Nitrogen in fecal bacterial, fiber, and soluble fractions of patients with cirrhosis: effects of lactulose and lactulose plus neomycin.

To determine how lactulose and lactulose plus neomycin might alter nitrogen metabolism in the colon we investigated the effect of these agents on the distribution of nitrogen in the bacterial, soluble, and fiber fractions of stool. The alterations in fecal nitrogen excretion were additionally correlated with changes in total body urea synthesis and degradation rates. Six patients with stable cirrhosis received a control diet alone followed by the administration of lactulose (56 +/- 6 gm/day), and eight similar patients received lactulose alone (63 +/- 5 gm/day) followed by the addition of neomycin (4 gm/day). Their feces were partitioned into individual fractions by physical separation. Lactulose administration increased nitrogen excreted in the bacterial fraction by 165% (from 0.52 +/- 0.14 gm/day to 1.38 +/- 0.21 gm/day) and by 135% in the soluble fraction (from 0.58 +/- 0.08 gm/day to 1.36 +/- 0.23 gm/day). When lactulose was supplemented with neomycin, the nitrogen content of the bacterial fraction decreased by 28%. Lactulose caused a 23% reduction in the urea production rate that was mainly accounted for by increase in fecal nitrogen excretion. The addition of neomycin caused a further reduction in urea production that was explained by an inhibition of urea degradation. These results demonstrate that a major effect of lactulose was to augment the incorporation of nitrogen into fecal bacteria although nitrogen in the soluble fraction also increased. The additional nitrogen excreted in the fecal bacterial and soluble fractions caused a reduction in urea synthesis.