Diurnal variation in urine [15N]urea content, estimates of whole body protein turnover, and isotope recycling in healthy meal-fed children with cystic fibrosis

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Abstract
The pattern of urinary urea excretion and labeling with 15N was examined in 8 meal-fed 6-9 yr old children, over a 3-day period using a simulated constant infusion of the label. The children had cystic fibrosis but were healthy and in a good nutritional status at the time of the study. Reciprocal diurnal patterns of urea excretion and [15N]urea enrichment were suitable for mathematical description. Urea excretion was maximal in the evening at .apprx. 2000 h and minimal at 0800 h, whereas the [15N]urea enrichment was maximal at about 0800 h and minimal at 2000 h. In addition to the diurnal variation the [15N]urea enrichment increased exponentially to a plateau or isotopic steady-state. The diurnal variation in [15N]urea enrichment resulted in large diurnal changes in the calculated rates of whole body amino nitrogen flux, synthesis and breakdown. Flux rates were .apprx. 44% higher in the evening than in the morning. Synthesis rates were 19% higher in the evening, whereas breakdown rates were 27% greater in the morning. Mean amino N flux rates were 1.28 (SD 0.13) g N .cntdot. kg-1 .cntdot. day-1. Isotope recycling was estimated from the slope of the [15N]urea enrichment curve between 30-54 h from the start of the study. There was a wide range in recycling, 2.9-19.4%, (mean 11.4, SD 5.4). Some of the biological and pharmacological importance of the diurnal variation in the protein metabolism is discussed.