Adiposity throughout Adulthood and Risk of Young-Onset Breast Cancer Tumor Subtypes in the Young Women's Health History Study.

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 = 1,381) study of invasive YOBC (ages <50 years), cases were identified from the Los Angeles County and Metropolitan Detroit Surveillance, Epidemiology, and End Results registries, 2010 to 2015. Area-based, frequency-matched controls were sampled from the 2010 Census. General adiposity [body mass index (BMI)] and central adiposity (waist circumference and waist-to-height ratio) across adulthood and covariates were collected from in-person interviews and measurements. ORs and 95% confidence intervals (CI) for adiposity and YOBC tumor subtypes [i.e., luminal A, luminal B, HER2+, and 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 nonsignificant. Similarly, adult overweight and obese BMIs were inversely associated with luminal A (OR = 0.66, 95% CI, 0.48-0.91 and OR = 0.59, 95% CI, 0.46-0.87, respectively), but not other subtypes. Conversely, larger waist circumference was associated with higher odds of luminal B and TN YOBC (OR = 1.48, 95% CI, 1.01-2.15 and OR = 2.48, 95% CI, 1.52-3.88, respectively), but not other subtypes (with similar results for weight-to-height ratio); highest odds were among parous women.

Conclusions: Findings show greater general adult adiposity is associated with reduced odds of luminal A YOBC, whereas 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.