The study, conducted by researchers from the National Cancer Institute and the University of Hawaii, examined 597 Asian-American women who had breast cancer and 966 women without the disease.
The researchers found that the women who consumed the most soy-based foods -- such as miso and tofu -- between the ages of 5 and 11 experienced a 58 percent reduction in 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 the study's lead researcher, Dr. Larissa Korde of the National Cancer Institute.
The researchers also found that women who regularly ate soy products as adolescents and adults reduced their breast cancer risk by 25 percent.
Though the researchers are uncertain what mechanism in soy is responsible for breast cancer protection, they suspect its high content of isoflavones may contribute. Soy isoflavones are natural plant chemicals that exert estrogen-like effects on the body.
Korde suspects that the timing of soy consumption -- as well as its healthy isoflavones -- is most crucial in breast cancer protection.
Natural health proponent Mike Adams, author of "The Seven Laws of Nutrition," says women and children who consume soy should try to limit their consumption to fermented soy products only, such as miso or properly processed tofu.