A study by U.S. National Cancer Institute has found that Asian-American women who consumed high amounts of soy during childhood appear to have reduced their risk for breast cancer.

Regina Ziegler, a senior investigator in the cancer epidemiology and genetics division at the cancer institute, said in an agency news release, "Historically, breast cancer incidence rates have been four to seven times higher among white women in the U.S. than in women in China or Japan."

In the study the researchers interviewed 597 Asian-American women with breast cancer and 966 healthy women including women of Chinese, Japanese and Filipino descent living in California or Hawaii.

The women’s mothers were asked about their daughter’s soy consumption in childhood and found that a high soy intake during childhood was associated with a 58% reduced risk of breast cancer. High soy consumption during adolescence and adulthood was associated with a 20 to 25 % reduced risk.

Lead investigator, Dr. Larissa Korde, a staff clinician at the institute's Clinical Genetics Branch, said in the news release, "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."

The researchers added, "However, when Asian women migrate to the U.S., their breast cancer risk rises over several generations and reaches that of U.S. white women, suggesting that modifiable factors, rather than genetics, are responsible for the international differences.

Precisely which environmental or lifestyle factors are involved, though, "remain elusive," she said. "Our study was designed to identify them."

The researchers suggested early soy intake may have a biological role in breast cancer prevention, though the exact underlying mechanism isn't known. "Soy isoflavones have estrogenic properties that may cause changes in breast tissue," Korde said. "Animal models suggest that ingestion of soy may result in earlier maturation of breast tissue and increased resistance to carcinogens."

It was too early to recommend dietary changes in girls' diets, Ziegler said. "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," said Ziegler. "The findings need to be replicated through additional research."

The study is reported in the journal Cancer Epidemiology, Biomarkers and Prevention.