Enhanced retention of urea-nitrogen in patients on total parenteral nutrition for intestinal failure

Urea, the main end product of human nitrogen metabolism, is either excreted in the urine or salvaged for further metabolic interaction, through the activity of the colonic microflora. In patients on TPN (total parenteral nutrition) the bowel is ‘defunctioned’ insofar as the microflora are deprived of exogenous substrate, and it might be expected that urea salvaging is reduced to low levels. The recovery of labelled urea-nitrogen was measured in 7 patients on TPN and compared with 4 normals. Following a single intravenous dose of 15N15N-urea, urine was collected for 48h and analysed for 15N. In normals the median retention of 15N was 17% (range 13–17) of the dose compared with 38% (24–52) in 5 TPN patients not on antibiotics (p < 0.05). TPN patients on antibiotics retained 11 and 15%. Despite the absence of exogenous substrate, the ‘defunctioned’ bowel of patients on TPN appears more active in hydrolysing urea than in normals, and the hydrolysis is sensitive to the effect of antibiotics.