A Physical Activity and Sitting Time Balance Index and All-Cause Mortality Risk

Introduction Achieving a healthier balance of more time spent in physical activity (PA) and less time in sedentary behaviour is now widely advocated for achieving multiple health benefits. This study introduces a Physical Activity and Sitting Time Balance Index (PASTBI), a potential risk identification tool addressing the interplay between PA and sedentary behaviour; and aims to explore its association with the risk of all-cause mortality in Australian adults. Methods This prospective cohort study analysed the Australian Diabetes, Obesity and Lifestyle Study (AusDiab) data on 5,836 Australian adults. The PASTBI was calculated by dividing the total duration of moderate-vigorous intensity PA (minutes/day) by the duration of sitting time (ST) (hours/day), both self-reported at baseline (2004-05). The PASTBI was expressed in quartiles ranging from Q1 – low PA/high ST to Q4 – high PA/low ST. The association between PASTBI and all-cause mortality was explored (in 2022) using the Cox proportional hazards regression models adjusted for socio-demographics, lifestyle factors, waist circumference, and the number of comorbidities. Results During 78,406 person-years of follow-up (median follow-up of 14.3 years), there were 885 deaths (15%). In the fully adjusted model, compared to those in the highest PASTBI category (Quartile 4 - high PA/low ST), participants from the lowest PASTBI category (Quartile 1 - low PA/high ST) were at a higher risk of all-cause mortality [HR (95% CI) = 1.47 (1.21-1.79)]. Conclusions A less favourable balance of time spent in PA and ST (as characterized by a parsimonious PASTBI index approach) was associated with a higher risk of all-cause mortality.