Phytoestrogens and Breast Cancer: Should French Recommendations Evolve?

SIMPLE SUMMARY: Health agencies in several countries have different recommendations regarding the consumption of soy isoflavones (phytoestrogens) in breast cancer (BC) survivors. This review is an analysis of prospective cohort studies published between 2009 and 2020 to determine whether there is a rationale for advising avoidance of soy in dietary supplements and/or foods in women with a history of BC. The endpoint was breast cancer recurrence and/or mortality, and the association with soy isoflavones intake was specifically targeted. No negative effect of soy isoflavones on BC mortality/recurrence was found. These natural products could have beneficial effects. These results coincide with other recent work and suggest that taking soy isoflavones is safe for breast cancer survivors. These data no longer seem to coincide with the French recommendations, which may need to be changed. ABSTRACT: Breast cancer (BC) occurs less frequently in Asia, where there is high soy consumption. It has been hypothesized that soy isoflavones could be protective against BC recurrence and mortality. At the same time, health organizations in several countries have differing recommendations for soy consumption (soy foods or dietary supplements) in BC survivors. The objective of this review is to analyze the literature and to determine whether it is justified to advise avoiding soy in dietary supplements and/or food in women with a history of BC. We conducted a systematic literature search with the Medline/Pubmed and Web of Science databases. Only prospective cohort studies published since 2009 were retained. The endpoint of studies was BC recurrence and/or mortality, and the association with soy isoflavone intake was specifically targeted. Seven studies were included. None of these studies found statistically significant adverse effects of soy consumption on BC recurrence or mortality (specific or all-cause). Overall, only one study was not able to find beneficial effects of soy intake on BC patients. The other studies concluded that there were positive associations but in very variable ways. Two studies found a decrease in BC recurrence associated with a higher isoflavone intake only for post-menopausal women. The other four studies concluded that there were positive associations regardless of menopausal status. Four studies showed better results on women with hormonal-sensitive cancer and/or patients receiving hormonal treatment. Only one found a stronger association for patients with ER-negative BC. No adverse effects of soy isoflavones on BC mortality/recurrence were found. Soy isoflavones may exert beneficial effects. These results coincide with other recent works and suggest that soy isoflavone intake is safe for BC survivors. Thus, these data no longer seem to coincide with the French recommendations, which could then be brought to evolve. However, in order to confirm the current results, larger studies are needed.