ABSTRACT
BACKGROUND: The role of adult adiposity in young-onset breast cancer (YOBC) subtype risk is not well understood. METHODS: In this population-based case(n=1812)-control(n=1381) study of invasive YOBC (aged <50 years), cases were identified from the Los Angeles County and Metropolitan Detroit SEER registries, 2010-2015. Area-based, frequency-matched controls were sampled from the 2010 Census. General adiposity (body mass index (BMI)) and central adiposity (waist circumference (WC), waist-to-height ratio (WHtR)) across adulthood and covariates were collected from in-person interviews and measurements. Odds ratios (ORs) and 95% confidence intervals (CIs) for adiposity and YOBC tumor subtypes (i.e., luminal A, luminal B, HER2+, triple negative (TN)) were calculated, overall and by parity, using multivariable weighted logistic regression. RESULTS: Obese young adult BMI was inversely associated with luminal A YOBC (OR=0.35, 95% CI 0.16-0.79); other subtype associations were non-significant. Similarly, adult overweight and obese BMIs were inversely associated with luminal A (respectively OR=0.66, 95% CI 0.48-0.91 and OR=0.59, 95% CI 0.46-0.87), but not other subtypes. Conversely, larger WC was associated with higher odds of luminal B and TN YOBC (respectively OR=1.48, 95% CI 1.01-2.15 and OR=2.48, 95% CI 1.52-3.88), but not other subtypes (with similar results for WHtR); highest odds were among parous women. CONCLUSIONS: Findings show greater general adult adiposity is associated with reduced odds of luminal A YOBC, while greater central adiposity is associated with increased odds of luminal B and TN YOBC, particularly among parous women. IMPACT: Additional studies of central adiposity and YOBC subtype risk, especially incorporating pregnancy history, are warranted.