Higher BMI at age 20 linked to lower postmenopausal breast cancer risk; mammographic density partly explains the link

# Study question Why would higher body mass index (BMI) at about age 20 be linked to lower postmenopausal breast cancer risk decades later, and can mammographic density explain that relationship?

# What the researchers used The team analyzed the UK PROCAS screening cohort: women aged 46–73 who attended routine mammography between 2009 and 2015. The main analysis set included 33,821 postmenopausal women with valid recalled weight at age 20. Participants provided lifestyle, reproductive, and family-history information. Mammographic density was measured two ways: a visual analog scale (VAS) percentage density scored by experienced readers, and an automated Volpara-derived fibroglandular volume (FGV).

# Main findings

• Over a median follow-up of 10.44 years, 1,261 breast cancers were recorded.
• Breast cancer incidence declined across BMI-at-20 categories: 4.5% for BMI below 18.5 kg/m² versus 2.6% for BMI 30 kg/m² or higher.

• BMI at cohort entry (midlife) correlated strongly with lower VAS, higher FGV, and greater breast fatty volume, and with higher breast cancer risk.

# How to interpret these patterns The results suggest adiposity at different life stages affects breast tissue differently. Early-adulthood adiposity (BMI at 20) relates to lower percentage mammographic density later in life, a factor that is linked to reduced postmenopausal breast cancer risk. Later-life adiposity (midlife BMI) is associated with higher fibroglandular volume after adjustment and with higher cancer risk. That pattern implies distinct biological or developmental pathways for adiposity at young adult versus midlife exposures.

# Practical implications

• Self-recalled weight at age 20 can carry predictive signal for later breast cancer risk when linked with mammographic density measures.
• Mammographic density measurement method matters: area-based VAS percentage density and volume-based FGV showed different associations with early and later adiposity.
• Risk assessment that considers body size across the life course and the specific density metric used in screening might provide clearer risk stratification, but this study does not prescribe changes to clinical practice.

# Bottom line Higher BMI at age 20 was associated with lower postmenopausal breast cancer incidence in this large screening cohort, and percentage mammographic density measured by VAS mediated part of that inverse association. Conversely, higher midlife BMI was linked to higher fibroglandular volume and higher cancer risk, pointing to different effects of adiposity at different ages.