Breast Cancer Prevention (PDQ®): Prevention - Health Professional Information [NCI] - Description of Evidence
The role of ovarian hormones in the development of breast cancer is demonstrated by studies of artificial menopause. Following ovarian ablation, breast cancer risk may be reduced as much as 75% depending on age, weight, and parity, with the greatest reduction for young, thin, nulliparous women.[10,11,12,13] The removal of one ovary also reduces the risk of breast cancer, but to a lesser degree than the removal of both ovaries.
Other hormonal changes also influence breast cancer risk. Childbirth is followed by a transient increase in risk and then a long-term reduction in risk, which is greater for younger women.[13,15,16] In one study, women who experienced a first full-term pregnancy before age 20 years were half as likely to develop breast cancer as nulliparous women or women who underwent a first full-term pregnancy at age 35 years or older.[17,18] Age at menarche also affects breast cancer risk. Women who experienced menarche at age 11 years or younger have about a 20% greater chance of developing breast cancer than women who experienced menarche at age 14 years or older. Women who experience late menopause also have increased risk. Reproductive risk factors may interact with more predisposing genotypes. In the Nurses' Health Study, the associations between age at first birth, menarche, and menopause and the development of breast cancer were observed only among women without a family history of breast cancer in a mother or sister. Breast-feeding is associated with a decreased risk of breast cancer.[21,22]
A number of studies suggest that endogenous estrogen and androgen levels are higher in women who develop breast cancer than in women who do not.[9,23,24] Methods shown to decrease endogenous estrogen include maintenance of ideal body weight (refer to the Obesity section of this summary for more information), adoption of a low-fat diet in postmenopausal women, and moderate exercise in adolescent girls. Whether such interventions will decrease breast cancer risk is worthy of study.
The inherited genetic profile of an individual influences susceptibility to mutagens and growth factors, which initiate or promote the carcinogenic process. Known genetic syndromes related to specific aberrant alleles account for approximately 5% of breast cancers. Identifying high-risk genes provides insight into breast cancer etiology and allows the development of preventive interventions for affected populations. (Refer to the PDQ summary on Genetics of Breast and Ovarian Cancer for more information.)