Association of the Physical Activity–Sitting Time Balance Index with Metabolic Syndrome and Depressive Symptoms in Korean Adults: KNHANES 2014–2024

Background/Objectives: The physical activity–sitting time balance index (PASTBI), the ratio of weekly physical activity to daily sitting time, has been linked to mortality. As a ratio, it implicitly assumes that proportional changes in the two behaviors carry equal and opposite weight. We tested this assumption while examining associations with metabolic syndrome and depressive symptoms. Methods: We analyzed 26,716 adults from the Korea National Health and Nutrition Examination Survey 2014–2024. Metabolic syndrome was defined by the revised National Cholesterol Education Program criteria and depressive symptoms by a Patient Health Questionnaire-9 score ≥ 10. Design-based logistic regression with log-transformed physical activity and sitting time tested the ratio constraint. Results: Each 1-standard-deviation increase in log-transformed PASTBI was associated with lower odds of metabolic syndrome (odds ratio [OR] 0.87, 95% confidence interval [CI] 0.84–0.91) and depressive symptoms (OR 0.89, 95% CI 0.83–0.96), and both associations remained essentially unchanged after mutual adjustment. Among active participants, doubling physical activity (OR 0.91) and sitting time (OR 1.10) were reciprocally associated with metabolic syndrome (ratio constraint p = 0.972). For depressive symptoms, only sitting time was associated (ratio constraint p = 0.011). Among inactive participants (13%), whose index was always zero, each doubling of sitting time was associated with depressive symptoms (OR 1.74, 95% CI 1.33–2.28). Conclusions: PASTBI was a valid summary for metabolic syndrome among active adults but not for depressive symptoms or inactive adults. Ratio-based indices should be verified for each outcome and population.