The Human Mammary Gland as a Target for Isoflavones: How Does the Relation Vary in Individuals with Different Ethnicity?

Abstract

Based on observational studies, it appears that soy food consumption provides protection against breast cancer primarily in Asian but not in Western populations. Given the problems in examining the effects of isoflavones directly in the human mammary gland, this review describes epidemiologic studies that investigated the association with biomarkers reflecting hormonal activity of isoflavones, in particular sex steroid levels, mammographic densities, nipple aspirate fluid, and tissue specimens from biopsies or surgeries. Three possible mechanisms that may be responsible for ethnic-specific health effects from these compounds are discussed: genetic variation in metabolic enzymes, timing of exposure, and intestinal metabolism by microbiota. Only a limited number of comparative studies and even fewer nutritional interventions have examined effects and addressed differences in biomarkers between Asian and Western populations. Investigations that looked at estrogens and mammographic densities as endpoints observed some associations in Asian women that were not seen in Caucasians. On the other hand, the low rate of nipple aspirate fluid production and a lack of breast tissue studies make it impossible to evaluate effects of isoflavones on these biomarkers in Asian women. Based on the current evidence, it appears likely that the timing of exposure is the most important determinant of beneficial health effects from soy foods. This may be the result of gut microbiota, which colonize the intestine during childhood and facilitates the hydrolysis of glycosides and the formation of equol from dadzein, a pathway that may result in beneficial health effects. The current evidence is insufficient to answer the question whether women of diverse ethnic groups experience distinct effects from soy isoflavones in breast tissue, but as knowledge about the role of early life nutrition and the development of gut microbiota increases, the potential for diverse metabolic pathways of isoflavones in individuals with different ethnic backgrounds and dietary exposures may be clarified.

Key words

soy isoflavones - breast tissue - cancer - ethnicity - estrogens - biomarkers

• References

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