Composition of fecal fiber from human subjects.

This research addresses two problems of fecal fiber analysis: the nitrogen in fecal neutral detergent fiber (NDF); and the impact of sodium sulfite on NDF yield and composition. Fecal samples for analysis were collected from healthy adults during a study of the effect of a wheat bran supplement on bowel function. When sodium sulfite, originally a component of neutral detergent solution, was omitted, the yield of fecal NDF, the concentration of apparent lignin in the NDF and the amount of nitrogen recovered in both the NDF residue and the lignin increased. These data support a previous report by Van Soest that the sulfite attacks lignin. We present evidence that the effect of sodium sulfite also could be one of inhibiting the formation of Maillard products, which are detected as lignin, during the reflux step of the NDF method. Three different fractions of nitrogen were identified in fecal NDF: the fraction eliminated by sodium sulfite; a fraction removed by dilute acid; and the fraction that remained in Klason lignin. Apparent NDF digestibilities were calculated several ways by using correction factors to account for this nitrogen. Only the subtraction of Klason lignin from fecal NDF eliminated the one negative digestibility we observed.