Probiotics Do Not Increase Phytoestrogen Bioavailability

MINNEAPOLIS--Probiotics do not protect soy phytoestrogens from metabolization by intestinal bacteria or increase plasma levels of isoflavones in postmenopausal women with or without a history of breast cancer, according to a study published in the August issue of The Journal of Nutrition (134, 8:1998-2003, 2004) (www.nutrition.org).

Twenty breast cancer survivors and 20 controls completed four six-week treatments of supplementation with soy protein (S), soy protein and probiotics (S+P), milk protein (M) and milk protein and probiotics (M+P). The probiotic supplement used in the study consisted of Lactobacillus acidophilus DDS+1, Bifidobacterium longum and 15 to 30 mg fructooligosaccharide/d.

Plasma phytoestrogen levels did not differ between controls and survivors or between the S and S+P diets. In addition, plasma concentrations and urinary excretion of equol (a substance produced in the intestine as a metabolite of soybeans and soy foods) in two subjects differed by more than seven-fold between the two diets.

The researchers concluded the probiotic supplement used in the study does not generally affect plasma isoflavone levels, although the significant differences between plasma and urinary equol in some subjects suggests equol producer status may be modifiable in some individuals.