In the past few years, some studies involving large groups of breast cancer survivors have shed more light on this issue.
A study in China looked at the association between soy consumption and risk of breast cancer recurrence in over 5,000 breast cancer survivors. The study found lower risk of recurrence in women who consumed more soy. This was true for women with both estrogen-receptor positive and negative breast cancers. This does not mean that soy lowered the risk of recurrence. The women in the study who ate more soy also tended to eat more fish and vegetables and to exercise more than the other women. These other healthy lifestyle habits may have had a beneficial effect.
A study in the United States showed similar results. The Women’s Healthy Eating and Living (WHEL) study examined the effect of eating soy foods on breast cancer recovery in over 3,000 breast cancer survivors. It found that soy intake was not related to a second breast cancer incident. A second incident included recurrences and new breast cancer diagnosis. This held true regardless of hormone receptor status.
The World Cancer Research Fund and American Institute of Cancer Research report for breast cancer survivors states that there is some (limited) evidence of links between better survival after breast cancer and eating foods containing soy.