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

Oral lactose-ureide is resistant to human digestive enzymes, but is ferment ed by the colonic microflora. Nine normal adults consuming a diet which pro vided 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 fr om lactose[N-15,N-15]ureide was determined by measuring the recovery of N-1 5 in stools and urine in various forms. About 80% of the label given as lac tose-[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 8 0% of the dose was completely fermented. At least 5% of the labelled urea w as absorbed and excreted as the intact molecule. Of the 15N derived from la ctose-[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 o f 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-caeca l transit time, but that other factors related to colonic bacterial metabol ism determine the duration and extent of hydrolysis of urea by urease enzym es. Lactose-ureide can be used to probe the metabolic activity of the colon ic microflora in normal individuals.