Metabolism of lactose-[13C]ureide and lactose-[15N,15N]ureide in normal adults consuming a diet marginally adequate in protein

Oral lactose-ureide is resistant to human digestive enzymes, but is fermented by the colonic microflora. Nine normal adults consuming a diet which provided 36 g of protein/day were given oral doses of lactose-[C-13]ureide and lactose-[N-15,N-15]ureide. The appearance on breath of (CO2)-C-13 derived from lactose-[13C]ureide was followed for 48 h. The fate of N-15 derived from lactose[N-15,N-15]ureide was determined by measuring the recovery of N-15 in stools and urine in various forms. About 80% of the label given as lactose-[C-13]ureide was recovered on the breath, and about 80% Of label given as lactose-[N-15,N-15]ureide was not recovered in stool, indicating that 80% of the dose was completely fermented. At least 5% of the labelled urea was absorbed and excreted as the intact molecule. Of the 15N derived from lactose-[N-15,N-15]ureide and available for further metabolic interaction, 67% was retained and 33% was excreted in urine. The time taken for [N-15,N-15]urea to appear in urine was similar for all subjects, but the appearance of either (CO2)-C-13 On the breath or [N-15,N-14]urea in urine varied. It is concluded that the hydrolysis of the sugar-urea bond may reflect oro-caecal transit time, but that other factors related to colonic bacterial metabolism determine the duration and extent of hydrolysis of urea by urease enzymes. Lactose-ureide can be used to probe the metabolic activity of the colonic microflora in normal individuals.