Intake of bean fiber, beans, and grains and reduced risk of hormone receptor‐negative breast cancer: the San Francisco Bay Area Breast Cancer Study

High dietary fiber intake has been associated with reduced breast cancer risk, but few studies considered tumor subtypes defined by estrogen receptor (ER) and progesterone receptor (PR) status or included racial/ethnic minority populations who vary in their fiber intake. We analyzed food frequency data from a population‐based case–control study, including 2135 breast cancer cases (1070 Hispanics, 493 African Americans, and 572 non‐Hispanic Whites (NHWs)) and 2571 controls (1391 Hispanics, 557 African Americans, and 623 NHWs). Odds ratios (OR) and 95% confidence intervals (CI) for breast cancer associated with fiber intake were calculated using unconditional logistic regression. Breast cancer risk associated with high intake (high vs. low quartile) of bean fiber (p‐trend = 0.01), total beans (p‐trend = 0.03), or total grains (p‐trend = 0.05) was reduced by 20%. Inverse associations were strongest for ER‐PR‐ breast cancer, with risk reductions associated with high intake ranging from 28 to 36%. For bean fiber, risk was reduced among foreign‐born Hispanics only, who had the highest fiber intake, whereas for grain intake, inverse associations were found among NHWs only. There was no evidence of association with fiber intake from vegetables and fruits or total intake of vegetables and fruits. A high dietary intake of bean fiber and fiber‐rich foods such as beans and grains may lower the risk of ER‐PR‐ breast cancer, an aggressive breast cancer subtype for which few risk factors have been identified.