The effect of a weight-reducing diet on the nitrogen metabolism of obese adolescents.

Rates of whole body amino nitrogen flux were measured in 16 obese adolescents undergoing weight reduction with a high protein low energy diet. The subjects received approximately 2.5 g of animal protein per day per kilogram ideal body weight and maintained nitrogen balance throughout the 18 days on the diet. Flux rates were calculated separately from the cumulative excretion of 15N in urinary ammonia and urea following the administration of a single dose of [15N]glycine. The pattern of 15N label appearance in urinary ammonia and urea nitrogen was followed for 72 h after the administration of [15N]glycine. Significant amounts of label continued to be excreted in both urinary ammonia and nitrogen for 36-48 h after label administration. The weight-reducing diet accelerated 15N cumulative excretion in urinary urea, but not in ammonia nitrogen compared with the control diet. Whole body nitrogen flux rates increased rapidly and significantly on the diet. Using the urea end product, this increase was evident on the 4th diet day, but not by the 7th or subsequent days. On the other hand, using the ammonia end product, flux rate increased markedly (p less than 0.0001) and remained elevated throughout the whole study. Our results demonstrate adaptive changes in whole body amino-nitrogen metabolism in response to the reducing diet. Different patterns of change are seen depending upon whether an ammonia or a urea end product is used. Our data thus add to the evidence for compartmentation of the body's amino-nitrogen pools.