Soy Isoflavones and Breast Tissue

GHENT, Belgium After intake of soy milk and soy supplements, isoflavones reach exposure levels in breast tissue at which potential health effects may occur, according to a study in The American Journal of Clinical Nutrition (2010;91(4):976-84). In this dietary intervention study, healthy women were randomly allocated to a soy milk (n=11; 16.98-mg genistein and 5.40-mg daidzein aglycone equivalents per dose), a soy supplement (n=10; 5.27-mg genistein and 17.56-mg daidzein aglycone equivalents per dose) or control (n=10) group. After a run-in period of four days, three doses of soy milk or soy supplements were taken daily for five days before an esthetic breast reduction.

After soy administration, genistein and total daidzein concentrations, which were expressed as aglycone equivalents, ranged from 135.1 to 2831 nmol/L and 105.1 to 1397 nmol/L, respectively, in hydrolyzed serum and from 92.33 to 493.8 pmol/g and 22.15 to 770.8 pmol/g, respectively, in hydrolyzed breast tissue. The major metabolites identified in non-hydrolyzed samples were genistein-7-O-glucuronide and daidzein-7-O-glucuronide, with an overall glucuronidation of 98 percent. Total isoflavones showed a breast adipose/glandular tissue distribution of 40:60, and their mean derived 17beta-estradiol equivalents toward estrogen receptor beta were 21 ± 4-fold and 40 ± 10-fold higher than the 17beta-estradiol concentrations in adipose and glandular fractions, respectively.