The impact of 24-h activity behaviors on physical and mental health in Chinese older adults: a compositional isochronous substitution analysis

To examine associations between the 24-h composition of movement behaviors (sedentary behavior [SB], light physical activity [LPA], moderate-to-vigorous physical activity [MVPA], and sleep) and physical and mental health in older adults using compositional data analysis. Data came from 4,150 adults aged ≥ 60 in the 2015 China Health and Nutrition Survey. Multiple‑balance isometric log‑ratio regression and compositional isotemporal substitution models were used to assess relative associations and the effect of time reallocation. The 24‑hour geometric mean composition was 43.1% sleep, 30.6% SB, 21.8% LPA, and 4.5% MVPA. LPA was positively associated with physical (β = 0.062, p < 0.001) and mental health (β = 0.117, p < 0.001), while sleep showed negative associations. SB and MVPA were not significantly associated. Reallocating 30 min from SB or sleep to LPA significantly improved both physical and mental health. Dose‑response analyses indicated greatest physical health benefit when replacing SB with about 25 min of LPA, while mental health benefits increased non‑linearly with duration. Replacing sedentary time or sleep with LPA, even in small amounts, is associated with better physical and mental health in older adults, supporting integrated 24‑hour activity guidelines that emphasize light‑intensity movement.