The effect of <scp>body mass index</scp> change on breast and uterine cancer risk in middle‐ and older‐aged adults

The effects of cofactors on the impact of body mass index (BMI) change on breast and endometrial cancer are rarely investigated. This cohort study included cancer-free women who underwent national health examinations in 2010-2014 (Cohort 1) and 2009-2013 (Cohort 2), with follow-up through 2021 and 2017, respectively. BMI at baseline and follow-up was combined and categorized into 25 BMI change patterns. Cohort 1 (2.24 million) had 30,323 breast and 5465 uterine corpus cancers, while Cohort 2 (1.46 million) had 8580 breast and 1170 uterine corpus cancers. In Cohort 1, persistent low BMI was associated with a reduced risk of both cancers, particularly in postmenopausal women and never-smokers. However, persistent overweight, obesity I/II, and BMI fluctuation (loss or gain) were linked to an increased risk of both cancers, particularly in postmenopausal women and ever-smokers. Among premenopausal women, substantial BMI loss was associated with increased breast cancer risk. In postmenopausal women, a transition from overweight to underweight (adjusted hazard ratio [aHR]: 4.63, 95% confidence interval [CI]: 1.15-18.59) and from overweight to obesity class II (aHR: 4.06, 95% CI: 1.01-16.30) significantly increased uterine cancer risk. Among smokers, most BMI changes did not show significant association, except for persistent obesity I (aHR: 1.57, 95% CI: 1.01-2.43) and progression from obesity class I to II (aHR: 4.03, 95% CI: 1.97-8.26). Cohort 2 findings were largely consistent with those from Cohort 1. In conclusion, persistent high BMI and further BMI gain were associated with increased risk of both cancers, particularly in postmenopausal women and never-smokers.