The collection, preservation, analysis and calculation of urinary purine derivatives were followed the guidelines of IAEA (1997). In brief, the diluted frozen urine samples were thawed and ultra-sonicated for 20 min to break the urinary precipitates, if any, and further diluted before analysis. The concentrations of allantoin, uric acid, and xanthine and hypoxanthine in the diluted urine samples were determined by the colorimetric methods as described in the IAEA (1997) manual. The daily intestinal flow of microbial nitrogen was calculated from total urinary purine derivatives excretion: The equation used to relate absorption of microbial purines (X, mmol/day) and excretion of purine derivatives in urine (Y; mmol/day) was Y=0.84×X+(0.150×W0.75e−0.25×X). The supply of microbial N (g/day) was calculated as follows: