Background: The evaluation of ammonia detoxification by pre-and probiotics by means of colonic lactose-[ 15 N 2 ]ureide ( 15 N-LU) degradation is of great interest both scientifically and in terms of nutrition physiology. Objective: Pre-and probiotics were supplemented in healthy adults to evaluate the effect of the ammonia metabolism in the human colon by means of 15 N-LU. Methods: A total of 14 participants aged 20-28 years daily received a regular diet either without (no treatment) or with supplementation of 30 g fibre of potatoes (FPs), 30 g wrinkle pea starch (WPS, resistant starch content: 12 and 70%, respectively) and 375 g Lactobacillus acidophilus (LC1) yoghurt, over a 10-day period in a randomised order. After 1 week, 5.7 mg/kg body weight 15 N-LU was administered together with breakfast. A venous blood sample was taken after 6 h. Urine and faeces were collected over a period of 48 and 72 h, respectively. The 15 N abundances were measured by isotope ratio mass spectrometry. Results: The mean renal 15 N-excretion differed significantly between the supplementation of FP and no treatment (32.5 versus 46.3%, P ¼ 0.034), FP and LC1 (32.5 versus 51.6%, P ¼ 0.001), and WPS and LC1 (38.5 versus 51.6%, P ¼ 0.048). The mean faecal 15 N-excretion amounted to 42.7% (no treatment), 59.7% (FP), 41.8% (WPS) and 44.0% (LC1). In comparison with no treatment, the urinary 15 NH 3 -enrichment was significantly decreased at 16 h after FP supplementation. Conclusion: The prebiotic intake of FP and WPS lowered the colonic generation and the renal excretion of toxic 15 NH 3 , respectively, when using 15 N-LU as a xenobiotic marker.