Jasmiina Rantala, Karri Seppä, Johan Eriksson, Sirpa Heinävaara, Tommi Härkänen, Pekka Jousilahti, Paul Knekt, Seppo Koskinen, Maarit Laaksonen, Nea Malila, Satu Männistö, Ossi Rahkonen, Heidi Ryynänen, Sanna Lappi-Heikkinen, Janne Pitkäniemi & METCA Study Group
Nov 29, 2025
"Abstract
Background
While various modifiable risk factors for the incidence of early-onset breast cancer (EOBC) have been identified, their association with breast cancer-specific mortality (BCSM) among patients is not well known.
Methods
To assess the association between modifiable factors and breast cancer-specific mortality, we estimated hazard ratios (HRs) for BCSM in relation to these factors in a cohort of 53,899 women with 380 EOBC (aged 18–49 years at diagnosis) cases and 77 EOBC-related deaths by using the Cox proportional hazards regression.
Results
Pre-diagnosis BMI > 25 kg/m2 was associated with an increased risk of BCSM (HR 1.85, 95% CI 1.04–3.30) compared to normal weight women.
Conclusions
Being overweight or obese before an early-onset breast cancer diagnosis is associated with increased breast cancer-specific mortality. Although the potential underlying biological mechanisms remain inconclusive, it likely involves complex hormonal alterations related to changes in body adiposity. As voluntary weight loss is generally not advised during cancer treatment, maintaining a healthy weight throughout adulthood is strongly encouraged to reduce numerous health risks, including excess mortality in young breast cancer patients."