Thiamin balance in the gastrointestinal tract of sheep.

Six 5- to 6-month-old sheep fitted with rumen fistulas and reentrant cannulas in the duodenum and in the ileum were adapted to two dies low in thiamin and containing different percentages of urea nitrogen. The sheep were subjected to seven thiamin balance experiments, with continuous feeding and total collection of duodenal and ideal contents and feces for periods of 6 day each. Thiamin and dry matter were determined in aliquots of collected digesta and feces. Daily thiamin intake was always less than .3 mg, but average daily flow of thiamin into the duodenum was between 1.53 and 3.46 milligrams. Microbial net synthesis of thiamin in the forestomach system was between 1.44 and 3.23 mg/day, so 90 to 96% of thiamin entering the duodenum was of microbial origin. Disappearance of thiamin from the small intestines approximately equaled thiamin net synthesis in the forestomachs, indicating high absorption of microbially produced thiamin. In five experiments, thiamin balance in the large intestines was positive, but no measurements were made of thiamin breakdown within, and absorption from, the large intestines.