2023 Volume 1 Issue 3 Pages 41-42
We previously reported that age-specific breast cancer incidence rates among Japanese women increased from 2005 to 2015 in a bimodal distribution pattern with pre- and postmenopausal peaks1). In that report, we presumed that the low obesity prevalence and high intake of soy products or isoflavones in Japan were involved in the bimodal pattern. Additionally, we demonstrated in an ecological study using national data that breast cancer incidence correlated positively with current drinking habits and past smoking habits in certain age groups, but not with obesity prevalence2). As of 22 October 2023, national data of the age-specific breast cancer incidence from 2003 to 2019 among Japanese women are available on the site of e-Stat (a portal site of Japanese Government Statistics)3).
As shown in Fig. 1 (left panel), the age-specific breast cancer (excluding carcinoma in situ) incidence rates of 5-year intervals continue to increase from 2004 through 2019 in Japan. The age-specific breast cancer incidence rates changed from data of national estimates to data based on the National Cancer Registry in 2016 and thereafter. The right panel of Fig. 1 shows annual changes in plant protein intake from 1999 to 2019 among Japanese women stratified by 10-year age groups3). Plant protein intake was smaller in younger women and decreased more obviously during the period in age-groups less than 60 years. This indicates a possible contribution of the reduced plant protein intake to the recent rising incidence of breast cancer. The inserted panel shows annual changes in the intake of soy products from 2011 to 2019, the only period for which data was available3). Despite the limited timeframe, the annual changes appeared to be similar in both plant protein and soy product intakes, suggesting that the changes from 1999 to 2019 in plant proteins may reflect those in soy products.
Whole foods rich in plant proteins have a positive impact on human health, and soy-based products are well-studied plant protein sources for cancer prevention4). It is interesting to note that inhibitory effects of soy products or isoflavones on breast cancer development have been observed only in Asian postmenopausal women5). In Japan, two reports from the Japan Public Health Center-based Prospective Study (JPHC Study) were somewhat inconsistent regarding the effect of soy products6,7). Thus, further cohort or intervention studies are needed to better understand the causal relationship between plant protein (or soy product) intake and breast cancer incidence in Japan.
