The purpose of this study is to evaluate how beverages containing Lactobacillus casei Shirota (BLS) and soy
isoflavone consumption since adolescence affected the incidence of breast cancer. In a population-based case-control
study, three hundred and six cases with breast cancer and 662 controls aged 40 to 55 were matched for age and residential
area and included in the analyses. Diet, lifestyle and other breast cancer risk factors were investigated using the selfadministered
questionnaire and interview. Odds ratios (ORs) of BLS and soy isoflavone consumption for breast cancer incidence
were independently and jointly estimated using a conditional logistic regression. The ORs of BLS consumption (≥
four times a week against < four times a week) was 0.65 and statistically significant (p = 0.048). The analysis of association
between soy consumption and breast cancer incidence showed the more the isoflavone consumption is, the lower the
odds of breast cancer becomes. Adjusted ORs for breast cancer in the second, the third and the fourth quartiles of soy consumption
against the first quartile were 0.76, 0.53 and 0.48, respectively (trend test, p = 0.0002). The BLS-isoflavone interaction
was not statistically significant; however, a biological interaction was suggested. Regular consumption of BLS
and isoflavones since adolescence was inversely associated with the incidence of breast cancer in Japanese women.
Keywords: Breast cancer, Lactobacillus casei Shirota, probiotic beverage, soy isoflavones.
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