Unilateral breast-feeding and breast cancer.

Women of fishing villages in Hong Kong by custom breast-feed with only the right breast. The hypothesis that the unsuckled breast may have an altered risk of cancer development was investigated. Records of the radiotherapy divisions in Hong Kong between 1958 and 1975 were searched, and breast-cancer patients were interviewed for a detailed history of lactation. The overall left/right ratio of cancer in the breasts of 2372 women with unilateral breast carcinoma was 0-97, indicating that breast cancer was equally distributed between the two sides. Of 73 patients with a history of exclusively one-sided breast-feeding, 27 of 34 patients aged 55 or over (79-4%) and 19 of 39 patients under age 55 (48-7%) had a carcinoma in the unsuckled breast. Comparisons of patients who had nursed unilaterally with nulliparous patients and with patients who had borne children but had not breast-fed indicated a highly significant increased risk of cancer in the unsuckled breast. No statistically significant differences in laterality of breast cancer were found in 52 patients who had for convenience nursed to a greater extent from one side than the other. This study indicates that in post-menopausal women who have breast-fed unilaterally, the risk of cancer is significantly higher in the unsuckled breast and that breast-feeding may help to protect the suckled breast against cancer.