Body mass index (BMI) is calculated as weight in kilograms divided by height in meters squared. The luminal A–like subtype includes estrogen receptor (ER)–positive, progesterone receptor (PR)–positive, and ERBB2/HER2-negative tumors; luminal B–subtype, all ER-positive and/or PR-positive tumors that are not luminal A–like (subtypes luminal B–like ERBB2/HER2-negative and luminal B–like ERBB2/HER2-positive); and nonluminal subtype, all ER-negative and PR-negative tumors, regardless of ERBB2/HER2 status (subtypes ERBB2/HER2 enriched: ER-negative, PR-negative, and ERBB2/HER2-positive; triple-negative: ER-negative, PR-negative, and ERBB2/HER2-negative).
aRepresents linear trend per 5.0-U of difference in BMI from 18.5 to 49.9 and are adjusted for attained age, cohort, year of birth, age at menarche, age at first birth, number of births, time since last birth, and family history of breast cancer. Estimates were obtained from 2 augmentation models. The first model included luminal A–like, luminal B–like, and nonluminal breast cancer as end points with tests for heterogeneity in effect by tumor type (for BMI at ages 18-24 years, P = .07; at ages 25-34 years, P = .002; at ages 35-44 years, P < .001; at ages 45-54, P < .001). Estimates for subtypes of luminal B–like and nonluminal breast cancer were obtained from a second model fitting luminal A–like, luminal B–like ERBB2/HER2-positive, luminal B–like ERBB2/HER2-negative, ERBB2/HER2-enriched, and triple-negative breast cancer as end points.