Reduced obligatory nitrogen loss in rats made obese by cafeteria feeding

Rats made obese by fat feeding have been shown to have a lower urinary mitrogen excretion when fasting than non-obese animals (1). However, it is not clear whether this reduced metabolism of protien is the effect (2) or the cause (3) of the obese state nor are its implications understood. We report here that the induction of obesity by cafeteria feeding in rats is also assosited with a lower fasting urinary nitrogen loss (FUNL) than relative to matched non-obese controls and that these changes are paralled by changes in obligatory nitrogen loss (ONL) of the animals when fed a protein free diet. Our results suggest that the reduced net nitrogen losses are a consequence of obesity and, since ONL is the dominant component of the protein requirements, that obese subjects may have a reduced protein requirement as well as a lower propensity to catabolise protein when fasting. Thus, the adiposity of the subject may need to be considered when estimates of protein requirement are made.