Soy during childhood linked to lower breast cancer risk

"Childhood soy intake was significantly associated with reduced breast cancer risk in our study, suggesting that the timing of soy intake may be especially critical,"​ said lead investigator, Larissa Korde, from the NCI's Clinical Genetics Branch.

Population studies have shown that a diet rich in soy is associated with fewer cases of breast cancer, linked to the presence of soy isoflavones. China has the world's lowest incidence and mortality from breast cancer - a disease that has over one million new cases every year worldwide.

The new research of Asian-American women adds to an ever-growing body of research supporting potential cancer-protecting properties of soy. Korde told attendees at the American Association for Cancer Research's Frontiers in Cancer Prevention Research​ meeting this week that soy consumption during childhood, adolescence and adult life was associated with a decreased risk of breast cancer, but that the strongest and most consistent effect was seen for childhood intake.

Korde and co-workers, including researchers from the University of Hawaii, the Northern California Cancer Center, and the University of Southern California, recruited 597 Asian-American women with breast cancer (cases) and 966 healthy women without the disease (controls). The participants were asked to answer questions about adolescent and adult diet and lifestyles.

Additionally, for a subset of 255 participants whose mothers were alive and living in the US, the mothers were asked about their daughter's early childhood exposures.

By comparing the highest and lowest soy intake values for soy-based foods such as tofu, miso and natto, Korde and co-workers calculated that women with the highest soy intake during childhood (ages 5 to 11) had a 58 per cent lower risk of breast cancer as adults as the women with the lowest soy intake as children.

The corresponding reductions for adolescent and adult intake were about 25 per cent, they said.

The underlying mechanism is not known, said the researchers, but they hypothesised that the oestrogenic effects of soy isoflavones cause changes in breast tissue during childhood that may decrease sensitivity to carcinogens later in life. A similar protective effect has been found in studies of overweight girls, perhaps because fat tissue also secretes oestrogens, said Korde.

"Hormonal exposures in adulthood, such as use of oestrogen and progesterone replacement therapy, are established breast cancer risk factors. However, a growing body of evidence suggests that hormonally related exposures early in life may also modify susceptibility to breast cancer,"​ said Korde.

Regina Ziegler, senior investigator on the study, added a note of caution, however, and said that it would be premature to recommend changes in childhood diet.

"This is the first study to evaluate childhood soy intake and subsequent breast cancer risk, and this one result is not enough for a public health recommendation,"​ she said. "The findings need to be replicated through additional research."​

Indeed, the study does have several limitations, including asking women to evaluate and quantify adolescent dietary intakes, as well as asking mothers about a daughter's dietary intake during childhood, the accuracy of both are dependent on the recall ability of the interviewees.

The research does fit with a growing body of research that supports the potential cancer-protecting properties of soy.

A recent animal study published in the journal Cancer Research​ (Vol. 66, Issue 2) reported that high dietary intake of soy protected against breast cancer in postmenopausal monkeys.

This supports another study from the University of Ulster that focussed on the inverse link between soy and breast cancer. In this study, funded by the EU's "Quality of Life and Management of Living Resources"​ project, soy isoflavones were reported to inhibit breast cancer cell invasion in vitro​.

NutraIngredients.com has not seen the full data presented by the National Cancer Institute scientists.