Prevalence of breast cancer-related risk factors in underweight premenopausal women: the Korea National Health and Nutrition Examination Survey IV–VI

Purpose: This study aimed to examine the prevalence and trends of breast cancer-related risk factors and characteristics in premenopausal underweight Korean women according to birth year cohort. Methods: Socioeconomic and breast cancer-related risk factors were investigated in 13,415 premenopausal women using nationwide cross-sectional surveys performed between 2007 and 2015. Underweight was defined as body mass index < 18.5 kg/m ² . Multivariable models were created using complex sample procedures. Results: Underweight women comprised 9.5% of the sample. Compared with those who were obese or of normal weight, underweight women were characterized by younger age, higher rate of metropolitan residence, higher economic status, more education, higher rates of non-manual employment and unmarried status, lower rate of early menarche, higher rates of nulliparity, lower parity, alcohol consumption, and never having breastfed, and lower levels of high physical activity. Multivariable analysis showed that underweight women had a significantly lower rate of early menarche, lower parity, higher nulliparity, older age at first delivery, and lower levels of high physical activity compared to premenopausal women with normal weight. These trends were more apparent among women born in recent years. Conclusions: Underweight Korean premenopausal women exhibit distinctive features associated with an increased risk of breast cancer, except for a lower rate of early menarche. These associations were prominent in recent generations. Assessment of the association between underweight and premenopausal breast cancer risk should focus on promoting healthy lifestyles to reduce breast cancer risk.