Women who eat soy products are much less likely to die from or develop a recurrence of breast cancer.
A paper in JAMA by Xiao Shu and colleagues followed up over 5000 Chinese breast cancer survivors aged 20 to 75 for up to 7 years. The study also involved collecting lifestyle data and dietary information including soy-based food intake.
After a median follow-up of 3.9 years there had been 444 deaths and 534 disease recurrences. But when the team organised the data according to soy intake the found a striking difference: the women who ate the largest amount of soy-based food per day were nearly 30% less likely to have died and 32% less likely to have developed cancer recurrence compared with women who ate the least. The effect showed a linear "dose-response" relationship, with the protective effect peaking at an intake of about 11g per day. The reason for the protective effect is not entirely clear but scientists suspect that it may be related to the presence of oestrogen-like isoflavone molecules that are present in soy and known as phyto-oestrogens.
Paradoxically, some scientists had expressed concerns that these molecules might actually aggravate breast cancer therapy, particularly for hormone-sensitive tumours which can be oestrogen-driven, but this study shows that's not true. Instead the scientists found that tamoxifen (which blocks the growth-promoting effects of oestrogens on cancer cells) and soy food could improve survival.
"We found that soy food is safe and was associated with lower mortality and recurrence among breast cancer patients," say the researchers. "This study suggests that moderate soy food intake is safe and potentially beneficial for women with breast cancer."
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