The capacity to convert the soy isoflavone daidzein to equol in vivo is presumably determined by an individual's intestinal microfloral populations; however, diet may also influence this conversion. The objectives of the present study were to determine whether a 1-mo supplementation of dietary fiber as wheat bran increases urinary equol excretion in equol excreters and stimulates equol production in nonexcreters and whether longer-term soy isoflavone intake increases equol production or alters overall urinary isoflavone excretion. First, we screened 74 women, ages 20-40 y, and determined their equol-excreter status. In these women, health and lifestyle patterns and habitual dietary intake did not differ according to equol-excreter status. Next, 26 of the women (13 equol excreters and 13 nonexcreters) were assigned (blocked on equol-excreter status) to either longer-term (1 mo) or short-term (4 d) soy protein supplementation. Within each soy treatment group, women participated in two 1-mo intervention periods (the exact length was determined by each woman's menstrual cycle) during which they consumed their usual diets supplemented daily with either 0 or 16 g dietary fiber in a randomized crossover design. A 1-mo washout period separated the two diet periods. Among the 19 women who completed both periods, fiber supplementation did not increase equol production in equol excreters or nonexcreters. In addition, isoflavonoid excretion did not differ by fiber dose or length of soy intervention. These results suggest that a daily 16 g-fiber dose as wheat bran and the addition of soy protein do not alter significantly the capacity of colonic microflora to produce equol.