March 24 (HealthDay News) -- Asian-American women who consumed high amounts of soy during childhood appear to have reduced their risk for breast cancer, a U.S. National Cancer Institute study has found.
The study included women of Chinese, Japanese and Filipino descent living in California or Hawaii.
"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," Regina Ziegler, a senior investigator in the cancer epidemiology and genetics division at the cancer institute, said in an agency news release.
"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 interviewed 597 Asian-American women with breast cancer and 966 healthy women. When possible, the women's mothers also were asked about their daughters' soy consumption in childhood.
The study found that high soy intake during childhood was associated with a 58 percent reduced risk of breast cancer, whereas high soy consumption during adolescence and adulthood was associated with a 20 to 25 percent reduced risk.
The study appears in the journal Cancer Epidemiology, Biomarkers & Prevention.
"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 lead investigator, Dr. Larissa Korde, a staff clinician at the institute's Clinical Genetics Branch, said in the news release.
Early soy intake may have a biological role in breast cancer prevention, the researchers suggested, 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."
Despite the study's findings, it's too early to recommend 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," she noted. "The findings need to be replicated through additional research."
SOURCE: American Association for Cancer Research, news release, March 24, 2009