#Grand Rounds

Does a Higher BMI Decrease Risk of Breast Cancer in Premenopausal Women?

BACKGROUND AND PURPOSE:

Increased BMI in postmenopausal women is associated with increased risk for breast cancer
Limitations in previous studies that assessed risk of adiposity in premenopause (case reports/small sample size)
The Premenopausal Breast Cancer Collaborative Group (JAMA Oncology, 2018) examined how BMI at different ages is associated with premenopausal breast cancer risk

Multicenter Prospective Cohort Study
- Pooled individual-level data from 19 prospective cohorts
BMI was recorded from ages 18-54 years
Median follow-up was 9.3 years per participant and incidence of breast cancer was recorded
Primary outcome: Invasive or in situ premenopausal breast cancer

758,592 premenopausal women were included
- 13,082 incident cases of breast cancer
BMI at ages 18 to 24 years compared to ages 45 to 54 years demonstrated a stronger inverse linear association with breast cancer risk
- Age 18 to 24: Hazard risk (HR) per 5 kg/m2 (5.0-U) difference 0.77 (95% CI, 0.73-0.80)
- Age 45 to 54: HR per 5.0-U difference, 0.88 (95% CI, 0.86-0.91)
Risk gradient between the highest and lowest BMI categories (BMI≥35.0 vs <17.0)
- There was a 4.2-fold risk between ages 18 to 24 years
- HR, 0.24 (95% CI, 0.14-0.40)
The inverse associations between BMI and breast cancer risk were consistent
- Among nonoverweight BMI levels
- Taking in to account other breast cancer risk factors
BMI at every age group was most strongly associated with ER and/or PR positive breast cancer

Greater BMI is associated with a reduced risk of premenopausal breast cancer
Associations were strongest for early adult BMI
The authors have “estimated 12% to 23% reduction in premenopausal breast cancer risk per 5 kg/m2 difference in BMI”
The authors do not recommend adiposity as a preventative measure. It rather reflects underlying hormonal mechanisms that need to be further elucidated

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