(Ivanhoe Newswire) -- The way breast tissue ages naturally can reduce the risk of breast cancer.

As most women age, the glands that produce milk (lobules) shut down. This process is known as involution. It is believed that breast cancer starts in the lobules, so if there are fewer of them and they are smaller, it decreases the risk. A new study reveals women who have complete shut down of their milk glands have half the risk of developing breast cancer.

"The risk of breast cancer increases among women who have no breast involution as they age," reports lead author Lynn Hartmann, M.D., an oncologist at Mayo Clinic in Rochester, Minn. "This study identifies specific groups of women who are at lower or higher risks of breast cancer due to their extent of involution and factors such as age, family cancer history, reproductive history, and types of benign breast disease."

Researchers found women with benign breast disease and little or no breast tissue involution were twice as likely to develop breast cancer as women whose breast glandular tissues had been completely replaced with connective and fatty tissue.

The findings may have implications for women taking hormone replacement therapy (HRT) -- which appears to slow down milk gland shut down. More women (22.9 percent) who never took hormone replacement therapy had complete involution of mammary tissue than women (20.3 percent) who were on estrogen, progesterone, or combinations of the two hormones.

Investigators also report women who gave birth to more than three children kept more breast lobules. More women (27 percent) who had no children had complete involution, but only 18 percent of women with four children did. Breast-feeding had no impact on involution.

Researchers conclude including involution along with other risk factors will better help oncologists predict a woman's risk of breast cancer.

SOURCE: Journal of the National Cancer Institute, 2006;98:1600-1607