"We did not see any evidence that soy intake after breast cancer increases the risk of recurrence or deaths [from breast cancer]," said Dr Xiao Ou Shu, a professor of epidemiology and medicine at Vanderbilt University Medical Center in Nashville.
"Our study indicated that soy food intake among breast cancer survivors is safe and may reduce the risk of recurrence," she said.
She emphasised she is talking about soy foods, such as tofu and soybeans, not soy supplements.
The research is to be presented at the American Association for Cancer Research's annual meeting in Orlando, Fla. Results of studies presented at meetings are considered preliminary until published in a peer-reviewed journal.
Breast cancer and soy
For the study, Shu evaluated data on 9,515 women who had participated in one of three studies of breast cancer survivors: Life After Cancer Epidemiology, Women's Healthy Eating and Living and the Shanghai Breast Cancer Survival studies.
Shu reviewed information from food questionnaires on the women's soy food intake. The average time between breast cancer diagnosis and soy food evaluation was about 14 months.
After a follow-up that averaged 7.4 years, Shu found 1,348 breast cancer recurrences and 1,171 deaths from breast cancer and other causes.
Compared to the women who ate the least soy, women in the upper 10th percentile group for soy food intake had a 35% reduced risk of recurrence.
Those who ate the most soy also had a 17% reduced risk of death from all causes during the follow-up, but that finding did not reach statistical significance, Shu said.
Eating soy after cancer is ok
Soy food consumption was considerably higher in the Shanghai group than among the US participants.
How much soy might be protective? "Women can get the level of soy isoflavones that is similar to the top 10% consumption level found in our US study population by consuming a cup of [soy] milk or half serving of tofu (2 oz.) per day," she said.
In the past, women who survived breast cancer tended to avoid soy foods, said Dr Marian Neuhouser, associate member of the Fred Hutchinson Cancer Research Center in Seattle.
The worry, she said, was that some soy foods can act as a weak phytoestrogen, and most breast cancers are oestrogen-receptor positive, needing oestrogen to grow.
"I think this study and a previous study support the idea that soy foods are safe for women with breast cancer," she said.
She, too, emphasised that the study is on soy foods, not supplements. Soy foods are a good source of low-fat protein, she said.
Soy contributes to an overall healthy diet pattern, and healthy diet patterns are linked with lower recurrence, she said.
"If someone has a latte with soy milk, it means they aren't having a latte with whole milk, which has a lot more fat in it," Neuhouser said.
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