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Lactose[15N,15N′]ureide is resistant to digestion but is fermented by colonic bacterial enzymes into [15N,15N′]urea and subsequently [15N]ammonia (NH3). The formed [15N]ammonia can either be taken by the microbiota and excreted via the feces or be absorbed through the mucosa and renally excreted after conversion to [15N,14N′]urea in the liver. The proportionate recovery of 15N in feces or urine indicates the degree of colonic protein fermentation.
