A woman's risk for breast cancer declines as she grows older, Mayo Clinic researchers conclude.

That's because as women age, their milk-producing glands (lobules) start to shut down. This natural process is called lobular regression, or involution. Since it's believed that breast cancer originates in the lobules, a reduction in their size and number reduces cancer risk.

The Mayo study of over 8,700 women found that those with benign breast disease and little or no involution were twice as likely to develop breast cancer compared to women whose breast glandular tissues had been replaced with connective and fatty tissue.

The researchers also found that 53 percent of women age 70 and older had complete involution.

Women who had never used hormone replacement therapy (HRT) were more likely to have complete involution than those who did have HRT (22.9 percent vs. 20.3 percent), the researchers add. Twenty-seven percent of women who had had no children had complete involution, compared to 18 percent of women who had more than three children.

Breast-feeding had no impact on involution, the study found.

The findings were published in the Nov. 15 issue of the Journal of the National Cancer Institute.

"Our study is the first to establish that that extent of breast involution relates directly to risk for breast cancer," oncologist and lead investigator Dr. Lynn Hartmann said in a prepared statement.

The study provides doctors with another important factor to help them predict a woman's risk for breast cancer, she said.