**Background:** Depressive symptoms and disorders are common and debilitating in older adults, contributing to chronic disease burden and increased mortality. Physical activity is known to reduce depression risk, but the minimal effective dose for older adults, especially those with chronic illness, is unclear. This study aimed to quantify the lowest dose of moderate-to-vigorous physical activity (MVPA) associated with reduced odds of depressive symptoms and major depression in older adults with and without chronic disease over 10 years, using data from The Irish Longitudinal Study on Ageing (TILDA).
**Methods:** This population-representative cohort study included 4016 participants aged ≥50 years (54.9% female; mean age 61.0 years at baseline) from TILDA waves 1–5 (2009–2018). Depressive symptoms were measured using the 8-item Centre for Epidemiological Studies Depression (CES-D) scale; major depression was defined as CES-D ≥9 and/or a major depressive episode on the Composite International Diagnostic Interview. Physical activity was self-reported via the International Physical Activity Questionnaire and expressed as MET-minutes per week. MVPA was categorized into 3-dose (low <600, moderate 600–<1200, high ≥1200 MET-min/wk), 5-dose (0, 1–<600, 600–<1200, 1200–<2400, ≥2400 MET-min/wk), and continuous MET-min/wk. A post hoc analysis further refined the 1–<600 category into 1–<200, 200–<400, and 400–<600 MET-min/wk. Mixed-effects regression with negative binomial link (for depressive symptoms) and logistic link (for major depression) were used, adjusting for wave, age, sex, education, smoking, alcohol use, and antidepressant medication. Analyses were stratified by chronic disease status.
**Key Results:** Over 10 years, depression rates increased from 8.2% to 12.2%, and antidepressant use rose from 6.1% to 10.3%. The proportion meeting WHO guidelines (≥600 MET-min/wk) declined from 51.1% to 40.9%. For depressive symptoms, a dose-response was observed: compared with low activity, high activity (≥1200 MET-min/wk) was associated with a 20% lower rate (AIRR 0.80; 95% CI 0.78–0.84). In the 5-dose analysis, rates were 7% lower for 600–<1200 (AIRR 0.93; 95% CI 0.88–0.99), 16% lower for 1200–<2400 (AIRR 0.84; 95% CI 0.80–0.89), and 23% lower for ≥2400 (AIRR 0.77; 95% CI 0.74–0.80) compared with 0 MET-min/wk. The post hoc analysis showed that 400–<600 MET-min/wk was associated with a 16% lower rate (AIRR 0.84; 95% CI 0.81–0.86). For major depression, high activity (≥1200) was associated with 44% lower odds (AOR 0.56; 95% CI 0.48–0.65); 600–<1200 MET-min/wk gave 44% lower odds (AOR 0.56; 95% CI 0.43–0.73); and ≥2400 gave 49% lower odds (AOR 0.51; 95% CI 0.43–0.62). The post hoc analysis found 400–<600 MET-min/wk associated with 43% lower odds (AOR 0.57; 95% CI 0.49–0.66). Among those with chronic disease, benefits for depressive symptoms appeared at 600–<1200 MET-min/wk (AIRR 0.92; 95% CI 0.86–0.98), while those without disease required ≥2400 MET-min/wk for significant reduction (AIRR 0.81; 95% CI 0.73–0.90). For major depression, those with disease showed reduced odds starting at 600–<1200 (AOR 0.56; 95% CI 0.42–0.74), while those without disease only showed benefit at ≥1200 (AOR 0.65; 95% CI 0.45–0.95 for high dose in 3-dose model). Continuous MET-min/wk showed no significant association.
**Clinical Implications:** This study suggests that even low doses of MVPA (400–<600 MET-min/wk, equivalent to ~100 minutes of brisk walking per week) are associated with reduced depressive symptoms and major depression in older adults, with greater doses providing larger benefits. The minimal effective dose may be lower than current WHO guidelines (≥600 MET-min/wk), which could be more achievable for older adults, especially those with chronic disease. Clinicians and public health agencies should consider promoting these lower, more acceptable doses to improve mental health outcomes in aging populations. However, the observational design precludes causality, and self-reported activity may introduce bias. Future research should use device-measured activity and include diverse populations.