Some epidemiologic studies reported an association between a low ratio of urinary 2-hydroxyestrogens (2-hydroxyestradiol + 2-hydroxyestrone) to 16alpha-hydroxyestrone (2:16OHE(1)) 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 (10(9) colony-forming units Lactobacillus acidophilus DDS(R)+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 2:16OHE(1) ratio than controls (P = 0.10). In the group as a whole, soy consumption tended to increase urinary 2-hydroxyestrogens (P = 0.07) and 16alpha-hydroxyestrone (P = 0.11) but had no effect on the urinary 2:16OHE(1) 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 2:16OHE(1) 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 2:16OHE(1) ratios in women with
breast cancer and suggest that soy consumption increases this ratio only in women who are
equol producers.