Introduction
The incidence of breast cancer in Iceland has increased by more than 50% over the past four decades. Temporal changes in body mass index (BMI) are likely to have partly contributed to the observed rise in incidence.
 
Objectives
To estimate the population attributable fraction (PAF) of breast cancer in Iceland attributable to body mass index (BMI) between 1980 and 2020, and to examine how this contribution has changed over time.
 
Methods
Data on BMI prevalence for women aged 40-69 years during the decades 1970-1979, 1980-1989, 1990-1999, and 2000-2009 were obtained from the Cancer Detection Cohort (CDC) of the Icelandic Cancer Society. We had available information on approximately 100,000 measurements. BMI was categorized into three groups: < 25, 25-30 (overweight), and > 30 (obesity). Population age distribution from Statistics Iceland was used to estimate the number of attributable cancers in each 5-year age–period combination. We obtained relevant risk estimates for the overweight (RR = 1.12) and obesity (RR = 1.25) categories from the literature and used a standardized formula for population attributable fraction (PAF) that allows for different risk estimates for different exposure levels. Using these estimates, population attributable fractions (PAFs) for overweight and obesity were calculated for each decade.
 
 
 
Results
The population attributable fraction (PAF) for overweight and obesity was 8.1% during 1970–1979 and increased modestly to 8.7% in 1980–1989. This upward trend continued into the 1990s, when the PAF reached 9.6%, followed by a slight leveling off at 9.4% in 2000–2009.
 
Conclusion
These findings suggest that other risk factors than overweight and obesity may play a more significant role in explaining the increase in breast cancer incidence over recent decades. Given that BMI contributes only a fraction of the attributable risk, further risk factors must be evaluated to account for the rise in breast cancer incidence.