The study, limited to Asian Americans, found that high soy intakes during adolescence and as adults were associated with a 20 to 25 per cent reduction.
Scientists from the US National Cancer Institute report their findings in the journal Cancer Epidemiology, Biomarkers and Prevention.
"Historically, breast cancer incidence rates have been four to seven times higher among white women in the US than in women in China or Japan,” explained lead researcher Regina Ziegler from the NCI Division of Cancer Epidemiology and Genetics.
“However, when Asian women migrate to the US, their breast cancer risk rises over several generations and reaches that of US white women, suggesting that modifiable factors, rather than genetics, are responsible for the international differences.
“These lifestyle or environmental factors remain elusive; our study was designed to identify them.”
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.
The researchers focused on women of Chinese, Japanese and Filipino descent and living in San Francisco, Oakland, Los Angeles or Hawaii. Ziegler and her co-workers interviewed 597 women with breast cancer and 966 healthy women.
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 added.
"Since the effects of childhood soy intake could not be explained by measures other than Asian lifestyle during childhood or adult life, early soy intake might itself be protective," said lead author Larissa Korde.
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 oestrogen.
"Soy isoflavones have estrogenic properties that may cause changes in breast tissue. Animal models suggest that ingestion of soy may result in earlier maturation of breast tissue and increased resistance to carcinogens," said Korde.
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.
In a final note of caution, Ziegler added: "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. The findings need to be replicated through additional research."
Source: Cancer Epidemiology, Biomarkers and Prevention Published online ahead of print, doi: 10.1158/1055-9965.EPI-08-0405“Childhood Soy Intake and Breast Cancer Risk in Asian American Women” Authors: L.A. Korde, A.H. Wu, T. Fears, A.M.Y. Nomura, D.W. West, L.N. Kolonel, M.C. Pike, R.N. Hoover, R.G. Ziegler