small, the coefficient of variation being <7%. We propose that FUNL may be used as a proxy for the measurement of ONL and hence protein requirements. The reported relationship may also provide a theoretical basis for the well known but enigmatic relationship between BMR and ONL.