Frailty status, but not physical activity volume, is associated with chronic disease, self-reported memory difficulty, and depressive mood in older Korean adults: a cross-sectional analysis of the 2019 Korea National Health and Nutrition Examination

Frailty in older adults increases vulnerability to chronic diseases, cognitive complaints, and depressive symptoms. Studies in Korean older adults have typically combined frailty status and physical activity into composite groupings that do not separate their associations. Low physical activity is also itself a frailty criterion, creating a structural dependency when it is the exposure. This cross-sectional study analyzed 1,329 community-dwelling adults aged 65 years or older using the 2019 Korea National Health and Nutrition Examination Survey. Frailty was assessed using modified Fried criteria; because the low leisure-time physical activity criterion was met by 99.4% of participants, the primary analysis used a four-item definition excluding it (robust, pre-frail, frail). Physical activity was measured using the Global Physical Activity Questionnaire and dichotomized at 600 MET-min/week. Self-reported memory difficulty and depressive mood were assessed using single items of the Health-Related Quality of Life Instrument with Eight Items (HINT-8). Analyses incorporated the complex survey design with Taylor-series linearization and fitted models containing physical activity, frailty status, and their interaction, with stratified estimates, direct testing of sex interaction, and four frailty definitions. Weighted, 47.4% were robust, 34.2% pre-frail, 18.4% frail. Compared with robust participants, adjusted odds ratios in frail participants were 2.76 (95% CI 1.70–4.47) for osteoarthritis, 4.50 (2.44–8.31) for memory difficulty, and 5.22 (2.15–12.65) for depressive mood. High versus low physical activity was not associated with osteoarthritis (0.80, 0.58–1.10), memory difficulty (0.90, 0.54–1.50), or depressive mood (0.86, 0.48–1.55), and no physical activity × frailty interaction was detected ( p = 0.381, 0.753, 0.167). Findings were robust to stratification, extended adjustment, and all four frailty definitions, and no sex interaction was detected. Among participants with high physical activity, 64.0% of volume derived from transport-related activity and 63.0% reported no leisure-time activity; only 15.0% of the sample reported any. Frailty status, rather than total physical activity volume, was associated with chronic disease, self-reported memory difficulty, and depressive mood in older Korean adults. Because activity volume derived predominantly from transport-related rather than leisure-time activity, the domain and type of activity may matter more than total volume for frailty-related health. Prospective studies using objective, domain-specific measurement are required.