The effect of soy consumption on the urinary 2:16-hydroxyestrone ratio in postmenopausal women depends on equol production status but is not influenced by probiotic consumption

Some epidemiologic studies reported an association between a low ratio of urinary 2-hydroxyestrogens (2-hydroxyestradiol + 2-hydroxyestrone) to 16α-hydroxyestrone (2:16OHE₁) and increased breast cancer risk. Some studies show that soy consumption increases this ratio, and it is suggested that this effect may reduce breast cancer risk. We hypothesized that consumption of probiotic bacteria would alter fecal bacteria and enzymes involved in soy isoflavone metabolism, thereby increasing isoflavone bioavailability and enhancing the beneficial effects of soy on estrogen metabolism. Breast cancer survivors (n = 20) and controls (n = 20) were given 4 treatments for 6 wk each, separated by 2-wk washout periods, in a randomized, crossover design: soy protein (26.6 ± 4.5 g protein/d containing 44.4 ± 7.5 mg isoflavones/d); soy protein + probiotics (10⁹ colony-forming units Lactobacillus acidophilus DDS®+1 & Bifidobacterium longum, 15-30 mg fructooligosaccharide/d); milk protein (26.6 ± 4.5 g protein/d); and milk protein + probiotics. Survivors tended to have a lower baseline urine 2116OHE₁ ratio than controls (P = 0.10). In the group as a whole, soy consumption tended to increase urinary 2-hydroxyestrogens (P = 0.07) and 16α-hydroxyestrone (P = 0.11) but had no effect on the urinary 2:16OHE₁ ratio. When subjects were divided into groups by plasma concentrations and urinary levels of the daidzein metabolite equol, soy increased urinary 2-hydroxyestrogens (P = 0.01) and the 2116OHE₁ ratio (P = 0.04) only in subjects with high plasma equol concentrations. None of these results were influenced by probiotic consumption. These results are consistent with studies that found lower urine 2:16OHE₁ ratios in women with breast cancer and suggest that soy consumption increases this ratio only in women who are equol producers.