Digestible fiber (gum arabic), nitrogen excretion and urea recycling in rats

The effect of dietary fiber and level of amino acid (AA) intake on 1) partitioning of N excretion between urine and feces, 2) level of dietary AA on urea N kinetics, and 3) reutilization of urea-originated N into essential AAs was determined. In the first experiment, rats were adapted to one of two isocaloric (928-mJ · kg^-1 · day^-1) and isonitrogenous (1.33-g N · kg^{- 1} · day^-1) liquid diets containing 0 or 9% of nonprotein energy as gum arabic (GA) for 7 days. The rats were further subdivided, with half from each group continuing their diet and the AA intake of the remainder reduced by half (0.67 g N · kg^-1 · day^-1). Daily N balance was determined over the next 5 days. For experiment 2, a catheter was inserted into the rats used for experiment 1, and 5-6 days later, urea production and urea N recycling were determined with [¹⁵N₂] urea. In experiment 3, as in experiment 1, rats were adapted to the reduced AA diet with and without GA and then given [¹⁵N₂]urea. Plasma lysine was then analyzed for ¹⁵N to determine whether AA synthesis by intestinal bacteria was a contributing source of AAs to the body and whether this process was increased with dietary fiber. Dietary fiber decreased urinary N excretion and increased fecal N excretion but had no net effect on N balance. On the adequate-AA diet, GA decreased urea production (523 ± 36 vs. 374 ± 47 mg N · rat^-1 · day^-1, p < 0.01), and urea N recycling (47 ± 4 vs. 30 ± 6 mg N · rat^-1 · day^-1, p < 0.01) was found. In contrast, with the reduced-AA diet, GA increased urea production (250 ± 27 vs. 335 ± 36 mg N · rat^-1 · day^-1, p < 0.11) and urea N recycling 21 ± 2 vs. 33 ± 4 mg N · rat^-1 · day^-1, p < 0.05). Use of [¹⁵N₂] urea underestimates urea recycling. Fiber increased the incorporation of ¹⁵N into lysine.