Breastfeeding provides substantial maternal and infant health benefits, including reduced risk of breast and ovarian cancers and improved postpartum recovery. However, evidence regarding breastfeeding duration and its association with body size phenotypes in postmenopausal women remains limited.

We analyzed data from 3,814 postmenopausal women using the Korean National Health and Nutrition Examination Survey (2019–2021). Body size phenotypes were classified as metabolically healthy normal weight (MHNW), metabolically abnormal normal weight (MANW), metabolically healthy obesity (MHO), and metabolically abnormal obesity (MAO) based on body mass index and metabolic syndrome criteria. Breastfeeding duration per child was categorized as non-breastfeeding, 1–6 months, 7–12 months, 13–18 months, and >18 months. Multivariate logistic regression analyses were performed adjusting for age, smoking, alcohol consumption, family income, education, residence, and physical activity.

Breastfeeding durations per child were: 11.3% (non-breastfeeders), 17.0% (1–6 months), 41.2% (7–12 months), 8.5% (13–18 months), and 15.0% (>18 months). Longer breastfeeding was associated with older age, higher BMI, elevated blood pressure, and lower income. After full adjustment, breastfeeding >18 months per child was significantly associated with the MAO phenotype (odds ratio 2.04; 95% CI 1.21–3.43) compared with non-breastfeeders.

In postmenopausal women, prolonged breastfeeding duration was independently associated with the metabolically abnormal obesity phenotype. These findings highlight the need for tailored metabolic health strategies in women with long breastfeeding histories.