Risk factors for breast cancer in postmenopausal Caucasian and Chinese women.

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Background. Striking differences exist between countries in the incidence of breast cancer. The causes of these differences are unknown, but because incidence rates change in migrants, they are thought to be due to environmental rather than genetic differences. The goal of this research is to identify factors responsible for international differences in breast cancer risk. Methods . We have recruited Caucasian women, Chinese migrants to Canada who had lived in the West for less than 10 years, and women of Chinese ancestry who were born in the West, or who migrated to the West before age 21. All subjects were postmenopausal and aged 50 years or more. We obtained information on epidemiological risk factors by interview, and measured anthropometric characteristics. Results. Average length of residence in the West was 61 years for Caucasians (N=413), 62 years for women of Chinese ancestry born in the West (N=63), 40 years for Chinese who migrated early in life (N=153), and 7 years for recent Chinese migrants (N=421). Compared to Caucasians, recent Chinese migrants, weighed on average 14 kg less, and were 6 cms shorter. Recent migrants had menarche 1 year later than Caucasians, had menopause 0.6 year earlier, and were more often parous. Use of hormone replacement therapy was reported by 28% of recent Chinese migrants and 49% of Caucasians. 8% of recent migrants had at least one first degree relative with breast cancer, compared to 17% of Caucasians. 3% of recent migrants and 63% of Caucasians reported to have alcohol consumption at least once per week for 6 months or longer. 2% of recent migrants had smoked at least 1 cigarette per day for 3 months or longer, compared to 48% of Caucasians. Women of Chinese ancestry born in the West and early Chinese migrants had values for most of these variables that were intermediate between those of Caucasians and recent Chinese migrants. Conclusions. The differences observed between the 4 groups in this study strongly suggest that differences in risk of breast cancer between them are, at least in part, the result of differences in exposure to several known risk factors for breast cancer, and that levels of these risk factors are influenced by environmental factors.