Age-specific incidence rates of breast cancer among Japanese women increasing in a conspicuous bimodal distribution pattern

Abstract

Breast cancer incidence rates are increasing in East and Southeast Asia, along with the westernisation of reproductive and lifestyle patterns. Such westernisation is thought to be involved in the cumulative exposure of breast tissue to both endogenous and exogenous oestrogen. Immigrant studies among Asian American women indicate that risk factors for breast cancer can be modified. When breast cancer incidence rates were compared with those of corpus uteri and colon among Japanese women in 2005, 2010 and 2015, it is of note that age-specific incidence rates of breast cancer in 5-year age groups clearly increased during the 10-year period in a bimodal distribution pattern, with two peaks in the 45–49 and 60–64 years age groups. From the relevant literature, it is inferred that the low prevalence of obesity and the intake of soy products or isoflavones among Japanese women may contribute to the bimodal distribution pattern by suppressing the extent of increase in breast cancer incidence rates among Japanese postmenopausal women. With regard to dietary habits relating to obesity, it has been globally reported that the intake of high-calorie foods is associated with the incidence of oestrogen receptor-positive breast cancer in both pre- and postmenopausal women, while high-carbohydrate or -milk intake that can enhance the secretion of insulin or insulin-like growth factor 1 is associated with that of oestrogen receptor-negative breast cancer mostly in postmenopausal women. Studies are needed to clarify the aetiology or modifiable factors behind the bimodal incidence rates of breast cancer among Japanese women.