Isotemporal Substitution Effect of 24-h Movement Behaviors on Well-Being, Cognition, and BMI Among Older Adults

<b>Background</b>: This study investigated the interdependent relationships among older adults' daily engagement in physical activity (PA), sedentary time (ST), sleep, and their well-being, cognition, and body mass index (BMI). <b>Method</b>: Forty healthy older adults (31 females; Mean [age] = 70.8 ± 5.58) were included in the analysis. Participants wore a Fitbit tracker for an average of 23 h a day, five days a week, over six months. The Fitbit device tracked lightly active time, active time, ST, and sleep durations. Quality of life and cognitive flexibility were assessed using validated instruments. BMI was calculated using participants' self-reported height and weight. A compositional analysis (CODA) investigated the codependent associations among these variables and model time reallocation between behaviors. <b>Results</b>: Regression models utilizing CODA indicated significant associations between the outcomes of BMI (<i>p</i> = 0.05; Adj. R² = 0.20), while cognitive flexibility and quality of life revealed no association (<i>p</i> > 0.05). Shifting 10 min from ST to active time is associated with a theoretical decrease of -0.76 (95% CI, -1.49 to -0.04) units in BMI. Similarly, reallocating 10 min from active time to ST is associated with a theoretical increase of 1.17 (95% CI, 0.03 to 2.3) units in BMI. Reallocating 10 min between other movement behaviors yielded no statistical significance. <b>Conclusions</b>: Our study highlights the importance of promoting active time to improve BMI in this population. Encouraging 10 min bouts of PA among older adults, in place of ST, is vital for improving national PA guideline adherence.