Asian-American women who ate a lot of soy as children had a 58 percent reduced risk of developing breast cancer, U.S. researchers said on Monday in a finding that suggests soy may have a protective effect.

"Childhood soy intake was significantly associated with reduced breast cancer risk in our study," said Dr. Larissa Korde of the National Cancer Institute, whose study appears in the journal Cancer Epidemiology, Biomarkers and Prevention.

Historically, breast cancer rates among white women in the United States are four to seven times higher than in women in China or Japan, Regina Ziegler of the National Cancer Institute said in a statement.

But when Asian women emigrate to the United States, their risk for breast cancer rises over several generations, suggesting something other than genetics was at play. Korde and colleagues checked to see if diet or other lifestyle factors could explain the differences.

They interviewed nearly 1,600 women of Chinese, Japanese and Filipino descent who were living in San Francisco, Oakland, or Los Angeles, California, or Hawaii. Some 600 had breast cancer and the rest were healthy.

If the women had mothers living in the United States, they asked the mothers about their daughter's soy consumption in childhood.

Women who consumed the highest amounts of soy in childhood had 58 percent less risk of breast cancer compared with those in the lowest groups.

The effect was weaker when adolescents or adults ate or drank a lot of soy, but the study still found a 20 to 25 percent reduction in risk.

The relationship between childhood soy consumption and reduced cancer risk held for all women in the study, regardless of family history of breast cancer.

The findings about childhood soy consumption suggest "the timing of soy intake may be especially critical," Korde said.

She said exactly why soy may protect is not known, but early soy consumption may interfere with the biology of breast cancer. Soy contains isoflavones with properties similar to the female sex hormone estrogen, which may alter breast tissue, she said in a statement.

Tests in animals show soy may help breast tissue mature earlier and better resist cancer-causing agents, she said.

But Ziegler said it is too early for parents to start adding soy to their daughters' diets.

"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, adding that the findings need to be replicated in other studies.

Breast cancer is diagnosed in 1.2 million men and women globally every year and kills 500,000.