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-[13C]ureide and lactose-[15N,15N]ureide. The appearance on breath of 13CO2 derived from lactose-[13C]ureide was followed for 48 h. The fate of 15N derived from lactose-[15N,15N]ureide was determined by measuring the recovery of 15N in stools and urine in various forms. About 80% of the label given as lactose-[13C]ureide was recovered on the breath, and about 80% of label given as lactose-[15N,15N]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-[15N,15N]ureide and available for further metabolic interaction, 67% was retained and 33% was excreted in urine. The time taken for [15N,15N]urea to appear in urine was similar for all subjects, but the appearance of either 13CO2 on the breath or [15N,14N]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.

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