We evaluated the effect of resistant starch (RS) and resistant starch with wheat bran (RS+WB) on the colonic ammonia metabolism in healthy volunteers using the biomarker (15)N-lactose ureide ((15)N-LU). Particularly, it was investigated whether this biomarker allowed to estimate differences in the site of fermentation. Ten volunteers were included in a placebo-controlled crossover study. They consumed in random order 2 x 15 g RS/day for 2 weeks and placebo for 2 weeks separated by 2 weeks wash-out. At baseline, on the first day of each intake period and after each intake period, they consumed a (15)N-labelled test meal and collected all urine in different fractions for 48 h. In ten other volunteers, the effect of 2 x 15 g RS/day and of 2 x 15 g RS + 2 x 6 g WB was compared. These volunteers collected urine and feces for 72 h. (15)N-content of urine and feces was measured using combustion-isotope ratio mass spectrometry. RS exerted a significant decrease in cumulative urinary (15)N-excretion which was different from placebo. The effect was most pronounced in the 6-24 h urine fraction which suggest fermentation in the proximal colon. The effect of RS+WB on cumulative urinary (15)N-excretion was not significantly different from the effect of RS. A less pronounced decrease in the 6-24 h fraction was observed suggesting less fermentation in the proximal colon whereas no indications for more distal fermentation were observed. Since about 80% of the cumulative urinary (15)N was recovered within 24 h, it was concluded that the biomarker (15)N-LU was useful to monitor processes in the proximal colon rather than in the distal colon.